validation: fix error msg for assign to 'let' and 'function parameter'

Bug: tint:903
Change-Id: Ie2220d533f2227be717f61fa702514e2aeb052c8
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56300
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/resolver/assignment_validation_test.cc

@@ -173,7 +173,8 @@ TEST_F(ResolverAssignmentValidationTest, AssignToConstant_Fail) {
   WrapInFunction(var, Assign(Expr(Source{{12, 34}}, "a"), 2));
 
   EXPECT_FALSE(r()->Resolve());
-  EXPECT_EQ(r()->error(), "12:34 error: cannot assign to value of type 'i32'");
+  EXPECT_EQ(r()->error(),
+            "12:34 error: cannot assign to const\nnote: 'a' is declared here:");
 }
 
 TEST_F(ResolverAssignmentValidationTest, AssignNonStorable_Fail) {

src/resolver/function_validation_test.cc

@@ -354,7 +354,9 @@ TEST_F(ResolverFunctionValidationTest, FunctionParamsConst) {
        {Assign(Expr(Source{{12, 34}}, "arg"), Expr(1)), Return()});
 
   EXPECT_FALSE(r()->Resolve());
-  EXPECT_EQ(r()->error(), "12:34 error: cannot assign to value of type 'i32'");
+  EXPECT_EQ(r()->error(),
+            "12:34 error: cannot assign to function parameter\nnote: 'arg' is "
+            "declared here:");
 }
 
 TEST_F(ResolverFunctionValidationTest, WorkgroupSize_Literal_BadType) {

src/resolver/resolver.cc

@@ -3670,12 +3670,31 @@ bool Resolver::ValidateAssignment(const ast::AssignmentStatement* a) {
   auto const* lhs_type = TypeOf(a->lhs());
   auto const* rhs_type = TypeOf(a->rhs());
 
+  if (auto* ident = a->lhs()->As<ast::IdentifierExpression>()) {
+    VariableInfo* var;
+    if (variable_stack_.get(ident->symbol(), &var)) {
+      if (var->kind == VariableKind::kParameter) {
+        AddError("cannot assign to function parameter", a->lhs()->source());
+        AddNote("'" + builder_->Symbols().NameFor(ident->symbol()) +
+                    "' is declared here:",
+                var->declaration->source());
+        return false;
+      }
+      if (var->declaration->is_const()) {
+        AddError("cannot assign to const", a->lhs()->source());
+        AddNote("'" + builder_->Symbols().NameFor(ident->symbol()) +
+                    "' is declared here:",
+                var->declaration->source());
+        return false;
+      }
+    }
+  }
+
   auto* lhs_ref = lhs_type->As<sem::Reference>();
   if (!lhs_ref) {
     // LHS is not a reference, so it has no storage.
     AddError("cannot assign to value of type '" + TypeNameOf(a->lhs()) + "'",
              a->lhs()->source());
-
     return false;
   }