don't remove dbg! in arbitrary expressions

[y21]

Fixes #9914

The dbg_macro lint replaces empty dbg! invocations with the empty string in its suggestion, which is not always valid code in certain contexts (e.g. let _ = dbg!(); becomes let _ = ;). This PR changes it to (), which should always be valid where dbg!() is valid (dbg!() with no arguments evaluates to ()).

It also special-cases "standalone" dbg!(); expression statements, where it will suggest removing the whole statement entirely like it did before.

changelog: [dbg_macro]: don't remove dbg!() in arbitrary expressions as it sometimes results in syntax errors

[rustbot]

r? @Jarcho

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[Jarcho]

Does this work properly when dbg!() is expanded from another macro? e.g. foo!(); where foo!() expands to dbg!().

[y21]

Does this work properly when dbg!() is expanded from another macro? e.g. foo!(); where foo!() expands to dbg!().

Hmm it seems like it breaks if the dbg!() is passed as an expr metavariable

macro_rules! foo {
    ($x:expr) => { $x; };
}

fn main() {
    foo!(dbg!());
}

warning: the `dbg!` macro is intended as a debugging tool
  --> test.rs:16:10
   |
16 |     foo!(dbg!());
   |          ^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#dbg_macro
note: the lint level is defined here
  --> test.rs:1:9
   |
1  | #![warn(clippy::dbg_macro)]
   |         ^^^^^^^^^^^^^^^^^
help: remove the invocation before committing it to a version control system
   |
16 -     foo!(dbg!());
16 +     foo!(
   |

so perhaps my way of checking if we're allowed to remove the dbg!() by checking if the parent node is a StmtKind::Semi is not that reliable.

[Jarcho]

I think the only thing you can do here is to start tokenizing from the end of the call site and see if the first non-whitespace token is a semicolon.

[y21]

Alright, it's not exactly pretty but it works.

[Jarcho]

It works, so...

Thank you. @bors r+

[bors]

📌 Commit f0be0ee has been approved by Jarcho

It is now in the queue for this repository.

[bors]

⌛ Testing commit f0be0ee with merge 815a07e...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Jarcho
Pushing 815a07e to master...