Declare a Rust parameter mut when the body assigns to it

Coder.Test/Languages/RustGeneratedSourceCompilesTests.cs

@@ -99,6 +99,7 @@ private static SourceFile Exemplar()
 		geometry.Members.Add(CompiledExemplar.OriginTable());
 		geometry.Members.Add(CompiledExemplar.Measure());
 		geometry.Members.Add(Boxed());
+		geometry.Members.Add(Clamp());
 		geometry.Members.Add(new CompileTimeAssertion
 		{
 			Condition = "std::mem::size_of::<i32>() == 4",
@@ -109,6 +110,23 @@ private static SourceFile Exemplar()
 		return file;
 	}
 
+	/// <summary>
+	/// A function whose body assigns to one of its parameters.
+	/// </summary>
+	/// <returns>The declaration.</returns>
+	/// <remarks>
+	/// Every other target lets a parameter be reassigned; Rust binds it immutably, and without a
+	/// <c>mut</c> on the declaration the assignment is error E0384.
+	/// </remarks>
+	private static FunctionDeclaration Clamp()
+	{
+		FunctionDeclaration clamp = new("clamp") { ReturnType = "int" };
+		clamp.Parameters.Add(new Parameter("value", "int"));
+		clamp.Body.Add(new AssignmentStatement(new VariableReference("value"), new LiteralExpression<int>(0)));
+		clamp.Body.Add(new ReturnStatement(new VariableReference("value")));
+		return clamp;
+	}
+
 	/// <summary>
 	/// A type written over one parameter, so its impl block has to carry the parameter too.
 	/// </summary>

Coder.Test/Languages/RustGeneratorTests.cs

@@ -61,6 +61,44 @@
 			StringComparison.Ordinal);
 	}
 
+	/// <summary>
+	/// Tests that a parameter the body assigns to is declared <c>mut</c>, and only that one, since
+	/// Rust binds a parameter immutably.
+	/// </summary>
+	[TestMethod]
+	public void ReassignedParameter_IsDeclaredMut()
+	{
+		FunctionDeclaration function = new("clamp") { ReturnType = "int" };
+		function.Parameters.Add(new Parameter("a", "int"));
+		function.Parameters.Add(new Parameter("b", "int"));
+		function.Body.Add(new AssignmentStatement(new VariableReference("a"), new VariableReference("b")));
+		function.Body.Add(new ReturnStatement(new VariableReference("a")));
+
+		StringAssert.Contains(
+			Generator.Generate(function),
+			"pub fn clamp(mut a: i32, b: i32) -> i32",
+			StringComparison.Ordinal);
+	}
+
+	/// <summary>
+	/// Tests that an assignment after a local has taken over a parameter's name is to the local, so
+	/// the parameter is left immutable.
+	/// </summary>
+	[TestMethod]
+	public void ParameterShadowedBeforeAssignment_IsNotDeclaredMut()
+	{
+		FunctionDeclaration function = new("shadow") { ReturnType = "int" };
+		function.Parameters.Add(new Parameter("a", "int"));
+		function.Body.Add(new VariableDeclaration("a", "int", new LiteralExpression<int>(1)));
+		function.Body.Add(new AssignmentStatement(new VariableReference("a"), new LiteralExpression<int>(2)));
+		function.Body.Add(new ReturnStatement(new VariableReference("a")));
+
+		StringAssert.Contains(
+			Generator.Generate(function),
+			"pub fn shadow(a: i32) -> i32",
+			StringComparison.Ordinal);
+	}
+
 	/// <summary>
 	/// Tests that an unrecognized type name is passed through, so a caller can name a real Rust type.
 	/// </summary>

Coder/Languages/RustGenerator.cs

@@ -117,6 +117,17 @@ public class RustGenerator : StandardLanguageGenerator
 	/// </remarks>
 	private string implBounds = string.Empty;
 
+	/// <summary>
+	/// The parameters of the function being written that its body assigns to.
+	/// </summary>
+	/// <remarks>
+	/// Held rather than passed because a parameter is written through the shared parameter list,
+	/// which knows nothing of the body. Rust binds a parameter immutably, so one the body assigns to
+	/// has to be declared <c>mut</c>, and one it does not must not be, or rustc warns that the
+	/// <c>mut</c> is unused.
+	/// </remarks>
+	private IReadOnlySet<string> reassignedParameters = new HashSet<string>(StringComparer.Ordinal);
+
 	/// <summary>
 	/// Spells type parameters where they are being declared, bounds and all.
 	/// </summary>
@@ -826,7 +837,11 @@ private void GenerateFunction(FunctionDeclaration funcDecl, CodeBlocker code, st
 
 		code.Write($"fn {SpellFunctionName(funcDecl)}{SpellParameterDeclarations(funcDecl.TypeParameters)}(");
 		WriteReceiver(funcDecl, enclosingType, code);
+
+		IReadOnlySet<string> outerReassigned = reassignedParameters;
+		reassignedParameters = ReassignedParameters(funcDecl);
 		GenerateParameterList(funcDecl.Parameters, code);
+		reassignedParameters = outerReassigned;
 		code.Write(")");
 
 		if (funcDecl.Kind == FunctionKind.Constructor)
@@ -960,6 +975,42 @@ private static bool ReturnsAValue(FunctionDeclaration funcDecl) =>
 		_ => funcDecl.Name ?? "unnamed_function",
 	};
 
+	/// <summary>
+	/// Finds the parameters a function's body assigns to before any local takes the name over.
+	/// </summary>
+	/// <param name="funcDecl">The function to inspect.</param>
+	/// <returns>The names of the parameters to declare <c>mut</c>.</returns>
+	/// <remarks>
+	/// A local declared with a parameter's name shadows it from there on, so an assignment after
+	/// that point is to the local, which is already <c>let mut</c>, and not to the parameter.
+	/// </remarks>
+	private static HashSet<string> ReassignedParameters(FunctionDeclaration funcDecl)
+	{
+		HashSet<string> parameters = new(
+			funcDecl.Parameters.Select(parameter => parameter.Name).OfType<string>(),
+			StringComparer.Ordinal);
+		HashSet<string> reassigned = new(StringComparer.Ordinal);
+
+		foreach (AstNode statement in funcDecl.Body)
+		{
+			switch (statement)
+			{
+				case VariableDeclaration local:
+					parameters.Remove(local.Name);
+					break;
+
+				case AssignmentStatement { Target: VariableReference target } when parameters.Contains(target.Name):
+					reassigned.Add(target.Name);
+					break;
+
+				default:
+					break;
+			}
+		}
+
+		return reassigned;
+	}
+
 	/// <inheritdoc/>
 	/// <remarks>
 	/// A parameter's default value is written beside it as a comment. Rust has no default arguments,
@@ -971,6 +1022,11 @@ protected override void GenerateParameter(Parameter parameter, CodeBlocker code,
 		Ensure.NotNull(parameter);
 		Ensure.NotNull(code);
 
+		if (parameter.Name is not null && reassignedParameters.Contains(parameter.Name))
+		{
+			code.Write("mut ");
+		}
+
 		code.Write($"{parameter.Name ?? $"param{position}"}: ");
 		code.Write(SpellType(parameter.Type ?? new TypeReference(UnknownTypeName)));