Rustfmt-ify expansion of macro

[emi2k01]

Just like cargo-expand, it'd be cool if Rust Analyzer had an option to format the expansion of macros.

[lilyball]

Either that, or allow me to run Format Document on the resulting editor. Trying to do so right now gives me a "Cannot edit in read-only editor" error (in VSCode). Probably formatting the initial expansion is better though.

[Veykril]

The problem here is that macros don't necessarily expand to something on its own that rustfmt can format(to my knowledge) so this wouldn't always work as we don't expand macros into the source file.

[bjorn3]

It should be possible to wrap the macro output in for example fn foo() { ... } or fn foo() { let ... = (); } depending on the context in which the macro expanded and then pass it to rustfmt. Macros never expand to raw tokenstreams.

[lilyball]

I did just realize yesterday that the expansion does still include $crate, which isn't legal outside of macros, and unfortunately rustfmt cares.

> echo 'fn foo() { $crate::print("Bar"); }' | rustfmt
error: expected expression, found `$`
 --> <stdin>:1:12
  |
1 | fn foo() { $crate::print("Bar"); }
  |            ^ expected expression

[Veykril]

Mmh ye that is a problem since we can't always replace them with the crate name its referring to since the crate in question isn't necessarily in the nameable dependencies of the current crate. We could replace $crate with some special valid identifier, rust rustfmt and then replace them it back in the output again I suppose?

[lnicola]

CC #7532