refactor(parser): provide better error messages for const modifiers…

… on class elements (#6353)

Quality-of-life improvement for invalid `const` modifiers on class elements. Instead of panicking, the parser will eat `const` and report it as an error.

```ts
class C {
  static const H = 1;
}
```

crates/oxc_parser/src/diagnostics.rs

@@ -169,6 +169,14 @@ pub fn class_declaration(span: Span) -> OxcDiagnostic {
         .with_label(span)
 }
 
+/// A class member cannot have the 'const' keyword. ts(1248)
+#[cold]
+pub fn const_class_member(span: Span) -> OxcDiagnostic {
+    ts_error("1248", "A class member cannot have the 'const' keyword.")
+        .with_help("Did you mean `readonly`?")
+        .with_label(span)
+}
+
 #[cold]
 pub fn binding_rest_element_last(span: Span) -> OxcDiagnostic {
     OxcDiagnostic::error("A rest element must be last in a destructuring pattern").with_label(span)

crates/oxc_parser/src/js/class.rs

@@ -185,7 +185,7 @@ impl<'a> ParserImpl<'a> {
 
         let span = self.start_span();
 
-        let modifiers = self.parse_modifiers(true, false, true);
+        let modifiers = self.parse_modifiers(true, true, true);
 
         let mut kind = MethodDefinitionKind::Method;
         let mut generator = false;
@@ -277,6 +277,10 @@ impl<'a> ParserImpl<'a> {
             self.error(diagnostics::optional_definite_property(optional_span.expand_right(1)));
         }
 
+        if modifiers.contains(ModifierKind::Const) {
+            self.error(diagnostics::const_class_member(key.span()));
+        }
+
         if let PropertyKey::PrivateIdentifier(private_ident) = &key {
             // `private #foo`, etc. is illegal
             if self.ts_enabled() {

tasks/coverage/snapshots/parser_misc.snap

@@ -165,10 +165,17 @@ Negative Passed: 20/20 (100.00%)
    ╰────
   help: Try insert a semicolon here
 
+  × TS(1248): A class member cannot have the 'const' keyword.
+   ╭─[misc/fail/oxc-4212-1.ts:1:17]
+ 1 │ class a { const enum b(); }
+   ·                 ────
+   ╰────
+  help: Did you mean `readonly`?
+
   × Expected a semicolon or an implicit semicolon after a statement, but found none
-   ╭─[misc/fail/oxc-4212-1.ts:1:16]
+   ╭─[misc/fail/oxc-4212-1.ts:1:21]
  1 │ class a { const enum b(); }
-   ·                ▲
+   ·                     ▲
    ╰────
   help: Try insert a semicolon here
 

tasks/coverage/snapshots/parser_typescript.snap

@@ -4730,23 +4730,32 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/salsa/private
    ·          ────────────────
    ╰────
 
+  × TS(1248): A class member cannot have the 'const' keyword.
+   ╭─[typescript/tests/cases/compiler/ClassDeclaration26.ts:2:18]
+ 1 │ class C {
+ 2 │     public const var export foo = 10;
+   ·                  ───
+ 3 │ 
+   ╰────
+  help: Did you mean `readonly`?
+
   × Expected a semicolon or an implicit semicolon after a statement, but found none
-   ╭─[typescript/tests/cases/compiler/ClassDeclaration26.ts:2:17]
+   ╭─[typescript/tests/cases/compiler/ClassDeclaration26.ts:2:21]
  1 │ class C {
  2 │     public const var export foo = 10;
-   ·                 ▲
+   ·                     ▲
  3 │ 
    ╰────
   help: Try insert a semicolon here
 
-  × Expected a semicolon or an implicit semicolon after a statement, but found none
-   ╭─[typescript/tests/cases/compiler/ClassDeclarationWithInvalidConstOnPropertyDeclaration.ts:2:15]
+  × TS(1248): A class member cannot have the 'const' keyword.
+   ╭─[typescript/tests/cases/compiler/ClassDeclarationWithInvalidConstOnPropertyDeclaration.ts:2:16]
  1 │ class AtomicNumbers {
  2 │   static const H = 1;
-   ·               ▲
+   ·                ─
  3 │ }
    ╰────
-  help: Try insert a semicolon here
+  help: Did you mean `readonly`?
 
   × A parameter property is only allowed in a constructor implementation.
    ╭─[typescript/tests/cases/compiler/MemberAccessorDeclaration15.ts:2:12]
@@ -6597,14 +6606,14 @@ Expect to Parse: tasks/coverage/typescript/tests/cases/conformance/salsa/private
  19 │     const c5 = 0;
     ╰────
 
-  × Expected a semicolon or an implicit semicolon after a statement, but found none
-   ╭─[typescript/tests/cases/compiler/constInClassExpression.ts:2:10]
+  × TS(1248): A class member cannot have the 'const' keyword.
+   ╭─[typescript/tests/cases/compiler/constInClassExpression.ts:2:11]
  1 │ let C = class {
  2 │     const a = 4;
-   ·          ▲
+   ·           ─
  3 │ };
    ╰────
-  help: Try insert a semicolon here
+  help: Did you mean `readonly`?
 
   × TS(1090): 'static' modifier cannot appear on a parameter.
    ╭─[typescript/tests/cases/compiler/constructorArgsErrors1.ts:2:18]