Unclosed string literals cause unhelpful diagnostic

Code

fn main() { "a"-a"; }

Current output

error: prefix `a` is unknown
 --> src/main.rs:1:17
  |
1 | fn main() { "a"-a"; }
  |                 ^ unknown prefix
  |
  = note: prefixed identifiers and literals are reserved since Rust 2021
help: consider inserting whitespace here
  |
1 | fn main() { "a"-a "; }
  |                  +

error[E0765]: unterminated double quote string
 --> src/main.rs:1:18
  |
1 | fn main() { "a"-a"; }
  |                  ^^^^

Desired output

error[E0765]: unterminated double quote string
 --> src/main.rs:1:18
  |
1 | fn main() { "a"-a"; }
  |               ^  ^^^^ here
  |               help: you may have meant to close this string instead: `"a""`
  = note: prefixed identifiers and literals are reserved since Rust 2021

Rationale and extra context

Confusing error when dealing with long unclosed string literals and command line options.

Other cases

No response

Anything else?

No response

Reproduce the case in src/main.rs with rustc and compare the current and desired diagnostics, then trace the compiler diagnostic responsible for the prefixed identifier and unterminated string messages. Done means the unclosed-string case emits the requested E0765 diagnostic, including the suggested closing string and note, without the misleading prefix error.