Avoid tearing dbg! prints

[orlp]

Fixes #136703.

I did attempt a solution which collects all expressions into a tuple tmp before iterating over that tuple using the unstable macro_metavar_expr tmp.${index()}, but that refused to compile even with #[allow_internal_unstable(macro_metavar_expr)] and/or #![feature(macro_metavar_expr)]:

error[E0658]: meta-variable expressions are unstable
   --> /Users/orlp/programming/rust/rust/library/std/src/macros.rs:394:34
    |
394 |                     &eager_eval.${index()} as &dyn $crate::fmt::Debug,
    |                                  ^^^^^^^^^
    |
    = note: see issue #83527 <https://github.com/rust-lang/rust/issues/83527> for more information
    = help: add `#![feature(macro_metavar_expr)]` to the crate attributes to enable

As mentioned in the code comment, the original proposed solution in the issue of simply holding the stderr lock is undesirable because it can lead to deadlocks.

[rustbot]

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[joboet]

You could also use concat! to create a single format template for all arguments (yeah, apparently this works).
Nevermind, you still wouldn't be able to name the variables...

[orlp]

@joboet The dbg! macro has a comment warning us precisely against doing that, even if we could name the variables:

// NOTE: We cannot use `concat!` to make a static string as a format argument
// of `eprintln!` because `file!` could contain a `{` or
// `$val` expression could be a block (`{ .. }`), in which case the `eprintln!`
// will be malformed.

[joboet]

That's different, that's warning against combining the output of file! into the format string – that's unnecessary nowadays anyway since the compiler will inline string literals into format_args! itself.

[orlp]

If you'd like to see the other approach I've tried you can see it here: abe2246

It should work in theory, it just refuses to compile because of the unstable macro feature despite enabling that feature, in a way I don't understand.

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[joboet]

I've found another solution: #149869.