Format string syntax

[lnicola]

If IntelliJ can do it, so can we.

[bjorn3]

Syntax highlighting is implemented, but syntax errors in format strings aren't shown.

[DJMcNab]

Another thought would be to link to the specific Display/Debug implementation when go to definition is activated on {}/{:?} (using the same infrastructure from #4558)

[Milo123459]

I'm going to have a look into implementing this (hopefully!).

[Veykril]

Go ahead, feel free to reach out here/on zulip if you have any questions or need directions(also see https://github.com/rust-analyzer/rust-analyzer/blob/master/crates/ide/src/syntax_highlighting/format.rs for how highlighting deals with this already)

[ChayimFriedman2]

Granted, format strings need special handling in many IDE features and break once a while, but is there still something to specifically track in this issue?

[lnicola]

I guess not. I'd love an assist to replace path expressions, though.