Stabilize fmt::from_fn

[coolreader18]

Resolves #146705, pending its FCP.

As discussed in that tracking issue and #117729, this splits fmt::from_fn out from the debug_closure_helpers feature.

[rustbot]

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[Kobzol]

r? libs-api

[bors]

☔ The latest upstream changes (presumably #146023) made this pull request unmergeable. Please resolve the merge conflicts.

[jmillikin]

(1) My understanding is that stabilizing a feature requires an FCP, which hasn't yet happened for the linked feature.

(2) It's not clear to me that any resistence to the feature as-is exists, so maybe the whole thing could be stabilized without needing to separate out from_fn?

I would be thrilled to have someone with sufficient permissions start an FCP for debug_closure_helpers.

[coolreader18]

(1) My understanding is that stabilizing a feature requires an FCP, which hasn't yet happened for the linked feature.

I think that's true, but I was imagining that the FCP would just happen here.

(2) It's not clear to me that any resistence to the feature as-is exists, so maybe the whole thing could be stabilized without needing to separate out from_fn?

I'd be fine with that - hopefully someone will start an FCP there.

[coolreader18]

I'm closing this for now with the idea that the full feature will hopefully get FCP'd.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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[RUSTC-TIMING] askama test:false 0.169
error[E0658]: use of unstable library feature `round_char_boundary`
   --> src/librustdoc/passes/lint/unescaped_backticks.rs:447:25
    |
447 |             ("...", dox.ceil_char_boundary(insert_index - context_before_max_len))
    |                         ^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #93743 <https://github.com/rust-lang/rust/issues/93743> for more information
    = help: add `#![feature(round_char_boundary)]` to the crate attributes to enable
    = note: this compiler was built on 2025-08-04; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature `round_char_boundary`
   --> src/librustdoc/passes/lint/unescaped_backticks.rs:452:25
    |
452 |             ("...", dox.floor_char_boundary(insert_index + context_after_max_len))
    |                         ^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #93743 <https://github.com/rust-lang/rust/issues/93743> for more information
    = help: add `#![feature(round_char_boundary)]` to the crate attributes to enable
    = note: this compiler was built on 2025-08-04; consider upgrading it if it is out of date