Consistent trait bounds for ExtractIf Debug impls

[dtolnay]

Closes #137654. Refer to that issue for a table of the 4 different impl signatures we previously had in the standard library for Debug impls of various ExtractIf iterator types.

The one we are standardizing on is the one so far only used by alloc::collections::linked_list::ExtractIf, which is no F: Debug bound, no F: FnMut bound, only T: Debug bound.

This PR applies the following signature changes:

/* alloc::collections::btree_map */

    pub struct ExtractIf<'a, K, V, F, A = Global>
    where
-       F: 'a + FnMut(&K, &mut V) -> bool,
        Allocator + Clone,

    impl Debug for ExtractIf<'a, K, V, F,
+       A,
    >
    where
        K: Debug,
        V: Debug,
-       F: FnMut(&K, &mut V) -> bool,
+       A: Allocator + Clone,

/* alloc::collections::btree_set */

    pub struct ExtractIf<'a, T, F, A = Global>
    where
-       T: 'a,
-       F: 'a + FnMut(&T) -> bool,
        Allocator + Clone,

    impl Debug for ExtractIf<'a, T, F, A>
    where
        T: Debug,
-       F: FnMut(&T) -> bool,
        A: Allocator + Clone,

/* alloc::collections::linked_list */

    impl Debug for ExtractIf<'a, T, F,
+       A,
    >
    where
        T: Debug,
+       A: Allocator,

/* alloc::vec */

    impl Debug for ExtractIf<'a, T, F, A>
    where
        T: Debug,
-       F: Debug,
        A: Allocator,
-       A: Debug,

/* std::collections::hash_map */

    pub struct ExtractIf<'a, K, V, F>
    where
-       F: FnMut(&K, &mut V) -> bool,

    impl Debug for ExtractIf<'a, K, V, F>
    where
+       K: Debug,
+       V: Debug,
-       F: FnMut(&K, &mut V) -> bool,

/* std::collections::hash_set */

    pub struct ExtractIf<'a, T, F>
    where
-       F: FnMut(&T) -> bool,

    impl Debug for ExtractIf<'a, T, F>
    where
+       T: Debug,
-       F: FnMut(&T) -> bool,

I have made the following changes to bring these types into better alignment with one another.

• Delete F: Debug bounds. These are especially problematic because Rust closures do not come with a Debug impl, rendering the impl useless.

• Delete A: Debug bounds. Allocator parameters are unstable for now, but in the future this would become an API commitment that we do not debug-print a representation of the allocator when printing an iterator.

• Delete F: FnMut bounds. Requires hashbrown PR: Drop FnMut trait bounds from ExtractIf data structures hashbrown#616. API commitment: we commit to not doing RefCell voodoo inside ExtractIf to have some way for its Debug impl (which takes &self) to call a FnMut closure, if this is even possible.

• Add T: Debug bounds (or K/V), even on Debug impls that do not currently make use of them, but might in the future. Breaking change. Must backport into Rust 1.87 (current beta) or do a de-stabilization PR in beta to delay those types by one release.

• Render using debug_struct + finish_non_exhaustive, instead of debug_tuple.

• Do not render the entire underlying collection.

• Show a "peek" field indicating the current position of the iterator.

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[rustbot]

These commits modify the library/Cargo.lock file. Unintentional changes to library/Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[dtolnay]

Marking as draft until rust-lang/hashbrown#616 ends up in a hashbrown release, but this is ready for review.

[rust-log-analyzer]

The job mingw-check-tidy failed! Check out the build log: (web) (plain)

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info: removing rustup binaries
info: rustup is uninstalled
##[group]Image checksum input
mingw-check-tidy
# We use the ghcr base image because ghcr doesn't have a rate limit
# and the mingw-check-tidy job doesn't cache docker images in CI.
FROM ghcr.io/rust-lang/ubuntu:22.04

ARG DEBIAN_FRONTEND=noninteractive
RUN apt-get update && apt-get install -y --no-install-recommends \
  g++ \
  make \
---

COPY host-x86_64/mingw-check/validate-toolstate.sh /scripts/
COPY host-x86_64/mingw-check/validate-error-codes.sh /scripts/

# NOTE: intentionally uses python2 for x.py so we can test it still works.
# validate-toolstate only runs in our CI, so it's ok for it to only support python3.
ENV SCRIPT TIDY_PRINT_DIFF=1 python2.7 ../x.py test \
           --stage 0 src/tools/tidy tidyselftest --extra-checks=py,cpp
#
# This file is autogenerated by pip-compile with Python 3.10
# by the following command:
#
#    pip-compile --allow-unsafe --generate-hashes reuse-requirements.in
---
#12 2.894 Building wheels for collected packages: reuse
#12 2.895   Building wheel for reuse (pyproject.toml): started
#12 3.105   Building wheel for reuse (pyproject.toml): finished with status 'done'
#12 3.106   Created wheel for reuse: filename=reuse-4.0.3-cp310-cp310-manylinux_2_35_x86_64.whl size=132719 sha256=5bb60f62728aaedff7162745ce743c7f2f55069b3e7f82e6a37d70df455797cc
#12 3.107   Stored in directory: /tmp/pip-ephem-wheel-cache-9rbhx5tf/wheels/3d/8d/0a/e0fc6aba4494b28a967ab5eaf951c121d9c677958714e34532
#12 3.109 Successfully built reuse
#12 3.109 Installing collected packages: boolean-py, binaryornot, tomlkit, reuse, python-debian, markupsafe, license-expression, jinja2, chardet, attrs
#12 3.526 Successfully installed attrs-23.2.0 binaryornot-0.4.4 boolean-py-4.0 chardet-5.2.0 jinja2-3.1.4 license-expression-30.3.0 markupsafe-2.1.5 python-debian-0.1.49 reuse-4.0.3 tomlkit-0.13.0
#12 3.526 WARNING: Running pip as the 'root' user can result in broken permissions and conflicting behaviour with the system package manager. It is recommended to use a virtual environment instead: https://pip.pypa.io/warnings/venv
#12 4.076 Collecting virtualenv
#12 4.138   Downloading virtualenv-20.30.0-py3-none-any.whl (4.3 MB)
#12 4.331      ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 4.3/4.3 MB 22.7 MB/s eta 0:00:00
#12 4.391 Collecting platformdirs<5,>=3.9.1
#12 4.399   Downloading platformdirs-4.3.7-py3-none-any.whl (18 kB)
#12 4.439 Collecting filelock<4,>=3.12.2
#12 4.447   Downloading filelock-3.18.0-py3-none-any.whl (16 kB)
#12 4.472 Collecting distlib<1,>=0.3.7
#12 4.479   Downloading distlib-0.3.9-py2.py3-none-any.whl (468 kB)
#12 4.492      ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 469.0/469.0 KB 44.5 MB/s eta 0:00:00
#12 4.573 Installing collected packages: distlib, platformdirs, filelock, virtualenv
#12 4.756 Successfully installed distlib-0.3.9 filelock-3.18.0 platformdirs-4.3.7 virtualenv-20.30.0
#12 4.756 WARNING: Running pip as the 'root' user can result in broken permissions and conflicting behaviour with the system package manager. It is recommended to use a virtual environment instead: https://pip.pypa.io/warnings/venv
#12 DONE 4.8s

#13 [7/8] COPY host-x86_64/mingw-check/validate-toolstate.sh /scripts/
#13 DONE 0.0s
---
DirectMap4k:      137152 kB
DirectMap2M:     7202816 kB
DirectMap1G:    11534336 kB
##[endgroup]
Executing TIDY_PRINT_DIFF=1 python2.7 ../x.py test            --stage 0 src/tools/tidy tidyselftest --extra-checks=py,cpp
+ TIDY_PRINT_DIFF=1 python2.7 ../x.py test --stage 0 src/tools/tidy tidyselftest --extra-checks=py,cpp
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.05s
##[endgroup]
WARN: currently no CI rustc builds have rustc debug assertions enabled. Please either set `rust.debug-assertions` to `false` if you want to use download CI rustc or set `rust.download-rustc` to `false`.
[TIMING] core::build_steps::tool::LibcxxVersionTool { target: x86_64-unknown-linux-gnu } -- 0.216
---
[TIMING] core::build_steps::tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
fmt: checked 5962 files
tidy check
tidy error: invalid source: "git+https://github.com/rust-lang/hashbrown?rev=refs%2Fpull%2F616%2Fhead#82cd86cc31a8f3a753dda0178f12946485e5fbbb"
tidy error: could not find allowed package `allocator-api2`
Remove from PERMITTED_DEPENDENCIES list if it is no longer used.
Go to `src/tools/tidy/src/deps.rs:235` for the list.
tidy: Skipping binary file check, read-only filesystem
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'virtualenv'
Requirement already satisfied: pip in ./build/venv/lib/python3.10/site-packages (25.0.1)
linting python files
All checks passed!
checking python file formatting
26 files already formatted
checking C++ file formatting
some tidy checks failed
Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:01:46
  local time: Sun Apr 13 20:34:39 UTC 2025
  network time: Sun, 13 Apr 2025 20:34:39 GMT
##[error]Process completed with exit code 1.
Post job cleanup.