Misleading diagnostic for macro error! when called with no parameters

[MarkSteinbrick95]

When calling the macro with no arguments error!():

error: Could not compile
warning: build failed, waiting for other jobs to finish...
error: expected identifier, found `,`
  --> mod.rs:37:15
   |
37 |               error!();
   |               ^^^^^^^^^
   |               |
   |               expected identifier
   |               help: remove this comma
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error: aborting due to previous error

error: Could not compile

The diagnostic should instruct the user that a string literal is required as a parameter to the macro.

[jonas-schievink]

The diagnostic should instruct the user that a string literal is required as a parameter to the macro.

error! is declared as taking $($arg:tt)*, so I don't think this is possible. If it would take a $fmt:lit instead, maybe.

[estebank]

Ran using -Zexternal-macro-backtrace:

error: expected identifier, found `,`
  --> <::log::macros::error macros>:4:28
   |
1  | / ( target : $ target : expr , $ ( $ arg : tt ) * ) => (
2  | | log ! ( target : $ target , $ crate :: Level :: Error , $ ( $ arg ) * ) ; ) ;
3  | | ( $ ( $ arg : tt ) * ) => (
4  | | log ! ( $ crate :: Level :: Error , $ ( $ arg ) * ) ; )
   | |____________________________^__________________________- in this expansion of `error!`
   |                              |
   |                              expected identifier
   |                              help: remove this comma
   |
  ::: src/main.rs:29:5
   |
29 |       error!();
   |       --------- in this macro invocation

Getting the compiler to behave correctly in this case will be difficult, but making the macro itself handle this case and have an appropriate compile time error might be reasonable. What does @rust-lang/libs think?

[dtolnay]

I opened rust-lang/log#322 to fix the problem on log's side. The compiler was largely doing the right thing and I consider it a problem with the way log's macro was written that resulted in a misleading error.

There is something going wrong in the compiler here too though. Minimized:

macro_rules! log {
    ($lvl:expr, $($arg:tt)+) => {}
}

fn main() {
    log!(Level::Error ,);
}

error: expected identifier, found `,`
 --> src/main.rs:6:17
  |
6 |     log!(Level::Error ,);
  |                 ^
  |                 |
  |                 expected identifier
  |                 help: remove this comma

The error points to something that is an identifier and says it expected an identifier 😞 and points to something that is not a comma and says remove this comma. 😢

[estebank]

I noticed that too. My guess is that it's just a token off by one error somewhere.

[estebank]

@dtolnay I can confirm that that incorrect diagnostic is caused by #52397 :(

It'd be the following without it

error: unexpected end of macro invocation
 --> file.rs:6:24
  |
1 | macro_rules! log {
  | ---------------- when calling this macro
...
6 |     log!(Level::Error ,);
  |                        ^ missing tokens in macro arguments