[JonathanBrouwer]

This PR makes applying the #[no_mangle] and #[export_name] attributes an error, rather than a warning.

It also makes applying the attributes to a provided trait function consistently an error.
This is necessary because the current behavior is very buggy, and it is not clear to me what the correct behavior should be.

History:

• Historically, export_name used to error on all targets except fields, arms, macro defs, and macro calls.
• Historically, no_mangle has always warned on invalid targets. It has always had buggy behaviour on provided trait functions.
• A year ago, Rework target checking for built-in attributes #145085 introduced a bug which allows export_name on provided trait functions. The behaviour is buggy.

The specific changes this PR makes are:

• export_name goes from warning to error on fields, arms, macro defs, and macro calls
• export_name goes from allowed to error on provided trait functions
• no_mangle goes from warning to error on all targets except the allowed targets: free functions, statics, inherent methods and trait impl methods.

Fixes #164571

cc @mejrs

[JonathanBrouwer]

This needs a crater run
@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 4d7c0cd (4d7c0cd298ace72490220301fa543d63a154f8ce)
Base parent: 0cefa27 (0cefa27fe3deee26fc98ee44249406358d9f0d26)

[JonathanBrouwer]

@craterbot check

[craterbot]

👌 Experiment pr-164789 created and queued.
🤖 Automatically detected try build 4d7c0cd
⚠️ Try build based on commit 5c4119b, but latest commit is 6717514. Did you forget to make a new try build?
🔍 You can check out the queue and this experiment's details.

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[mejrs]

rustc_codegen_ssa has a bunch of code defending against invalid codegen attrs, being able to clean that up would be nice. (for example, #159374 adds some)

[rust-bors]

☔ The latest upstream changes (presumably #164894) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

[craterbot]

🚧 Experiment pr-164789 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more