UX: highlight the diff when a signature mismatch occurs

Today I made a typo in my function signatures. It took me longer than I wanted to identify I was missing an &mut. The command line output of rustc handles this rather nicely: it renders the mismatched signatures using bold text. It'd be great if rust-analyzer could similarly apply render tricks to draw attention to the diff.

Another thing that stands out is that the signature diffs in rust-analyzer are not aligned, which makes it harder to scan for differences. I'm not sure to which degree VS Code allows for this, but if it it's at all possible it would be a huge boon.

Thanks!

Screenshots

Command line output

The missing &mut stands out clearly here and I can proceed immediately to fixing it. Also notice how the signatures are aligned.

Rust-analyzer output

Spotting the missing &mut took me about 30 seconds, getting me out of my flow of writing code.

Start by investigating rust-analyzer’s signature-mismatch diagnostic rendering and the VS Code/LSP capabilities mentioned in the report. Done means differing signature text is visually emphasized and the signatures are aligned enough to scan, matching the improvements illustrated by the supplied screenshots.