Recover from typo where == is used in place of =

[chenyukang]

Fixes #101477

[rust-highfive]

r? @fee1-dead

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[fee1-dead]

Nice work! LGTM pending error message improvements. Also, could this be using SessionDiagnostics instead? cc @davidtwco

[fee1-dead]

Suggested change

	let mut err = self.struct_span_err(self.token.span, "expect `=`, found `==`");
	err.span_suggestion_short(self.token.span, "write `=` instead of `==`", "=", appl)
	let mut err = self.struct_span_err(self.token.span, "unexpected `==`");
	err.span_suggestion_short(self.token.span, "try using `=` instead", "=", appl)

[chenyukang]

SessionDiagnostic will print the suggestion in the same line, so it will print like this:

error: unexpected `==`
 --> src/test/ui/parser/issue-101477-enum.rs:6:7
  |
6 |     B == 2 //~ ERROR unexpected `==`
  |       ^^ help: try using `=` instead `==`: `=`

I think it's better in this format?

error: unexpected `==`
 --> src/test/ui/parser/issue-101477-enum.rs:6:7
  |
6 |     B == 2 //~ ERROR unexpected `==`
  |       ^^ help: replace `==` with a equal symbol: `=`

[chenyukang]

Resolved with @compiler-errors with advices.

[davidtwco]

Also, could this be using SessionDiagnostics instead? cc @davidtwco

This could absolutely use SessionDiagnostic, I'd appreciate it if it did - see #100717.

[chenyukang]

Also, could this be using SessionDiagnostics instead? cc @davidtwco

This could absolutely use SessionDiagnostic, I'd appreciate it if it did - see #100717.

@davidtwco
I'm fixing it to use SessionDiagnostic, I have a question when using it:

How can I keep returning an Err to avoid extra useless diagnostics?

-            let mut err = self.struct_span_err(self.token.span, "unexpected `==`");
-            err.span_suggestion_short(self.token.span, "try using `=` instead `==`", "=", appl)
-                .emit();
-            return Err(err);
+            self.sess.emit_err(UseEqInstead { span: self.token.span });
+            return Ok(true);

I use return Ok now and there will be a extra diagnostics which I want to remove.

--- a/src/test/ui/parser/issue-101477-let.stderr
+++ b/src/test/ui/parser/issue-101477-let.stderr
@@ -2,7 +2,13 @@ error: unexpected `==`
   --> $DIR/issue-101477-let.rs:4:11
    |
 LL |     let x == 2;
-   |           ^^ help: try using `=` instead `==`
+   |           ^^ help: replace `==` with a equal symbol: `=`

-error: aborting due to previous error
+error: expected expression, found `==`
+  --> $DIR/issue-101477-let.rs:4:11
+   |
+LL |     let x == 2;
+   |           ^^ expected expression
+
+error: aborting due to 2 previous errors

[chenyukang]

Seems we use this struct to avoid later errors:

/// Useful type to use with `Result<>` indicate that an error has already
/// been reported to the user, so no need to continue checking.
#[derive(Clone, Copy, Debug, Encodable, Decodable, Hash, PartialEq, Eq, PartialOrd, Ord)]
#[derive(HashStable_Generic)]
pub struct ErrorGuaranteed(());

But parse_let_expr is:

 fn parse_let_expr(&mut self) -> PResult<'a, P<Expr>> {
        ...

        self.expect(&token::Eq)?;
        
        ...
        Ok(self.mk_expr(span, ExprKind::Let(pat, expr, span)))
    }

[chenyukang]

Got it:

  let mut err = self.sess.create_err(UseEqInstead { span: self.token.span });
  err.emit();
  return Err(err);

[rustbot]

rustc_error_messages was changed

cc @davidtwco, @compiler-errors, @JohnTitor, @estebank, @TaKO8Ki

[fee1-dead]

@bors r+

[bors]

📌 Commit ddb225f has been approved by fee1-dead

It is now in the queue for this repository.