diff --git a/OpenRA.Game/Actor.cs b/OpenRA.Game/Actor.cs
index 9f608f8815..f90d0b3465 100644
--- a/OpenRA.Game/Actor.cs
+++ b/OpenRA.Game/Actor.cs
@@ -25,7 +25,7 @@ namespace OpenRA
 {
 	public sealed class Actor : IScriptBindable, IScriptNotifyBind, ILuaTableBinding, ILuaEqualityBinding, ILuaToStringBinding, IEquatable<Actor>, IDisposable
 	{
-		internal struct SyncHash
+		internal readonly struct SyncHash
 		{
 			public readonly ISync Trait;
 			readonly Func<object, int> hashFunction;
diff --git a/OpenRA.Game/CPos.cs b/OpenRA.Game/CPos.cs
index 35caf2d52a..9ef4afaa98 100644
--- a/OpenRA.Game/CPos.cs
+++ b/OpenRA.Game/CPos.cs
@@ -16,7 +16,7 @@ using OpenRA.Scripting;
 
 namespace OpenRA
 {
-	public struct CPos : IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaEqualityBinding, ILuaTableBinding, IEquatable<CPos>
+	public readonly struct CPos : IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaEqualityBinding, ILuaTableBinding, IEquatable<CPos>
 	{
 		// Coordinates are packed in a 32 bit signed int
 		// X and Y are 12 bits (signed): -2048...2047
diff --git a/OpenRA.Game/CVec.cs b/OpenRA.Game/CVec.cs
index 4a7910d976..ee7e721eb8 100644
--- a/OpenRA.Game/CVec.cs
+++ b/OpenRA.Game/CVec.cs
@@ -17,7 +17,7 @@ using OpenRA.Scripting;
 
 namespace OpenRA
 {
-	public struct CVec : IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaUnaryMinusBinding, ILuaEqualityBinding, ILuaTableBinding, IEquatable<CVec>
+	public readonly struct CVec : IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaUnaryMinusBinding, ILuaEqualityBinding, ILuaTableBinding, IEquatable<CVec>
 	{
 		public readonly int X, Y;
 
diff --git a/OpenRA.Game/Graphics/Animation.cs b/OpenRA.Game/Graphics/Animation.cs
index c7e0bf61ea..63e00520d7 100644
--- a/OpenRA.Game/Graphics/Animation.cs
+++ b/OpenRA.Game/Graphics/Animation.cs
@@ -52,7 +52,7 @@ namespace OpenRA.Graphics
 		public int CurrentFrame => backwards ? CurrentSequence.Length - frame - 1 : frame;
 		public Sprite Image => CurrentSequence.GetSprite(CurrentFrame, facingFunc());
 
-		public IRenderable[] Render(WPos pos, WVec offset, int zOffset, PaletteReference palette)
+		public IRenderable[] Render(WPos pos, in WVec offset, int zOffset, PaletteReference palette)
 		{
 			var tintModifiers = CurrentSequence.IgnoreWorldTint ? TintModifiers.IgnoreWorldTint : TintModifiers.None;
 			var alpha = CurrentSequence.GetAlpha(CurrentFrame);
@@ -68,7 +68,7 @@ namespace OpenRA.Graphics
 			return new IRenderable[] { imageRenderable };
 		}
 
-		public IRenderable[] RenderUI(WorldRenderer wr, int2 pos, WVec offset, int zOffset, PaletteReference palette, float scale = 1f)
+		public IRenderable[] RenderUI(WorldRenderer wr, int2 pos, in WVec offset, int zOffset, PaletteReference palette, float scale = 1f)
 		{
 			scale *= CurrentSequence.Scale;
 			var screenOffset = (scale * wr.ScreenVectorComponents(offset)).XY.ToInt2();
@@ -87,7 +87,7 @@ namespace OpenRA.Graphics
 			return new IRenderable[] { imageRenderable };
 		}
 
-		public Rectangle ScreenBounds(WorldRenderer wr, WPos pos, WVec offset)
+		public Rectangle ScreenBounds(WorldRenderer wr, WPos pos, in WVec offset)
 		{
 			var scale = CurrentSequence.Scale;
 			var xy = wr.ScreenPxPosition(pos) + wr.ScreenPxOffset(offset);
diff --git a/OpenRA.Game/Graphics/Model.cs b/OpenRA.Game/Graphics/Model.cs
index 1eb3526ca7..b724c0428e 100644
--- a/OpenRA.Game/Graphics/Model.cs
+++ b/OpenRA.Game/Graphics/Model.cs
@@ -29,7 +29,7 @@ namespace OpenRA.Graphics
 		Rectangle AggregateBounds { get; }
 	}
 
-	public struct ModelRenderData
+	public readonly struct ModelRenderData
 	{
 		public readonly int Start;
 		public readonly int Count;
diff --git a/OpenRA.Game/Graphics/ModelAnimation.cs b/OpenRA.Game/Graphics/ModelAnimation.cs
index 6bc96f060b..2485a10a81 100644
--- a/OpenRA.Game/Graphics/ModelAnimation.cs
+++ b/OpenRA.Game/Graphics/ModelAnimation.cs
@@ -14,7 +14,7 @@ using OpenRA.Primitives;
 
 namespace OpenRA.Graphics
 {
-	public struct ModelAnimation
+	public readonly struct ModelAnimation
 	{
 		public readonly IModel Model;
 		public readonly Func<WVec> OffsetFunc;
diff --git a/OpenRA.Game/Graphics/PlatformInterfaces.cs b/OpenRA.Game/Graphics/PlatformInterfaces.cs
index f4fc561b4f..0e2fd853b3 100644
--- a/OpenRA.Game/Graphics/PlatformInterfaces.cs
+++ b/OpenRA.Game/Graphics/PlatformInterfaces.cs
@@ -145,7 +145,7 @@ namespace OpenRA
 		TriangleList,
 	}
 
-	public struct Range<T>
+	public readonly struct Range<T>
 	{
 		public readonly T Start, End;
 		public Range(T start, T end) { Start = start; End = end; }
diff --git a/OpenRA.Game/Graphics/Renderable.cs b/OpenRA.Game/Graphics/Renderable.cs
index 8935d05a7e..f3970cbf9c 100644
--- a/OpenRA.Game/Graphics/Renderable.cs
+++ b/OpenRA.Game/Graphics/Renderable.cs
@@ -21,7 +21,7 @@ namespace OpenRA.Graphics
 		bool IsDecoration { get; }
 
 		IRenderable WithZOffset(int newOffset);
-		IRenderable OffsetBy(WVec offset);
+		IRenderable OffsetBy(in WVec offset);
 		IRenderable AsDecoration();
 
 		IFinalizedRenderable PrepareRender(WorldRenderer wr);
diff --git a/OpenRA.Game/Graphics/SpriteRenderable.cs b/OpenRA.Game/Graphics/SpriteRenderable.cs
index e478e17d44..2953a9518a 100644
--- a/OpenRA.Game/Graphics/SpriteRenderable.cs
+++ b/OpenRA.Game/Graphics/SpriteRenderable.cs
@@ -55,7 +55,7 @@ namespace OpenRA.Graphics
 
 		public IPalettedRenderable WithPalette(PaletteReference newPalette) { return new SpriteRenderable(sprite, pos, offset, zOffset, newPalette, scale, alpha, tint, tintModifiers, isDecoration); }
 		public IRenderable WithZOffset(int newOffset) { return new SpriteRenderable(sprite, pos, offset, newOffset, palette, scale, alpha, tint, tintModifiers, isDecoration); }
-		public IRenderable OffsetBy(WVec vec) { return new SpriteRenderable(sprite, pos + vec, offset, zOffset, palette, scale, alpha, tint, tintModifiers, isDecoration); }
+		public IRenderable OffsetBy(in WVec vec) { return new SpriteRenderable(sprite, pos + vec, offset, zOffset, palette, scale, alpha, tint, tintModifiers, isDecoration); }
 		public IRenderable AsDecoration() { return new SpriteRenderable(sprite, pos, offset, zOffset, palette, scale, alpha, tint, tintModifiers, true); }
 
 		public IModifyableRenderable WithAlpha(float newAlpha)
diff --git a/OpenRA.Game/Graphics/TargetLineRenderable.cs b/OpenRA.Game/Graphics/TargetLineRenderable.cs
index ecc542abcd..fada77017d 100644
--- a/OpenRA.Game/Graphics/TargetLineRenderable.cs
+++ b/OpenRA.Game/Graphics/TargetLineRenderable.cs
@@ -35,7 +35,14 @@ namespace OpenRA.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return new TargetLineRenderable(waypoints, color); }
-		public IRenderable OffsetBy(WVec vec) { return new TargetLineRenderable(waypoints.Select(w => w + vec), color); }
+
+		public IRenderable OffsetBy(in WVec vec)
+		{
+			// Lambdas can't use 'in' variables, so capture a copy for later
+			var offset = vec;
+			return new TargetLineRenderable(waypoints.Select(w => w + offset), color);
+		}
+
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Game/Graphics/UISpriteRenderable.cs b/OpenRA.Game/Graphics/UISpriteRenderable.cs
index 0930881138..23e57cce7b 100644
--- a/OpenRA.Game/Graphics/UISpriteRenderable.cs
+++ b/OpenRA.Game/Graphics/UISpriteRenderable.cs
@@ -44,7 +44,7 @@ namespace OpenRA.Graphics
 
 		public IPalettedRenderable WithPalette(PaletteReference newPalette) { return new UISpriteRenderable(sprite, effectiveWorldPos, screenPos, zOffset, newPalette, scale, alpha); }
 		public IRenderable WithZOffset(int newOffset) { return this; }
-		public IRenderable OffsetBy(WVec vec) { return this; }
+		public IRenderable OffsetBy(in WVec vec) { return this; }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Game/Graphics/Vertex.cs b/OpenRA.Game/Graphics/Vertex.cs
index 6893a3bee4..87b1a3f194 100644
--- a/OpenRA.Game/Graphics/Vertex.cs
+++ b/OpenRA.Game/Graphics/Vertex.cs
@@ -14,7 +14,7 @@ using System.Runtime.InteropServices;
 namespace OpenRA.Graphics
 {
 	[StructLayout(LayoutKind.Sequential)]
-	public struct Vertex
+	public readonly struct Vertex
 	{
 		// 3d position
 		public readonly float X, Y, Z;
diff --git a/OpenRA.Game/Graphics/WorldRenderer.cs b/OpenRA.Game/Graphics/WorldRenderer.cs
index 97b87baa3b..b793241ded 100644
--- a/OpenRA.Game/Graphics/WorldRenderer.cs
+++ b/OpenRA.Game/Graphics/WorldRenderer.cs
@@ -373,7 +373,7 @@ namespace OpenRA.Graphics
 		}
 
 		// For scaling vectors to pixel sizes in the model renderer
-		public float3 ScreenVectorComponents(WVec vec)
+		public float3 ScreenVectorComponents(in WVec vec)
 		{
 			return new float3(
 				(float)TileSize.Width * vec.X / TileScale,
@@ -382,13 +382,13 @@ namespace OpenRA.Graphics
 		}
 
 		// For scaling vectors to pixel sizes in the model renderer
-		public float[] ScreenVector(WVec vec)
+		public float[] ScreenVector(in WVec vec)
 		{
 			var xyz = ScreenVectorComponents(vec);
 			return new[] { xyz.X, xyz.Y, xyz.Z, 1f };
 		}
 
-		public int2 ScreenPxOffset(WVec vec)
+		public int2 ScreenPxOffset(in WVec vec)
 		{
 			// Round to nearest pixel
 			var xyz = ScreenVectorComponents(vec);
diff --git a/OpenRA.Game/Input/Hotkey.cs b/OpenRA.Game/Input/Hotkey.cs
index 4264ae86be..b510cebaca 100644
--- a/OpenRA.Game/Input/Hotkey.cs
+++ b/OpenRA.Game/Input/Hotkey.cs
@@ -13,7 +13,7 @@ using System;
 
 namespace OpenRA
 {
-	public struct Hotkey : IEquatable<Hotkey>
+	public readonly struct Hotkey : IEquatable<Hotkey>
 	{
 		public static Hotkey Invalid = new Hotkey(Keycode.UNKNOWN, Modifiers.None);
 		public bool IsValid()
diff --git a/OpenRA.Game/MPos.cs b/OpenRA.Game/MPos.cs
index 8892713223..b376311318 100644
--- a/OpenRA.Game/MPos.cs
+++ b/OpenRA.Game/MPos.cs
@@ -14,7 +14,7 @@ using OpenRA.Primitives;
 
 namespace OpenRA
 {
-	public struct MPos : IEquatable<MPos>
+	public readonly struct MPos : IEquatable<MPos>
 	{
 		public readonly int U, V;
 
@@ -66,7 +66,7 @@ namespace OpenRA
 	/// <summary>
 	/// Projected map position
 	/// </summary>
-	public struct PPos : IEquatable<PPos>
+	public readonly struct PPos : IEquatable<PPos>
 	{
 		public readonly int U, V;
 
diff --git a/OpenRA.Game/Map/Map.cs b/OpenRA.Game/Map/Map.cs
index c122c6cfa9..6bd505a182 100644
--- a/OpenRA.Game/Map/Map.cs
+++ b/OpenRA.Game/Map/Map.cs
@@ -24,7 +24,7 @@ using OpenRA.Traits;
 
 namespace OpenRA
 {
-	struct BinaryDataHeader
+	readonly struct BinaryDataHeader
 	{
 		public readonly byte Format;
 		public readonly uint TilesOffset;
@@ -1225,7 +1225,7 @@ namespace OpenRA
 			return AllEdgeCells.Random(rand);
 		}
 
-		public WDist DistanceToEdge(WPos pos, WVec dir)
+		public WDist DistanceToEdge(WPos pos, in WVec dir)
 		{
 			var projectedPos = pos - new WVec(0, pos.Z, pos.Z);
 			var x = dir.X == 0 ? int.MaxValue : ((dir.X < 0 ? ProjectedTopLeft.X : ProjectedBottomRight.X) - projectedPos.X) / dir.X;
diff --git a/OpenRA.Game/Map/MapCoordsRegion.cs b/OpenRA.Game/Map/MapCoordsRegion.cs
index d4d813330e..156643faea 100644
--- a/OpenRA.Game/Map/MapCoordsRegion.cs
+++ b/OpenRA.Game/Map/MapCoordsRegion.cs
@@ -14,7 +14,7 @@ using System.Collections.Generic;
 
 namespace OpenRA
 {
-	public struct MapCoordsRegion : IEnumerable<MPos>
+	public readonly struct MapCoordsRegion : IEnumerable<MPos>
 	{
 		public struct MapCoordsEnumerator : IEnumerator<MPos>
 		{
diff --git a/OpenRA.Game/Map/MapGrid.cs b/OpenRA.Game/Map/MapGrid.cs
index 2e8483f153..3abab39569 100644
--- a/OpenRA.Game/Map/MapGrid.cs
+++ b/OpenRA.Game/Map/MapGrid.cs
@@ -22,7 +22,7 @@ namespace OpenRA
 	public enum RampSplit { Flat, X, Y }
 	public enum RampCornerHeight { Low = 0, Half = 1, Full = 2 }
 
-	public struct CellRamp
+	public readonly struct CellRamp
 	{
 		public readonly int CenterHeightOffset;
 		public readonly WVec[] Corners;
diff --git a/OpenRA.Game/Map/TileReference.cs b/OpenRA.Game/Map/TileReference.cs
index 0adfe7a047..693218f893 100644
--- a/OpenRA.Game/Map/TileReference.cs
+++ b/OpenRA.Game/Map/TileReference.cs
@@ -11,7 +11,7 @@
 
 namespace OpenRA
 {
-	public struct TerrainTile
+	public readonly struct TerrainTile
 	{
 		public readonly ushort Type;
 		public readonly byte Index;
@@ -27,7 +27,7 @@ namespace OpenRA
 		public override string ToString() { return Type + "," + Index; }
 	}
 
-	public struct ResourceTile
+	public readonly struct ResourceTile
 	{
 		public readonly byte Type;
 		public readonly byte Index;
diff --git a/OpenRA.Game/Primitives/ActionQueue.cs b/OpenRA.Game/Primitives/ActionQueue.cs
index 09bdcff491..f0489d2cf8 100644
--- a/OpenRA.Game/Primitives/ActionQueue.cs
+++ b/OpenRA.Game/Primitives/ActionQueue.cs
@@ -65,7 +65,7 @@ namespace OpenRA.Primitives
 		}
 	}
 
-	struct DelayedAction : IComparable<DelayedAction>
+	readonly struct DelayedAction : IComparable<DelayedAction>
 	{
 		public readonly long Time;
 		public readonly Action Action;
diff --git a/OpenRA.Game/Primitives/BitSet.cs b/OpenRA.Game/Primitives/BitSet.cs
index 9881325441..0aa85de83a 100644
--- a/OpenRA.Game/Primitives/BitSet.cs
+++ b/OpenRA.Game/Primitives/BitSet.cs
@@ -75,7 +75,7 @@ namespace OpenRA.Primitives
 		}
 	}
 
-	public struct BitSet<T> : IEnumerable<string>, IEquatable<BitSet<T>> where T : class
+	public readonly struct BitSet<T> : IEnumerable<string>, IEquatable<BitSet<T>> where T : class
 	{
 		readonly BitSetIndex bits;
 
diff --git a/OpenRA.Game/Primitives/Color.cs b/OpenRA.Game/Primitives/Color.cs
index 13819d6b7e..72ba06d12e 100644
--- a/OpenRA.Game/Primitives/Color.cs
+++ b/OpenRA.Game/Primitives/Color.cs
@@ -15,7 +15,7 @@ using OpenRA.Scripting;
 
 namespace OpenRA.Primitives
 {
-	public struct Color : IEquatable<Color>, IScriptBindable
+	public readonly struct Color : IEquatable<Color>, IScriptBindable
 	{
 		readonly long argb;
 
diff --git a/OpenRA.Game/Primitives/Int32Matrix4x4.cs b/OpenRA.Game/Primitives/Int32Matrix4x4.cs
index a4117b5ed9..b6eef02a4f 100644
--- a/OpenRA.Game/Primitives/Int32Matrix4x4.cs
+++ b/OpenRA.Game/Primitives/Int32Matrix4x4.cs
@@ -13,7 +13,7 @@ using System;
 
 namespace OpenRA
 {
-	public struct Int32Matrix4x4 : IEquatable<Int32Matrix4x4>
+	public readonly struct Int32Matrix4x4 : IEquatable<Int32Matrix4x4>
 	{
 		public readonly int M11, M12, M13, M14, M21, M22, M23, M24, M31, M32, M33, M34, M41, M42, M43, M44;
 
diff --git a/OpenRA.Game/Primitives/LongBitSet.cs b/OpenRA.Game/Primitives/LongBitSet.cs
index ba0e958572..6e78e2bb84 100644
--- a/OpenRA.Game/Primitives/LongBitSet.cs
+++ b/OpenRA.Game/Primitives/LongBitSet.cs
@@ -92,7 +92,7 @@ namespace OpenRA.Primitives
 	}
 
 	// Opitmized BitSet to be used only when guaranteed to be no more than 64 values.
-	public struct LongBitSet<T> : IEnumerable<string>, IEquatable<LongBitSet<T>> where T : class
+	public readonly struct LongBitSet<T> : IEnumerable<string>, IEquatable<LongBitSet<T>> where T : class
 	{
 		readonly long bits;
 
diff --git a/OpenRA.Game/Primitives/PlayerDictionary.cs b/OpenRA.Game/Primitives/PlayerDictionary.cs
index 74d1cde195..d032ead5f0 100644
--- a/OpenRA.Game/Primitives/PlayerDictionary.cs
+++ b/OpenRA.Game/Primitives/PlayerDictionary.cs
@@ -17,7 +17,7 @@ namespace OpenRA.Primitives
 	/// <summary>
 	/// Provides a mapping of players to values, as well as fast lookup by player index.
 	/// </summary>
-	public struct PlayerDictionary<T> : IReadOnlyList<T>, IReadOnlyDictionary<Player, T> where T : class
+	public readonly struct PlayerDictionary<T> : IReadOnlyList<T>, IReadOnlyDictionary<Player, T> where T : class
 	{
 		readonly T[] valueByPlayerIndex;
 		readonly Dictionary<Player, T> valueByPlayer;
diff --git a/OpenRA.Game/Primitives/Polygon.cs b/OpenRA.Game/Primitives/Polygon.cs
index cf9b51b364..a4fee9b440 100644
--- a/OpenRA.Game/Primitives/Polygon.cs
+++ b/OpenRA.Game/Primitives/Polygon.cs
@@ -14,13 +14,13 @@ using System.Linq;
 
 namespace OpenRA.Primitives
 {
-	public struct Polygon
+	public readonly struct Polygon
 	{
 		public static readonly Polygon Empty = new Polygon(Rectangle.Empty);
 
 		public readonly Rectangle BoundingRect;
 		public readonly int2[] Vertices;
-		bool isRectangle;
+		readonly bool isRectangle;
 
 		public Polygon(Rectangle bounds)
 		{
diff --git a/OpenRA.Game/Primitives/Size.cs b/OpenRA.Game/Primitives/Size.cs
index 06f05b2b33..36f1e3a7d4 100644
--- a/OpenRA.Game/Primitives/Size.cs
+++ b/OpenRA.Game/Primitives/Size.cs
@@ -13,7 +13,7 @@ using System;
 
 namespace OpenRA.Primitives
 {
-	public struct Size : IEquatable<Size>
+	public readonly struct Size : IEquatable<Size>
 	{
 		public readonly int Width;
 		public readonly int Height;
diff --git a/OpenRA.Game/Primitives/float2.cs b/OpenRA.Game/Primitives/float2.cs
index bffb3d6c10..ed62e4fe46 100644
--- a/OpenRA.Game/Primitives/float2.cs
+++ b/OpenRA.Game/Primitives/float2.cs
@@ -18,7 +18,7 @@ namespace OpenRA
 {
 	[SuppressMessage("StyleCop.CSharp.NamingRules", "SA1300:ElementMustBeginWithUpperCaseLetter", Justification = "Mimic a built-in type alias.")]
 	[StructLayout(LayoutKind.Sequential)]
-	public struct float2 : IEquatable<float2>
+	public readonly struct float2 : IEquatable<float2>
 	{
 		public readonly float X, Y;
 
diff --git a/OpenRA.Game/Primitives/int2.cs b/OpenRA.Game/Primitives/int2.cs
index 8543ca412f..b4e50105fe 100644
--- a/OpenRA.Game/Primitives/int2.cs
+++ b/OpenRA.Game/Primitives/int2.cs
@@ -16,7 +16,7 @@ using OpenRA.Primitives;
 namespace OpenRA
 {
 	[SuppressMessage("StyleCop.CSharp.NamingRules", "SA1300:ElementMustBeginWithUpperCaseLetter", Justification = "Mimic a built-in type alias.")]
-	public struct int2 : IEquatable<int2>
+	public readonly struct int2 : IEquatable<int2>
 	{
 		public readonly int X, Y;
 		public int2(int x, int y) { X = x; Y = y; }
diff --git a/OpenRA.Game/SelectableExts.cs b/OpenRA.Game/SelectableExts.cs
index 09d509665b..ee38c4a089 100644
--- a/OpenRA.Game/SelectableExts.cs
+++ b/OpenRA.Game/SelectableExts.cs
@@ -73,7 +73,7 @@ namespace OpenRA.Traits
 			return actors.MaxByOrDefault(a => CalculateActorSelectionPriority(a.Info, a.MouseBounds, selectionPixel, modifiers));
 		}
 
-		static long CalculateActorSelectionPriority(ActorInfo info, Polygon bounds, int2 selectionPixel, Modifiers modifiers)
+		static long CalculateActorSelectionPriority(ActorInfo info, in Polygon bounds, int2 selectionPixel, Modifiers modifiers)
 		{
 			if (bounds.IsEmpty)
 				return info.SelectionPriority(modifiers);
diff --git a/OpenRA.Game/Support/PerfSample.cs b/OpenRA.Game/Support/PerfSample.cs
index f7348be8ce..0137cdabd0 100644
--- a/OpenRA.Game/Support/PerfSample.cs
+++ b/OpenRA.Game/Support/PerfSample.cs
@@ -14,7 +14,7 @@ using System.Diagnostics;
 
 namespace OpenRA.Support
 {
-	public struct PerfSample : IDisposable
+	public readonly struct PerfSample : IDisposable
 	{
 		readonly string item;
 		readonly long ticks;
diff --git a/OpenRA.Game/Support/VariableExpression.cs b/OpenRA.Game/Support/VariableExpression.cs
index df7a6584b3..09a745dc09 100644
--- a/OpenRA.Game/Support/VariableExpression.cs
+++ b/OpenRA.Game/Support/VariableExpression.cs
@@ -157,7 +157,7 @@ namespace OpenRA.Support
 			Invalid = ~0
 		}
 
-		struct TokenTypeInfo
+		readonly struct TokenTypeInfo
 		{
 			public readonly string Symbol;
 			public readonly Precedence Precedence;
diff --git a/OpenRA.Game/TraitDictionary.cs b/OpenRA.Game/TraitDictionary.cs
index 5df5bb91cf..881510dfca 100644
--- a/OpenRA.Game/TraitDictionary.cs
+++ b/OpenRA.Game/TraitDictionary.cs
@@ -249,7 +249,7 @@ namespace OpenRA
 				}
 			}
 
-			struct AllEnumerable : IEnumerable<TraitPair<T>>
+			readonly struct AllEnumerable : IEnumerable<TraitPair<T>>
 			{
 				readonly TraitContainer<T> container;
 				public AllEnumerable(TraitContainer<T> container) { this.container = container; }
diff --git a/OpenRA.Game/Traits/World/ScreenMap.cs b/OpenRA.Game/Traits/World/ScreenMap.cs
index 2e4144d197..87157533ba 100644
--- a/OpenRA.Game/Traits/World/ScreenMap.cs
+++ b/OpenRA.Game/Traits/World/ScreenMap.cs
@@ -18,7 +18,7 @@ using OpenRA.Primitives;
 
 namespace OpenRA.Traits
 {
-	public struct ActorBoundsPair
+	public readonly struct ActorBoundsPair
 	{
 		public readonly Actor Actor;
 		public readonly Polygon Bounds;
diff --git a/OpenRA.Game/WAngle.cs b/OpenRA.Game/WAngle.cs
index 1ba06268b9..8f4c6e88a9 100644
--- a/OpenRA.Game/WAngle.cs
+++ b/OpenRA.Game/WAngle.cs
@@ -19,7 +19,7 @@ namespace OpenRA
 	/// <summary>
 	/// 1D angle - 1024 units = 360 degrees.
 	/// </summary>
-	public struct WAngle : IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaEqualityBinding, IEquatable<WAngle>
+	public readonly struct WAngle : IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaEqualityBinding, IEquatable<WAngle>
 	{
 		public readonly int Angle;
 		public int AngleSquared => (int)Angle * Angle;
diff --git a/OpenRA.Game/WDist.cs b/OpenRA.Game/WDist.cs
index b14c01cac6..1d494afdba 100644
--- a/OpenRA.Game/WDist.cs
+++ b/OpenRA.Game/WDist.cs
@@ -21,7 +21,7 @@ namespace OpenRA
 	/// <summary>
 	/// 1d world distance - 1024 units = 1 cell.
 	/// </summary>
-	public struct WDist : IComparable, IComparable<WDist>, IEquatable<WDist>, IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaEqualityBinding, ILuaTableBinding
+	public readonly struct WDist : IComparable, IComparable<WDist>, IEquatable<WDist>, IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaEqualityBinding, ILuaTableBinding
 	{
 		public readonly int Length;
 		public long LengthSquared => (long)Length * Length;
diff --git a/OpenRA.Game/WPos.cs b/OpenRA.Game/WPos.cs
index 3beb20ef41..0ae9c2de59 100644
--- a/OpenRA.Game/WPos.cs
+++ b/OpenRA.Game/WPos.cs
@@ -18,7 +18,7 @@ using OpenRA.Scripting;
 
 namespace OpenRA
 {
-	public struct WPos : IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaEqualityBinding, ILuaTableBinding, IEquatable<WPos>
+	public readonly struct WPos : IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaEqualityBinding, ILuaTableBinding, IEquatable<WPos>
 	{
 		public readonly int X, Y, Z;
 
@@ -27,24 +27,24 @@ namespace OpenRA
 
 		public static readonly WPos Zero = new WPos(0, 0, 0);
 
-		public static explicit operator WVec(WPos a) { return new WVec(a.X, a.Y, a.Z); }
+		public static explicit operator WVec(in WPos a) { return new WVec(a.X, a.Y, a.Z); }
 
-		public static WPos operator +(WPos a, WVec b) { return new WPos(a.X + b.X, a.Y + b.Y, a.Z + b.Z); }
-		public static WPos operator -(WPos a, WVec b) { return new WPos(a.X - b.X, a.Y - b.Y, a.Z - b.Z); }
-		public static WVec operator -(WPos a, WPos b) { return new WVec(a.X - b.X, a.Y - b.Y, a.Z - b.Z); }
+		public static WPos operator +(in WPos a, in WVec b) { return new WPos(a.X + b.X, a.Y + b.Y, a.Z + b.Z); }
+		public static WPos operator -(in WPos a, in WVec b) { return new WPos(a.X - b.X, a.Y - b.Y, a.Z - b.Z); }
+		public static WVec operator -(in WPos a, in WPos b) { return new WVec(a.X - b.X, a.Y - b.Y, a.Z - b.Z); }
 
-		public static bool operator ==(WPos me, WPos other) { return me.X == other.X && me.Y == other.Y && me.Z == other.Z; }
-		public static bool operator !=(WPos me, WPos other) { return !(me == other); }
+		public static bool operator ==(in WPos me, in WPos other) { return me.X == other.X && me.Y == other.Y && me.Z == other.Z; }
+		public static bool operator !=(in WPos me, in WPos other) { return !(me == other); }
 
 		/// <summary>
 		/// Returns the linear interpolation between points 'a' and 'b'
 		/// </summary>
-		public static WPos Lerp(WPos a, WPos b, int mul, int div) { return a + (b - a) * mul / div; }
+		public static WPos Lerp(in WPos a, in WPos b, int mul, int div) { return a + (b - a) * mul / div; }
 
 		/// <summary>
 		/// Returns the linear interpolation between points 'a' and 'b'
 		/// </summary>
-		public static WPos Lerp(WPos a, WPos b, long mul, long div)
+		public static WPos Lerp(in WPos a, in WPos b, long mul, long div)
 		{
 			// The intermediate variables may need more precision than
 			// an int can provide, so we can't use WPos.
@@ -55,7 +55,7 @@ namespace OpenRA
 			return new WPos(x, y, z);
 		}
 
-		public static WPos LerpQuadratic(WPos a, WPos b, WAngle pitch, int mul, int div)
+		public static WPos LerpQuadratic(in WPos a, in WPos b, WAngle pitch, int mul, int div)
 		{
 			// Start with a linear lerp between the points
 			var ret = Lerp(a, b, mul, div);
diff --git a/OpenRA.Game/WVec.cs b/OpenRA.Game/WVec.cs
index cee6271692..f398e24fe6 100644
--- a/OpenRA.Game/WVec.cs
+++ b/OpenRA.Game/WVec.cs
@@ -17,7 +17,7 @@ using OpenRA.Support;
 
 namespace OpenRA
 {
-	public struct WVec : IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaUnaryMinusBinding, ILuaEqualityBinding, ILuaTableBinding, IEquatable<WVec>
+	public readonly struct WVec : IScriptBindable, ILuaAdditionBinding, ILuaSubtractionBinding, ILuaUnaryMinusBinding, ILuaEqualityBinding, ILuaTableBinding, IEquatable<WVec>
 	{
 		public readonly int X, Y, Z;
 
@@ -26,17 +26,17 @@ namespace OpenRA
 
 		public static readonly WVec Zero = new WVec(0, 0, 0);
 
-		public static WVec operator +(WVec a, WVec b) { return new WVec(a.X + b.X, a.Y + b.Y, a.Z + b.Z); }
-		public static WVec operator -(WVec a, WVec b) { return new WVec(a.X - b.X, a.Y - b.Y, a.Z - b.Z); }
-		public static WVec operator -(WVec a) { return new WVec(-a.X, -a.Y, -a.Z); }
-		public static WVec operator /(WVec a, int b) { return new WVec(a.X / b, a.Y / b, a.Z / b); }
-		public static WVec operator *(int a, WVec b) { return new WVec(a * b.X, a * b.Y, a * b.Z); }
-		public static WVec operator *(WVec a, int b) { return b * a; }
+		public static WVec operator +(in WVec a, in WVec b) { return new WVec(a.X + b.X, a.Y + b.Y, a.Z + b.Z); }
+		public static WVec operator -(in WVec a, in WVec b) { return new WVec(a.X - b.X, a.Y - b.Y, a.Z - b.Z); }
+		public static WVec operator -(in WVec a) { return new WVec(-a.X, -a.Y, -a.Z); }
+		public static WVec operator /(in WVec a, int b) { return new WVec(a.X / b, a.Y / b, a.Z / b); }
+		public static WVec operator *(int a, in WVec b) { return new WVec(a * b.X, a * b.Y, a * b.Z); }
+		public static WVec operator *(in WVec a, int b) { return b * a; }
 
-		public static bool operator ==(WVec me, WVec other) { return me.X == other.X && me.Y == other.Y && me.Z == other.Z; }
-		public static bool operator !=(WVec me, WVec other) { return !(me == other); }
+		public static bool operator ==(in WVec me, in WVec other) { return me.X == other.X && me.Y == other.Y && me.Z == other.Z; }
+		public static bool operator !=(in WVec me, in WVec other) { return !(me == other); }
 
-		public static int Dot(WVec a, WVec b) { return a.X * b.X + a.Y * b.Y + a.Z * b.Z; }
+		public static int Dot(in WVec a, in WVec b) { return a.X * b.X + a.Y * b.Y + a.Z * b.Z; }
 		public long LengthSquared => (long)X * X + (long)Y * Y + (long)Z * Z;
 		public int Length => (int)Exts.ISqrt(LengthSquared);
 		public long HorizontalLengthSquared => (long)X * X + (long)Y * Y;
@@ -73,9 +73,9 @@ namespace OpenRA
 			}
 		}
 
-		public static WVec Lerp(WVec a, WVec b, int mul, int div) { return a + (b - a) * mul / div; }
+		public static WVec Lerp(in WVec a, in WVec b, int mul, int div) { return a + (b - a) * mul / div; }
 
-		public static WVec LerpQuadratic(WVec a, WVec b, WAngle pitch, int mul, int div)
+		public static WVec LerpQuadratic(in WVec a, in WVec b, WAngle pitch, int mul, int div)
 		{
 			// Start with a linear lerp between the points
 			var ret = Lerp(a, b, mul, div);
diff --git a/OpenRA.Game/World.cs b/OpenRA.Game/World.cs
index e0bae82628..718ba0c274 100644
--- a/OpenRA.Game/World.cs
+++ b/OpenRA.Game/World.cs
@@ -576,7 +576,7 @@ namespace OpenRA
 		}
 	}
 
-	public struct TraitPair<T> : IEquatable<TraitPair<T>>
+	public readonly struct TraitPair<T> : IEquatable<TraitPair<T>>
 	{
 		public readonly Actor Actor;
 		public readonly T Trait;
diff --git a/OpenRA.Mods.Cnc/Activities/VoxelHarvesterDockSequence.cs b/OpenRA.Mods.Cnc/Activities/VoxelHarvesterDockSequence.cs
index 072646da4f..e83d595fd9 100644
--- a/OpenRA.Mods.Cnc/Activities/VoxelHarvesterDockSequence.cs
+++ b/OpenRA.Mods.Cnc/Activities/VoxelHarvesterDockSequence.cs
@@ -20,7 +20,7 @@ namespace OpenRA.Mods.Cnc.Activities
 		readonly WithVoxelUnloadBody body;
 		readonly WithDockingOverlay spriteOverlay;
 
-		public VoxelHarvesterDockSequence(Actor self, Actor refinery, WAngle dockAngle, bool isDragRequired, WVec dragOffset, int dragLength)
+		public VoxelHarvesterDockSequence(Actor self, Actor refinery, WAngle dockAngle, bool isDragRequired, in WVec dragOffset, int dragLength)
 			: base(self, refinery, dockAngle, isDragRequired, dragOffset, dragLength)
 		{
 			body = self.Trait<WithVoxelUnloadBody>();
diff --git a/OpenRA.Mods.Cnc/Graphics/TeslaZapRenderable.cs b/OpenRA.Mods.Cnc/Graphics/TeslaZapRenderable.cs
index 498ab09170..2a170b4f6c 100644
--- a/OpenRA.Mods.Cnc/Graphics/TeslaZapRenderable.cs
+++ b/OpenRA.Mods.Cnc/Graphics/TeslaZapRenderable.cs
@@ -44,7 +44,7 @@ namespace OpenRA.Mods.Cnc.Graphics
 		WVec cachedLength;
 		IEnumerable<IFinalizedRenderable> cache;
 
-		public TeslaZapRenderable(WPos pos, int zOffset, WVec length, string image, string brightSequence, int brightZaps, string dimSequence, int dimZaps, string palette)
+		public TeslaZapRenderable(WPos pos, int zOffset, in WVec length, string image, string brightSequence, int brightZaps, string dimSequence, int dimZaps, string palette)
 		{
 			this.pos = pos;
 			this.zOffset = zOffset;
@@ -72,7 +72,7 @@ namespace OpenRA.Mods.Cnc.Graphics
 		}
 
 		public IRenderable WithZOffset(int newOffset) { return new TeslaZapRenderable(pos, zOffset, length, image, brightSequence, brightZaps, dimSequence, dimZaps, palette); }
-		public IRenderable OffsetBy(WVec vec) { return new TeslaZapRenderable(pos + vec, zOffset, length, image, brightSequence, brightZaps, dimSequence, dimZaps, palette); }
+		public IRenderable OffsetBy(in WVec vec) { return new TeslaZapRenderable(pos + vec, zOffset, length, image, brightSequence, brightZaps, dimSequence, dimZaps, palette); }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Activities/Air/Fly.cs b/OpenRA.Mods.Common/Activities/Air/Fly.cs
index c4e8239698..480bd54ada 100644
--- a/OpenRA.Mods.Common/Activities/Air/Fly.cs
+++ b/OpenRA.Mods.Common/Activities/Air/Fly.cs
@@ -60,7 +60,7 @@ namespace OpenRA.Mods.Common.Activities
 			this.minRange = minRange;
 		}
 
-		public static void FlyTick(Actor self, Aircraft aircraft, WAngle desiredFacing, WDist desiredAltitude, WVec moveOverride, bool idleTurn = false)
+		public static void FlyTick(Actor self, Aircraft aircraft, WAngle desiredFacing, WDist desiredAltitude, in WVec moveOverride, bool idleTurn = false)
 		{
 			var dat = self.World.Map.DistanceAboveTerrain(aircraft.CenterPosition);
 			var move = aircraft.Info.CanSlide ? aircraft.FlyStep(desiredFacing) : aircraft.FlyStep(aircraft.Facing);
diff --git a/OpenRA.Mods.Common/Activities/Air/Land.cs b/OpenRA.Mods.Common/Activities/Air/Land.cs
index 4532af072d..5bfd64dc8c 100644
--- a/OpenRA.Mods.Common/Activities/Air/Land.cs
+++ b/OpenRA.Mods.Common/Activities/Air/Land.cs
@@ -46,10 +46,10 @@ namespace OpenRA.Mods.Common.Activities
 		public Land(Actor self, in Target target, WDist landRange, WAngle? facing = null, Color? targetLineColor = null)
 			: this(self, target, landRange, WVec.Zero, facing, targetLineColor: targetLineColor) { }
 
-		public Land(Actor self, in Target target, WVec offset, WAngle? facing = null, Color? targetLineColor = null)
+		public Land(Actor self, in Target target, in WVec offset, WAngle? facing = null, Color? targetLineColor = null)
 			: this(self, target, WDist.Zero, offset, facing, targetLineColor: targetLineColor) { }
 
-		public Land(Actor self, in Target target, WDist landRange, WVec offset, WAngle? facing = null, CPos[] clearCells = null, Color? targetLineColor = null)
+		public Land(Actor self, in Target target, WDist landRange, in WVec offset, WAngle? facing = null, CPos[] clearCells = null, Color? targetLineColor = null)
 		{
 			aircraft = self.Trait<Aircraft>();
 			this.target = target;
diff --git a/OpenRA.Mods.Common/Activities/HarvesterDockSequence.cs b/OpenRA.Mods.Common/Activities/HarvesterDockSequence.cs
index bbda40ac8d..27eef65fc9 100644
--- a/OpenRA.Mods.Common/Activities/HarvesterDockSequence.cs
+++ b/OpenRA.Mods.Common/Activities/HarvesterDockSequence.cs
@@ -33,7 +33,7 @@ namespace OpenRA.Mods.Common.Activities
 
 		protected DockingState dockingState;
 
-		public HarvesterDockSequence(Actor self, Actor refinery, WAngle dockAngle, bool isDragRequired, WVec dragOffset, int dragLength)
+		public HarvesterDockSequence(Actor self, Actor refinery, WAngle dockAngle, bool isDragRequired, in WVec dragOffset, int dragLength)
 		{
 			dockingState = DockingState.Turn;
 			Refinery = refinery;
diff --git a/OpenRA.Mods.Common/Activities/SpriteHarvesterDockSequence.cs b/OpenRA.Mods.Common/Activities/SpriteHarvesterDockSequence.cs
index afc5bc89ce..7a616a08dc 100644
--- a/OpenRA.Mods.Common/Activities/SpriteHarvesterDockSequence.cs
+++ b/OpenRA.Mods.Common/Activities/SpriteHarvesterDockSequence.cs
@@ -20,7 +20,7 @@ namespace OpenRA.Mods.Common.Activities
 		readonly WithDockingAnimationInfo wda;
 		protected bool dockAnimPlayed;
 
-		public SpriteHarvesterDockSequence(Actor self, Actor refinery, WAngle dockAngle, bool isDragRequired, WVec dragOffset, int dragLength)
+		public SpriteHarvesterDockSequence(Actor self, Actor refinery, WAngle dockAngle, bool isDragRequired, in WVec dragOffset, int dragLength)
 			: base(self, refinery, dockAngle, isDragRequired, dragOffset, dragLength)
 		{
 			wsb = self.Trait<WithSpriteBody>();
diff --git a/OpenRA.Mods.Common/EditorBrushes/EditorResourceBrush.cs b/OpenRA.Mods.Common/EditorBrushes/EditorResourceBrush.cs
index 9449410e71..2a04c3da0e 100644
--- a/OpenRA.Mods.Common/EditorBrushes/EditorResourceBrush.cs
+++ b/OpenRA.Mods.Common/EditorBrushes/EditorResourceBrush.cs
@@ -108,7 +108,7 @@ namespace OpenRA.Mods.Common.Widgets
 		}
 	}
 
-	struct CellResource
+	readonly struct CellResource
 	{
 		public readonly CPos Cell;
 		public readonly ResourceTile ResourceTile;
diff --git a/OpenRA.Mods.Common/FileFormats/InstallShieldCABCompression.cs b/OpenRA.Mods.Common/FileFormats/InstallShieldCABCompression.cs
index ff326cf82e..6a447e75d9 100644
--- a/OpenRA.Mods.Common/FileFormats/InstallShieldCABCompression.cs
+++ b/OpenRA.Mods.Common/FileFormats/InstallShieldCABCompression.cs
@@ -35,7 +35,7 @@ namespace OpenRA.Mods.Common.FileFormats
 			Next = 0x2
 		}
 
-		struct FileGroup
+		readonly struct FileGroup
 		{
 			public readonly string Name;
 			public readonly uint FirstFile;
@@ -55,7 +55,7 @@ namespace OpenRA.Mods.Common.FileFormats
 			}
 		}
 
-		struct CabDescriptor
+		readonly struct CabDescriptor
 		{
 			public readonly long FileTableOffset;
 			public readonly uint FileTableSize;
@@ -78,7 +78,7 @@ namespace OpenRA.Mods.Common.FileFormats
 			}
 		}
 
-		struct DirectoryDescriptor
+		readonly struct DirectoryDescriptor
 		{
 			public readonly string Name;
 
@@ -94,7 +94,7 @@ namespace OpenRA.Mods.Common.FileFormats
 			}
 		}
 
-		struct FileDescriptor
+		readonly struct FileDescriptor
 		{
 			public readonly uint Index;
 			public readonly CABFlags Flags;
@@ -142,7 +142,7 @@ namespace OpenRA.Mods.Common.FileFormats
 			}
 		}
 
-		struct CommonHeader
+		readonly struct CommonHeader
 		{
 			public const long Size = 16;
 			public readonly uint Version;
@@ -159,7 +159,7 @@ namespace OpenRA.Mods.Common.FileFormats
 			}
 		}
 
-		struct VolumeHeader
+		readonly struct VolumeHeader
 		{
 			public readonly uint DataOffset;
 			public readonly uint DataOffsetHigh;
diff --git a/OpenRA.Mods.Common/FileSystem/InstallShieldPackage.cs b/OpenRA.Mods.Common/FileSystem/InstallShieldPackage.cs
index a9576fabf8..cb75e20e84 100644
--- a/OpenRA.Mods.Common/FileSystem/InstallShieldPackage.cs
+++ b/OpenRA.Mods.Common/FileSystem/InstallShieldPackage.cs
@@ -21,7 +21,7 @@ namespace OpenRA.Mods.Common.FileSystem
 	{
 		public sealed class InstallShieldPackage : IReadOnlyPackage
 		{
-			public struct Entry
+			public readonly struct Entry
 			{
 				public readonly uint Offset;
 				public readonly uint Length;
diff --git a/OpenRA.Mods.Common/Graphics/BeamRenderable.cs b/OpenRA.Mods.Common/Graphics/BeamRenderable.cs
index 96e5a02a97..8e2bffee6d 100644
--- a/OpenRA.Mods.Common/Graphics/BeamRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/BeamRenderable.cs
@@ -24,7 +24,7 @@ namespace OpenRA.Mods.Common.Graphics
 		readonly WDist width;
 		readonly Color color;
 
-		public BeamRenderable(WPos pos, int zOffset, WVec length, BeamRenderableShape shape, WDist width, Color color)
+		public BeamRenderable(WPos pos, int zOffset, in WVec length, BeamRenderableShape shape, WDist width, Color color)
 		{
 			this.pos = pos;
 			this.zOffset = zOffset;
@@ -39,7 +39,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return new BeamRenderable(pos, zOffset, length, shape, width, color); }
-		public IRenderable OffsetBy(WVec vec) { return new BeamRenderable(pos + vec, zOffset, length, shape, width, color); }
+		public IRenderable OffsetBy(in WVec vec) { return new BeamRenderable(pos + vec, zOffset, length, shape, width, color); }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Graphics/CircleAnnotationRenderable.cs b/OpenRA.Mods.Common/Graphics/CircleAnnotationRenderable.cs
index c32c638d62..f2989cf6cb 100644
--- a/OpenRA.Mods.Common/Graphics/CircleAnnotationRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/CircleAnnotationRenderable.cs
@@ -39,7 +39,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return new CircleAnnotationRenderable(centerPosition, radius, width, color, filled); }
-		public IRenderable OffsetBy(WVec vec) { return new CircleAnnotationRenderable(centerPosition + vec, radius, width, color, filled); }
+		public IRenderable OffsetBy(in WVec vec) { return new CircleAnnotationRenderable(centerPosition + vec, radius, width, color, filled); }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Graphics/ContrailRenderable.cs b/OpenRA.Mods.Common/Graphics/ContrailRenderable.cs
index c4b2e2ba8c..59dc897efb 100644
--- a/OpenRA.Mods.Common/Graphics/ContrailRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/ContrailRenderable.cs
@@ -50,7 +50,14 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return new ContrailRenderable(world, (WPos[])trail.Clone(), width, next, length, skip, color, newOffset); }
-		public IRenderable OffsetBy(WVec vec) { return new ContrailRenderable(world, trail.Select(pos => pos + vec).ToArray(), width, next, length, skip, color, zOffset); }
+
+		public IRenderable OffsetBy(in WVec vec)
+		{
+			// Lambdas can't use 'in' variables, so capture a copy for later
+			var offset = vec;
+			return new ContrailRenderable(world, trail.Select(pos => pos + offset).ToArray(), width, next, length, skip, color, zOffset);
+		}
+
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Graphics/DetectionCircleAnnotationRenderable.cs b/OpenRA.Mods.Common/Graphics/DetectionCircleAnnotationRenderable.cs
index b871192330..fa1c479f22 100644
--- a/OpenRA.Mods.Common/Graphics/DetectionCircleAnnotationRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/DetectionCircleAnnotationRenderable.cs
@@ -52,7 +52,7 @@ namespace OpenRA.Mods.Common.Graphics
 				trailCount, trailSeparation, trailAngle, color, width, borderColor, borderWidth);
 		}
 
-		public IRenderable OffsetBy(WVec vec)
+		public IRenderable OffsetBy(in WVec vec)
 		{
 			return new DetectionCircleAnnotationRenderable(centerPosition + vec, radius, zOffset,
 				trailCount, trailSeparation, trailAngle, color, width, borderColor, borderWidth);
diff --git a/OpenRA.Mods.Common/Graphics/IsometricSelectionBarsAnnotationRenderable.cs b/OpenRA.Mods.Common/Graphics/IsometricSelectionBarsAnnotationRenderable.cs
index 718f0405ac..3a36aa5203 100644
--- a/OpenRA.Mods.Common/Graphics/IsometricSelectionBarsAnnotationRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/IsometricSelectionBarsAnnotationRenderable.cs
@@ -54,7 +54,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return this; }
-		public IRenderable OffsetBy(WVec vec) { return new IsometricSelectionBarsAnnotationRenderable(pos + vec, actor, bounds); }
+		public IRenderable OffsetBy(in WVec vec) { return new IsometricSelectionBarsAnnotationRenderable(pos + vec, actor, bounds); }
 		public IRenderable AsDecoration() { return this; }
 
 		void DrawExtraBars(WorldRenderer wr)
diff --git a/OpenRA.Mods.Common/Graphics/IsometricSelectionBoxAnnotationRenderable.cs b/OpenRA.Mods.Common/Graphics/IsometricSelectionBoxAnnotationRenderable.cs
index 520d7bead3..7d827e8d05 100644
--- a/OpenRA.Mods.Common/Graphics/IsometricSelectionBoxAnnotationRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/IsometricSelectionBoxAnnotationRenderable.cs
@@ -34,14 +34,14 @@ namespace OpenRA.Mods.Common.Graphics
 		readonly Polygon bounds;
 		readonly Color color;
 
-		public IsometricSelectionBoxAnnotationRenderable(Actor actor, Polygon bounds, Color color)
+		public IsometricSelectionBoxAnnotationRenderable(Actor actor, in Polygon bounds, Color color)
 		{
 			pos = actor.CenterPosition;
 			this.bounds = bounds;
 			this.color = color;
 		}
 
-		public IsometricSelectionBoxAnnotationRenderable(WPos pos, Polygon bounds, Color color)
+		public IsometricSelectionBoxAnnotationRenderable(WPos pos, in Polygon bounds, Color color)
 		{
 			this.pos = pos;
 			this.bounds = bounds;
@@ -54,7 +54,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return this; }
-		public IRenderable OffsetBy(WVec vec) { return new IsometricSelectionBoxAnnotationRenderable(pos + vec, bounds, color); }
+		public IRenderable OffsetBy(in WVec vec) { return new IsometricSelectionBoxAnnotationRenderable(pos + vec, bounds, color); }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Graphics/LineAnnotationRenderable.cs b/OpenRA.Mods.Common/Graphics/LineAnnotationRenderable.cs
index ffaa20b0a1..48f6827514 100644
--- a/OpenRA.Mods.Common/Graphics/LineAnnotationRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/LineAnnotationRenderable.cs
@@ -44,7 +44,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return new LineAnnotationRenderable(start, end, width, startColor, endColor); }
-		public IRenderable OffsetBy(WVec vec) { return new LineAnnotationRenderable(start + vec, end + vec, width, startColor, endColor); }
+		public IRenderable OffsetBy(in WVec vec) { return new LineAnnotationRenderable(start + vec, end + vec, width, startColor, endColor); }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Graphics/ModelActorPreview.cs b/OpenRA.Mods.Common/Graphics/ModelActorPreview.cs
index 2e2adcbbc0..fcbcfea85d 100644
--- a/OpenRA.Mods.Common/Graphics/ModelActorPreview.cs
+++ b/OpenRA.Mods.Common/Graphics/ModelActorPreview.cs
@@ -29,7 +29,7 @@ namespace OpenRA.Mods.Common.Graphics
 		readonly WVec offset;
 		readonly int zOffset;
 
-		public ModelPreview(ModelAnimation[] components, WVec offset, int zOffset, float scale, WAngle lightPitch, WAngle lightYaw,
+		public ModelPreview(ModelAnimation[] components, in WVec offset, int zOffset, float scale, WAngle lightPitch, WAngle lightYaw,
 			float[] lightAmbientColor, float[] lightDiffuseColor, WAngle cameraPitch,
 			PaletteReference colorPalette, PaletteReference normalsPalette, PaletteReference shadowPalette)
 		{
diff --git a/OpenRA.Mods.Common/Graphics/ModelRenderable.cs b/OpenRA.Mods.Common/Graphics/ModelRenderable.cs
index 7f5e3a57ef..7748f9f99f 100644
--- a/OpenRA.Mods.Common/Graphics/ModelRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/ModelRenderable.cs
@@ -90,7 +90,7 @@ namespace OpenRA.Mods.Common.Graphics
 				palette, normalsPalette, shadowPalette, alpha, tint, tintModifiers);
 		}
 
-		public IRenderable OffsetBy(WVec vec)
+		public IRenderable OffsetBy(in WVec vec)
 		{
 			return new ModelRenderable(
 				models, pos + vec, zOffset, camera, scale,
diff --git a/OpenRA.Mods.Common/Graphics/PolygonAnnotationRenderable.cs b/OpenRA.Mods.Common/Graphics/PolygonAnnotationRenderable.cs
index d70600596f..64ee6656f5 100644
--- a/OpenRA.Mods.Common/Graphics/PolygonAnnotationRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/PolygonAnnotationRenderable.cs
@@ -35,7 +35,14 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return new PolygonAnnotationRenderable(vertices, effectivePos, width, color); }
-		public IRenderable OffsetBy(WVec vec) { return new PolygonAnnotationRenderable(vertices.Select(v => v + vec).ToArray(), effectivePos + vec, width, color); }
+
+		public IRenderable OffsetBy(in WVec vec)
+		{
+			// Lambdas can't use 'in' variables, so capture a copy for later
+			var offset = vec;
+			return new PolygonAnnotationRenderable(vertices.Select(v => v + offset).ToArray(), effectivePos + vec, width, color);
+		}
+
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Graphics/RailgunRenderable.cs b/OpenRA.Mods.Common/Graphics/RailgunRenderable.cs
index 5d6bbda416..22eebc0c8b 100644
--- a/OpenRA.Mods.Common/Graphics/RailgunRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/RailgunRenderable.cs
@@ -45,7 +45,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return new RailgunHelixRenderable(pos, newOffset, railgun, info, ticks); }
-		public IRenderable OffsetBy(WVec vec) { return new RailgunHelixRenderable(pos + vec, zOffset, railgun, info, ticks); }
+		public IRenderable OffsetBy(in WVec vec) { return new RailgunHelixRenderable(pos + vec, zOffset, railgun, info, ticks); }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Graphics/RangeCircleAnnotationRenderable.cs b/OpenRA.Mods.Common/Graphics/RangeCircleAnnotationRenderable.cs
index 4939e7b60e..421e05ddf6 100644
--- a/OpenRA.Mods.Common/Graphics/RangeCircleAnnotationRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/RangeCircleAnnotationRenderable.cs
@@ -44,7 +44,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return new RangeCircleAnnotationRenderable(centerPosition, radius, newOffset, color, width, borderColor, borderWidth); }
-		public IRenderable OffsetBy(WVec vec) { return new RangeCircleAnnotationRenderable(centerPosition + vec, radius, zOffset, color, width, borderColor, borderWidth); }
+		public IRenderable OffsetBy(in WVec vec) { return new RangeCircleAnnotationRenderable(centerPosition + vec, radius, zOffset, color, width, borderColor, borderWidth); }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Graphics/SelectionBarsAnnotationRenderable.cs b/OpenRA.Mods.Common/Graphics/SelectionBarsAnnotationRenderable.cs
index 626073c0ac..4da12b435c 100644
--- a/OpenRA.Mods.Common/Graphics/SelectionBarsAnnotationRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/SelectionBarsAnnotationRenderable.cs
@@ -45,7 +45,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return this; }
-		public IRenderable OffsetBy(WVec vec) { return new SelectionBarsAnnotationRenderable(pos + vec, actor, decorationBounds); }
+		public IRenderable OffsetBy(in WVec vec) { return new SelectionBarsAnnotationRenderable(pos + vec, actor, decorationBounds); }
 		public IRenderable AsDecoration() { return this; }
 
 		void DrawExtraBars(WorldRenderer wr, float2 start, float2 end)
diff --git a/OpenRA.Mods.Common/Graphics/SelectionBoxAnnotationRenderable.cs b/OpenRA.Mods.Common/Graphics/SelectionBoxAnnotationRenderable.cs
index 9ae16c64e3..d5551ca384 100644
--- a/OpenRA.Mods.Common/Graphics/SelectionBoxAnnotationRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/SelectionBoxAnnotationRenderable.cs
@@ -36,7 +36,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return this; }
-		public IRenderable OffsetBy(WVec vec) { return new SelectionBoxAnnotationRenderable(pos + vec, decorationBounds, color); }
+		public IRenderable OffsetBy(in WVec vec) { return new SelectionBoxAnnotationRenderable(pos + vec, decorationBounds, color); }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Graphics/TextAnnotationRenderable.cs b/OpenRA.Mods.Common/Graphics/TextAnnotationRenderable.cs
index 83cb6a405e..be91b6933a 100644
--- a/OpenRA.Mods.Common/Graphics/TextAnnotationRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/TextAnnotationRenderable.cs
@@ -47,7 +47,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return new TextAnnotationRenderable(font, pos, zOffset, color, text); }
-		public IRenderable OffsetBy(WVec vec) { return new TextAnnotationRenderable(font, pos + vec, zOffset, color, text); }
+		public IRenderable OffsetBy(in WVec vec) { return new TextAnnotationRenderable(font, pos + vec, zOffset, color, text); }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/Graphics/UIModelRenderable.cs b/OpenRA.Mods.Common/Graphics/UIModelRenderable.cs
index 8a549bb9b5..86caa3ded8 100644
--- a/OpenRA.Mods.Common/Graphics/UIModelRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/UIModelRenderable.cs
@@ -65,7 +65,7 @@ namespace OpenRA.Mods.Common.Graphics
 		}
 
 		public IRenderable WithZOffset(int newOffset) { return this; }
-		public IRenderable OffsetBy(WVec vec) { return this; }
+		public IRenderable OffsetBy(in WVec vec) { return this; }
 		public IRenderable AsDecoration() { return this; }
 
 		static readonly float[] GroundNormal = { 0, 0, 1, 1 };
diff --git a/OpenRA.Mods.Common/Graphics/UITextRenderable.cs b/OpenRA.Mods.Common/Graphics/UITextRenderable.cs
index 5bc02e494a..5e8486ae2a 100644
--- a/OpenRA.Mods.Common/Graphics/UITextRenderable.cs
+++ b/OpenRA.Mods.Common/Graphics/UITextRenderable.cs
@@ -49,7 +49,7 @@ namespace OpenRA.Mods.Common.Graphics
 		public bool IsDecoration => true;
 
 		public IRenderable WithZOffset(int newOffset) { return new UITextRenderable(font, effectiveWorldPos, screenPos, zOffset, color, text); }
-		public IRenderable OffsetBy(WVec vec) { return new UITextRenderable(font, effectiveWorldPos + vec, screenPos, zOffset, color, text); }
+		public IRenderable OffsetBy(in WVec vec) { return new UITextRenderable(font, effectiveWorldPos + vec, screenPos, zOffset, color, text); }
 		public IRenderable AsDecoration() { return this; }
 
 		public IFinalizedRenderable PrepareRender(WorldRenderer wr) { return this; }
diff --git a/OpenRA.Mods.Common/HitShapes/Capsule.cs b/OpenRA.Mods.Common/HitShapes/Capsule.cs
index 640a715c79..22a87038d8 100644
--- a/OpenRA.Mods.Common/HitShapes/Capsule.cs
+++ b/OpenRA.Mods.Common/HitShapes/Capsule.cs
@@ -61,7 +61,7 @@ namespace OpenRA.Mods.Common.HitShapes
 			OuterRadius = Radius + new WDist(Math.Max(PointA.Length, PointB.Length));
 		}
 
-		public WDist DistanceFromEdge(WVec v)
+		public WDist DistanceFromEdge(in WVec v)
 		{
 			var p = new int2(v.X, v.Y);
 
diff --git a/OpenRA.Mods.Common/HitShapes/Circle.cs b/OpenRA.Mods.Common/HitShapes/Circle.cs
index 03abb8c1c8..50f2cac8f6 100644
--- a/OpenRA.Mods.Common/HitShapes/Circle.cs
+++ b/OpenRA.Mods.Common/HitShapes/Circle.cs
@@ -40,7 +40,7 @@ namespace OpenRA.Mods.Common.HitShapes
 				throw new YamlException("VerticalTopOffset must be equal to or higher than VerticalBottomOffset.");
 		}
 
-		public WDist DistanceFromEdge(WVec v)
+		public WDist DistanceFromEdge(in WVec v)
 		{
 			return new WDist(Math.Max(0, v.Length - Radius.Length));
 		}
diff --git a/OpenRA.Mods.Common/HitShapes/IHitShape.cs b/OpenRA.Mods.Common/HitShapes/IHitShape.cs
index 34ee80ecde..8b02adbc4d 100644
--- a/OpenRA.Mods.Common/HitShapes/IHitShape.cs
+++ b/OpenRA.Mods.Common/HitShapes/IHitShape.cs
@@ -18,7 +18,7 @@ namespace OpenRA.Mods.Common.HitShapes
 	{
 		WDist OuterRadius { get; }
 
-		WDist DistanceFromEdge(WVec v);
+		WDist DistanceFromEdge(in WVec v);
 		WDist DistanceFromEdge(WPos pos, WPos origin, WRot orientation);
 
 		void Initialize();
diff --git a/OpenRA.Mods.Common/HitShapes/Polygon.cs b/OpenRA.Mods.Common/HitShapes/Polygon.cs
index edbef8d67f..7c0d2438cc 100644
--- a/OpenRA.Mods.Common/HitShapes/Polygon.cs
+++ b/OpenRA.Mods.Common/HitShapes/Polygon.cs
@@ -81,7 +81,7 @@ namespace OpenRA.Mods.Common.HitShapes
 			return (ac - ap).LengthSquared;
 		}
 
-		public WDist DistanceFromEdge(WVec v)
+		public WDist DistanceFromEdge(in WVec v)
 		{
 			var p = new int2(v.X, v.Y);
 			var z = Math.Abs(v.Z);
diff --git a/OpenRA.Mods.Common/HitShapes/Rectangle.cs b/OpenRA.Mods.Common/HitShapes/Rectangle.cs
index 6bd2da2a48..c5b4824f32 100644
--- a/OpenRA.Mods.Common/HitShapes/Rectangle.cs
+++ b/OpenRA.Mods.Common/HitShapes/Rectangle.cs
@@ -103,7 +103,7 @@ namespace OpenRA.Mods.Common.HitShapes
 			};
 		}
 
-		public WDist DistanceFromEdge(WVec v)
+		public WDist DistanceFromEdge(in WVec v)
 		{
 			var r = new WVec(
 				Math.Max(Math.Abs(v.X - center.X) - quadrantSize.X, 0),
diff --git a/OpenRA.Mods.Common/Pathfinder/CellInfo.cs b/OpenRA.Mods.Common/Pathfinder/CellInfo.cs
index 184c22d7bd..1200644310 100644
--- a/OpenRA.Mods.Common/Pathfinder/CellInfo.cs
+++ b/OpenRA.Mods.Common/Pathfinder/CellInfo.cs
@@ -25,7 +25,7 @@ namespace OpenRA.Mods.Common.Pathfinder
 	/// <summary>
 	/// Stores information about nodes in the pathfinding graph
 	/// </summary>
-	public struct CellInfo
+	public readonly struct CellInfo
 	{
 		/// <summary>
 		/// The cost to move from the start up to this node
diff --git a/OpenRA.Mods.Common/Pathfinder/PathGraph.cs b/OpenRA.Mods.Common/Pathfinder/PathGraph.cs
index 4edb22fd52..54f20b72cf 100644
--- a/OpenRA.Mods.Common/Pathfinder/PathGraph.cs
+++ b/OpenRA.Mods.Common/Pathfinder/PathGraph.cs
@@ -49,7 +49,7 @@ namespace OpenRA.Mods.Common.Pathfinder
 		Actor Actor { get; }
 	}
 
-	public struct GraphConnection
+	public readonly struct GraphConnection
 	{
 		public static readonly CostComparer ConnectionCostComparer = CostComparer.Instance;
 
diff --git a/OpenRA.Mods.Common/Projectiles/Missile.cs b/OpenRA.Mods.Common/Projectiles/Missile.cs
index 0d9db120ee..080376a31a 100644
--- a/OpenRA.Mods.Common/Projectiles/Missile.cs
+++ b/OpenRA.Mods.Common/Projectiles/Missile.cs
@@ -739,7 +739,7 @@ namespace OpenRA.Mods.Common.Projectiles
 			return desiredVFacing;
 		}
 
-		WVec HomingTick(World world, WVec tarDistVec, int relTarHorDist)
+		WVec HomingTick(World world, in WVec tarDistVec, int relTarHorDist)
 		{
 			int predClfHgt = 0;
 			int predClfDist = 0;
diff --git a/OpenRA.Mods.Common/Scripting/ScriptTriggers.cs b/OpenRA.Mods.Common/Scripting/ScriptTriggers.cs
index c670e62d79..38d2c5fe79 100644
--- a/OpenRA.Mods.Common/Scripting/ScriptTriggers.cs
+++ b/OpenRA.Mods.Common/Scripting/ScriptTriggers.cs
@@ -49,7 +49,7 @@ namespace OpenRA.Mods.Common.Scripting
 
 		readonly List<Triggerable>[] triggerables = Exts.MakeArray(Enum.GetValues(typeof(Trigger)).Length, _ => new List<Triggerable>());
 
-		struct Triggerable : IDisposable
+		readonly struct Triggerable : IDisposable
 		{
 			public readonly LuaFunction Function;
 			public readonly ScriptContext Context;
diff --git a/OpenRA.Mods.Common/Traits/BodyOrientation.cs b/OpenRA.Mods.Common/Traits/BodyOrientation.cs
index edf74258cf..dfda816ec0 100644
--- a/OpenRA.Mods.Common/Traits/BodyOrientation.cs
+++ b/OpenRA.Mods.Common/Traits/BodyOrientation.cs
@@ -26,7 +26,7 @@ namespace OpenRA.Mods.Common.Traits
 		[Desc("Fudge the coordinate system angles to simulate non-top-down perspective in mods with square cells.")]
 		public readonly bool UseClassicPerspectiveFudge = true;
 
-		public WVec LocalToWorld(WVec vec)
+		public WVec LocalToWorld(in WVec vec)
 		{
 			// Rotate by 90 degrees
 			if (!UseClassicPerspectiveFudge)
@@ -96,7 +96,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		public WAngle CameraPitch => info.CameraPitch;
 
-		public WVec LocalToWorld(WVec vec)
+		public WVec LocalToWorld(in WVec vec)
 		{
 			return info.LocalToWorld(vec);
 		}
diff --git a/OpenRA.Mods.Common/Traits/Conditions/ExternalCondition.cs b/OpenRA.Mods.Common/Traits/Conditions/ExternalCondition.cs
index 10065dc376..807188294c 100644
--- a/OpenRA.Mods.Common/Traits/Conditions/ExternalCondition.cs
+++ b/OpenRA.Mods.Common/Traits/Conditions/ExternalCondition.cs
@@ -40,7 +40,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class ExternalCondition : ITick, INotifyCreated
 	{
-		struct TimedToken
+		readonly struct TimedToken
 		{
 			public readonly int Expires;
 			public readonly int Token;
diff --git a/OpenRA.Mods.Common/Traits/HitShape.cs b/OpenRA.Mods.Common/Traits/HitShape.cs
index b756bacc2d..ca7df6b2a9 100644
--- a/OpenRA.Mods.Common/Traits/HitShape.cs
+++ b/OpenRA.Mods.Common/Traits/HitShape.cs
@@ -108,7 +108,7 @@ namespace OpenRA.Mods.Common.Traits
 			}
 		}
 
-		WVec CalculateTargetableOffset(Actor self, WVec offset)
+		WVec CalculateTargetableOffset(Actor self, in WVec offset)
 		{
 			var localOffset = offset;
 			var quantizedBodyOrientation = orientation.QuantizeOrientation(self, self.Orientation);
diff --git a/OpenRA.Mods.Common/Traits/World/ActorMap.cs b/OpenRA.Mods.Common/Traits/World/ActorMap.cs
index 4df65205f1..11b61b0863 100644
--- a/OpenRA.Mods.Common/Traits/World/ActorMap.cs
+++ b/OpenRA.Mods.Common/Traits/World/ActorMap.cs
@@ -246,7 +246,7 @@ namespace OpenRA.Mods.Common.Traits
 			}
 		}
 
-		struct ActorsAtEnumerable : IEnumerable<Actor>
+		readonly struct ActorsAtEnumerable : IEnumerable<Actor>
 		{
 			readonly InfluenceNode node;
 			public ActorsAtEnumerable(InfluenceNode node) { this.node = node; }
diff --git a/OpenRA.Mods.Common/Traits/World/Locomotor.cs b/OpenRA.Mods.Common/Traits/World/Locomotor.cs
index 8fa69fb0b9..1a6bfd8264 100644
--- a/OpenRA.Mods.Common/Traits/World/Locomotor.cs
+++ b/OpenRA.Mods.Common/Traits/World/Locomotor.cs
@@ -126,7 +126,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class Locomotor : IWorldLoaded
 	{
-		struct CellCache
+		readonly struct CellCache
 		{
 			public readonly LongBitSet<PlayerBitMask> Immovable;
 			public readonly LongBitSet<PlayerBitMask> Crushable;
diff --git a/OpenRA.Mods.Common/Traits/World/ResourceRenderer.cs b/OpenRA.Mods.Common/Traits/World/ResourceRenderer.cs
index e396bbabf2..edb2e16fd7 100644
--- a/OpenRA.Mods.Common/Traits/World/ResourceRenderer.cs
+++ b/OpenRA.Mods.Common/Traits/World/ResourceRenderer.cs
@@ -200,7 +200,7 @@ namespace OpenRA.Mods.Common.Traits
 
 		public ResourceType GetRenderedResourceType(CPos cell) { return RenderContent[cell].Type; }
 
-		public struct RendererCellContents
+		public readonly struct RendererCellContents
 		{
 			public readonly string Variant;
 			public readonly ResourceType Type;
diff --git a/OpenRA.Mods.Common/Traits/World/ShroudRenderer.cs b/OpenRA.Mods.Common/Traits/World/ShroudRenderer.cs
index 92188bac97..e39a6d4777 100644
--- a/OpenRA.Mods.Common/Traits/World/ShroudRenderer.cs
+++ b/OpenRA.Mods.Common/Traits/World/ShroudRenderer.cs
@@ -80,7 +80,7 @@ namespace OpenRA.Mods.Common.Traits
 			All = Top | Right | Bottom | Left
 		}
 
-		struct TileInfo
+		readonly struct TileInfo
 		{
 			public readonly float3 ScreenPosition;
 			public readonly byte Variant;
diff --git a/OpenRA.Mods.Common/Traits/World/WeatherOverlay.cs b/OpenRA.Mods.Common/Traits/World/WeatherOverlay.cs
index 0cab64f7e1..b112f0ee1c 100644
--- a/OpenRA.Mods.Common/Traits/World/WeatherOverlay.cs
+++ b/OpenRA.Mods.Common/Traits/World/WeatherOverlay.cs
@@ -73,7 +73,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class WeatherOverlay : ITick, IRenderAboveWorld, INotifyViewportZoomExtentsChanged
 	{
-		struct Particle
+		readonly struct Particle
 		{
 			public readonly float2 Pos;
 			public readonly int Size;
@@ -103,7 +103,7 @@ namespace OpenRA.Mods.Common.Traits
 				TailColor = Color.FromArgb(info.LineTailAlphaValue, Color.R, Color.G, Color.B);
 			}
 
-			Particle(Particle source)
+			Particle(in Particle source)
 			{
 				Pos = source.Pos;
 				Size = source.Size;
@@ -117,13 +117,13 @@ namespace OpenRA.Mods.Common.Traits
 				TailColor = source.TailColor;
 			}
 
-			public Particle(Particle source, float2 pos)
+			public Particle(in Particle source, float2 pos)
 				: this(source)
 			{
 				Pos = pos;
 			}
 
-			public Particle(Particle source, float2 pos, int swingDirection, float swingOffset)
+			public Particle(in Particle source, float2 pos, int swingDirection, float swingOffset)
 				: this(source)
 			{
 				Pos = pos;
diff --git a/OpenRA.Mods.D2k/PackageLoaders/D2kSoundResources.cs b/OpenRA.Mods.D2k/PackageLoaders/D2kSoundResources.cs
index 0f0fe5060c..ac6c6e724b 100644
--- a/OpenRA.Mods.D2k/PackageLoaders/D2kSoundResources.cs
+++ b/OpenRA.Mods.D2k/PackageLoaders/D2kSoundResources.cs
@@ -21,7 +21,7 @@ namespace OpenRA.Mods.D2k.PackageLoaders
 	{
 		sealed class D2kSoundResources : IReadOnlyPackage
 		{
-			struct Entry
+			readonly struct Entry
 			{
 				public readonly uint Offset;
 				public readonly uint Length;