suggest semi on expr mac!() good as stmt mac!().

Author: Centril

src/librustc_expand/mbe/macro_rules.rs

@@ -86,6 +86,7 @@ fn suggest_slice_pat(e: &mut DiagnosticBuilder<'_>, site_span: Span, parser: &Pa
 fn emit_frag_parse_err(
  mut e: DiagnosticBuilder<'_>,
  parser: &Parser<'_>,
+ orig_parser: &mut Parser<'_>,
  site_span: Span,
  macro_ident: ast::Ident,
  arm_span: Span,
@@ -118,6 +119,21 @@ fn emit_frag_parse_err(
  AstFragmentKind::Pat if macro_ident.name == sym::vec => {
  suggest_slice_pat(&mut e, site_span, parser);
  }
+ // Try a statement if an expression is wanted but failed and suggest adding `;` to call.
+ AstFragmentKind::Expr => match parse_ast_fragment(orig_parser, AstFragmentKind::Stmts) {
+ Err(mut err) => err.cancel(),
+ Ok(_) => {
+ e.note(
+ "the macro call doesn't expand to an expression, but it can expand to a statement",
+ );
+ e.span_suggestion_verbose(
+ site_span.shrink_to_hi(),
+ "add `;` to interpret the expansion as a statement",
+ ";".to_string(),
+ Applicability::MaybeIncorrect,
+ );
+ }
+ },
  _ => annotate_err_with_kind(&mut e, kind, site_span),
  };
  e.emit();
@@ -126,10 +142,11 @@ fn emit_frag_parse_err(
 impl<'a> ParserAnyMacro<'a> {
  crate fn make(mut self: Box<ParserAnyMacro<'a>>, kind: AstFragmentKind) -> AstFragment {
  let ParserAnyMacro { site_span, macro_ident, ref mut parser, arm_span } = *self;
+ let snapshot = &mut parser.clone();
  let fragment = match parse_ast_fragment(parser, kind) {
  Ok(f) => f,
  Err(err) => {
- emit_frag_parse_err(err, parser, site_span, macro_ident, arm_span, kind);
+ emit_frag_parse_err(err, parser, snapshot, site_span, macro_ident, arm_span, kind);
  return kind.dummy(site_span);
  }
  };

src/test/ui/macros/issue-34421-mac-expr-bad-stmt-good-add-semi.rs

@@ -0,0 +1,15 @@
+macro_rules! make_item {
+ ($a:ident) => {
+ struct $a;
+ }; //~^ ERROR expected expression
+ //~| ERROR expected expression
+}
+
+fn a() {
+ make_item!(A)
+}
+fn b() {
+ make_item!(B)
+}
+
+fn main() {}

src/test/ui/macros/issue-34421-mac-expr-bad-stmt-good-add-semi.stderr

@@ -0,0 +1,34 @@
+error: expected expression, found keyword `struct`
+ --> $DIR/issue-34421-mac-expr-bad-stmt-good-add-semi.rs:3:9
+ |
+LL | struct $a;
+ | ^^^^^^ expected expression
+...
+LL | make_item!(A)
+ | ------------- in this macro invocation
+ |
+ = note: the macro call doesn't expand to an expression, but it can expand to a statement
+ = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+help: add `;` to interpret the expansion as a statement
+ |
+LL | make_item!(A);
+ | ^
+
+error: expected expression, found keyword `struct`
+ --> $DIR/issue-34421-mac-expr-bad-stmt-good-add-semi.rs:3:9
+ |
+LL | struct $a;
+ | ^^^^^^ expected expression
+...
+LL | make_item!(B)
+ | ------------- in this macro invocation
+ |
+ = note: the macro call doesn't expand to an expression, but it can expand to a statement
+ = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+help: add `;` to interpret the expansion as a statement
+ |
+LL | make_item!(B);
+ | ^
+
+error: aborting due to 2 previous errors
+

src/test/ui/macros/macro-in-expression-context-2.stderr

@@ -6,6 +6,12 @@ LL | macro_rules! empty { () => () }
 ...
 LL | _ => { empty!() }
  | ^^^^^^^^ expected expression
+ |
+ = note: the macro call doesn't expand to an expression, but it can expand to a statement
+help: add `;` to interpret the expansion as a statement
+ |
+LL | _ => { empty!(); }
+ | ^
 
 error: aborting due to previous error