WIP: Add lint for String in diagnostic implementation

[clubby789]

Implementing the check described in #108162 (comment) so we can see what happens. Implemented as a lint so it can be #[allow]'d
cc @Nilstrieb

@rustbot label +A-translation
r? ghost

[Nilstrieb]

a little disappointed that this doesn't contain a String ☺️

[clubby789]

It might be more useful to instead lint expressions constructing diagnostics so we can see if we're calling a potentially problematic function to construct the string. There's quite a few legitimate use cases here

[rust-log-analyzer]

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git config --global core.autocrlf false
shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
env:
  CI_JOB_NAME: x86_64-gnu-llvm-14
---
   |
87 |     pub us: String,
   |             ^^^^^^
   |
   = note: this could indicate incorrectly eagerly converting to a string
   = note: `-D rustc::string-in-diagnostic` implied by `-D warnings`
error: use of String in diagnostic
  --> compiler/rustc_session/src/errors.rs:93:13
   |
93 |     pub us: String,
---

error: use of String in diagnostic
   --> compiler/rustc_session/src/errors.rs:255:17
    |
255 |     pub suffix: String,
    |
    = note: this could indicate incorrectly eagerly converting to a string

error: use of String in diagnostic
---

error: use of String in diagnostic
   --> compiler/rustc_session/src/errors.rs:274:17
    |
274 |     pub suffix: String,
    |
    = note: this could indicate incorrectly eagerly converting to a string

error: use of String in diagnostic

[bors]

☔ The latest upstream changes (presumably #108682) made this pull request unmergeable. Please resolve the merge conflicts.