Add Wuseless-cast

scripts/diagnostics.txt

@@ -697,6 +697,7 @@ warning negative-timing-limit NegativeTimingLimit "timing check limit value '{}'
 warning sign-conversion SignConversion "implicit conversion changes signedness from {} to {}"
 warning float-bool-conv FloatBoolConv "implicit conversion from floating point type {} to boolean value"
 warning int-bool-conv IntBoolConv "implicit conversion from {} to boolean value"
+warning useless-cast UselessCast "useless cast from {} to the same type"
 
 subsystem Statements
 error ReturnNotInSubroutine "return statement is only valid inside task and function blocks"
@@ -1054,7 +1055,8 @@ group default = { real-underflow real-overflow vector-overflow int-overflow unco
 
 group extra = { empty-member empty-stmt dup-import pointless-void-cast case-gen-none case-gen-dup
                 unused-result format-real ignored-slice task-ignored width-trunc dup-attr event-const
-                ineffective-sign port-width-trunc constant-conversion float-bool-conv int-bool-conv }
+                ineffective-sign port-width-trunc constant-conversion float-bool-conv int-bool-conv
+                useless-cast }
 
 group pedantic = { empty-pattern implicit-net-port nonstandard-escape-code nonstandard-generate format-multibit-strength
                    nonstandard-sys-func nonstandard-foreach nonstandard-dist }

scripts/warning_docs.txt

@@ -1117,3 +1117,12 @@ module m;
     initial if (i + 2) begin end // Did you mean i + 2 != 0 or something else?
 endmodule
 ```
+
+-Wuseless-cast
+An explicit cast converts an expression to the same type, so the cast does nothing.
+```
+module m;
+    int i, j;
+    assign i = int'(j);
+endmodule
+```

source/ast/expressions/AssignmentExpressions.cpp

@@ -805,6 +805,7 @@ Expression& ConversionExpression::fromSyntax(Compilation& compilation,
         if (!Bitstream::checkClassAccess(*type, context, targetExpr.sourceRange)) {
             return badExpr(compilation, result());
         }
+
         if (operand.kind == ExpressionKind::Streaming) {
             if (!Bitstream::isBitstreamCast(*type,
                                             operand.as<StreamingConcatenationExpression>())) {
@@ -827,6 +828,11 @@ Expression& ConversionExpression::fromSyntax(Compilation& compilation,
         return *result(ConversionKind::BitstreamCast);
     }
 
+    if (type->isMatching(*operand.type)) {
+        context.addDiag(diag::UselessCast, syntax.apostrophe.location())
+            << *operand.type << targetExpr.sourceRange << operand.sourceRange;
+    }
+
     return *result();
 }
 

tests/unittests/ast/EvalTests.cpp

@@ -931,7 +931,7 @@ TEST_CASE("Dynamic string ops") {
     CHECK(session.eval("{\"Hi\",str2}").str() == "Hiaaaaa");
     CHECK(session.eval("str2 = {\"Hi\", \"Bye\"}").str() == "HiBye");
 
-    CHECK(session.eval("string'(str1)").str() == "a");
+    CHECK(session.eval("str1").str() == "a");
 
     session.eval("byte ba[] = \"asdf\";");
     CHECK(session.eval("string'(ba)").str() == "asdf");

tests/unittests/ast/MemberTests.cpp

@@ -2065,7 +2065,7 @@ module m(input [4:0] a, output [4:0] b, z[6], output [5:0] l, I.m foo, I bar);
 
         if (k < 2) (a => z[1]) = 1;
         if (1 < 2) (a => z[2]) = 1;
-        if (int'(g) == 1) (a => z[3]) = 1;
+        if (byte'(g) == 1) (a => z[3]) = 1;
         if (+g == 1) (a => z[4]) = 1;
         if (g inside { 1, 2 }) (a => z[5]) = 1;
 

tests/unittests/ast/WarningTests.cpp

@@ -769,3 +769,23 @@ endmodule
     CHECK(diags[0].code == diag::FloatBoolConv);
     CHECK(diags[1].code == diag::IntBoolConv);
 }
+
+TEST_CASE("Useless cast warnings") {
+    auto tree = SyntaxTree::fromText(R"(
+module m;
+    int i, j;
+    assign i = int'(j);
+
+    logic [3:0] k, l;
+    assign k = 4'(l);
+endmodule
+)");
+
+    Compilation compilation;
+    compilation.addSyntaxTree(tree);
+
+    auto& diags = compilation.getAllDiagnostics();
+    REQUIRE(diags.size() == 2);
+    CHECK(diags[0].code == diag::UselessCast);
+    CHECK(diags[1].code == diag::UselessCast);
+}