Stabilize feature result_flattening

[Rynibami]

Stabilizes the Result::flatten method

Implementations

• Implementation Result::flatten: Add Result<Result<T, E>, E>::flatten -> Result<T, E> #70140
• Implementation const Result::flatten: make unstable Result::flatten a const fn #130692
• Update stabilization attribute macros (this PR)

Stabilization process

• Created this PR suggested by @RalfJung
• FCP (haven't found any, is it applicable here?)
• Close issue Tracking Issue for Result::flatten (result_flattening) #70142

[rustbot]

r? @jhpratt

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[jhpratt]

Yes, FCP is necessary and will be done on this PR. There's nothing you need to do on that front.

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[RalfJung]

Uses of this attribute should always be called out in the PR as they require special attention. However, this one should be fine, for the same reason as all the other rustc_allow_const_fn_unstable(const_precise_live_drops): if we change how const_precise_live_drops works, we'll ensure it keeps accepting these cases.

Cc @rust-lang/wg-const-eval