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

[Noratrieb]

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]

The job x86_64-gnu-llvm-14 failed! Check out the build log: (web)(plain)

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Download action repository 'actions/checkout@v3' (SHA:ac593985615ec2ede58e132d2e21d2b1cbd6127c)
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Complete job name: PR (x86_64-gnu-llvm-14, false, ubuntu-20.04-xl)
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.