feat: hint at unterminated strings in unknown prefix errors

[avrabe]

Hi! This is my first contribution to rust-analyzer - I hope I'm following the right approach.

Problem

When rust-analyzer encounters "unknown literal prefix" errors, they can sometimes be caused by unterminated string literals earlier in the code. This makes debugging confusing because the error points to a distant location rather than the actual missing quote.

For example:

fn main() {
    println\!("hello world   // missing closing quote
    let result = zero_function();  // error reported here: unknown literal prefix `zero`
}

Solution

Added simple quote balance detection to provide helpful hints when this situation occurs:

• When an "unknown literal prefix" error happens, check for unbalanced quotes in the preceding 500 characters
• If odd number of quotes found, add a note suggesting to check for unterminated string literals
• If even number, show normal error message

Before:

error: unknown literal prefix `zero`

After:

error: unknown literal prefix `zero` (note: check for unterminated string literal)

Implementation

• File: crates/parser/src/lexed_str.rs
• Lines added: 8 (very minimal change)
• Performance: Only checks recent text when error occurs
• Safety: Non-breaking, only enhances existing error messages

The implementation follows rust-analyzer's style of simple, reliable improvements. I tried to keep it as minimal as possible while still being helpful.

I'd really appreciate any feedback on the approach or implementation! Thank you for considering this contribution.

[ChayimFriedman2]

Seems reasonable, but please add a test. You can see other lexer tests at crates/parser/test_data/lexer/err/.