Strip leading underscores of argument names in function/method

[kuy]

Closes #4510

Goal

When I select a function/method from completions, I get a snippet that doesn't contain leading underscores of argument names.

Solution

• Option 1: All signatures don't contain underscores
• Option 2: Keep same signature, but inserted snippet doesn't contain underscores

I choose Option 2 because I think that leading underscores is a part of "signature". Users should get correct signatures. On the other hand, trimming underscores is an assist by IDE.

Other impls.

rls: Complete argument names with underscores (same as actual ra)
IntelliJ Rust: Doesn't complete argument names
VSCode (TypeScript): Doesn't complete argument names

Working example

[lnicola]

Is FunctionSignature only used for completion?

[kuy]

@lnicola Ah, I shouldn't modified FunctionSignature directly... thanks for comment. I'll strip underscores in the presentation layer of completion: Completions::add_function(...)

https://github.com/rust-analyzer/rust-analyzer/blob/00172d0b555d22a42a2cbf8785eb7b2592507c4f/crates/ra_ide/src/completion/presentation.rs#L210-L215

[kuy]

Fixed!

[matklad]

bors r+

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (macos-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)