Add compiler warning for double semicolon

[hellow554]

This could should emit an error:

pub fn foo() {
    return;;
}

I think something similar to this would be a good one:

warning: redundant `;` detected
 --> src/main.rs:X:XX
  |
LL|         return;;
  |                ^
  |                |
  |                help: remove this `;` (this should be a suggestion so rustfix can fix it automatically)

[hellow554]

@rustbot modify labels: A-diagnostics C-enhancement E-easy

[hbina]

Hi, I would like to resolve this issue. Do you have any lead/pointers that I can use to start?

[hellow554]

I'm not entirely sure, I only started recently with hacking the compiler. I think this can be easily done by looking at

rust/src/libsyntax/parse/lexer/mod.rs

Lines 947 to 950 in 49d139c

	';' => {
	self.bump();
	Ok(token::Semi)
	}

You could do something similar to this:

';' => { 
    self.bump(); 
    if self.ch_is(';') {
        self.bump();
        let mut err = self.sess.span_diagnostic.struct_span_warn(self.sp, "redundant `;` detected");
        ...
        err.emit();
    }
    Ok(token::Semi) 
} 

Of course you should also detect more than one, e.g. return ;;;. Also make sure it works for let a = "";;;;;;.

[hbina]

is it safe to just perform a while loop?
i am not sure how reliable this solution could be.
Especially if there's a space like ; ;

Regardless how to say that this ;;;;;; entirely line is faulty, not just the first character?

[hellow554]

is it safe to just perform a while loop?

Go and try it.

Especially if there's a space like ; ;

That could be a problem indeed, because spaces are treated seperatly.

Regardless how to say that this ;;;;;; entirely line is faulty, not just the first character?

You mean something like

fn foo() {
    ;;;;;;;;;;;;;;
}

? I think this should be a different warning. Something like "sole `;` detected", because it does not end any statement at all and should have no right to exist.

I think a person which is more privileged than me should say a word or too as well :) I'm not a rust lang member

[hellow554]

Instead of using the lexer level, you could also use the token level, because the lexer converts the bytestream to tokens (as you can see with Ok(token::Semi)) and if you detect two Semi tokens next to each other, you could emit a warning there. Should be also possible. But I'm not entirely sure where to do that.

[Centril]

cc @petrochenkov

This should be done as new a lint pass (redundant_semicolon) or included in an existing one.