rust-clippy Add `needless_conversion_for

Adds a lint to flag trait-behavior preserving calls where the receiver/argument could be used directly. For example, the lint flags the call to as_path in the following code, because &lint_file_path could be used instead: https://github.com/rust-lang/rust-clippy/blob/14dfc0359717c1576f6b7060614f9bbd331fb6a7/clippy_dev/src/new_lint.rs#L436

Organization

The PR is currently organized as five commits:

DisallowedPath -> ConfPath; REPLACEMENTS_ALLOWED -> REPLACEABLE: Shorten some configuration-related names for types used by this lint.
Move some functions into clippy_utils::ty: Move some functions from unnecessary_to_owned and needless_conversion_for_generic_args (cc: @Jarcho) into the clippy_utils::ty module.
Expand comment in replace_types: Emphasize that replace_types cannot change a function's output type (see below).
Add needless_conversion_for_trait lint; get uitest to pass: The PR's main commit.
Fix adjacent code: Fix Clippy code flagged by the lint.

Relationship to `unnecessary_to_owned`

Some of unnecessary_to_owned's functionality is subsumed by this lint. However, replace_types, which needless_conversion_for_trait uses, does not allow a function's output type to change. unnecessary_to_owned does not have this restriction. So, to ensure the two lints do not flag the same code, unnecessary_to_owned now requires that a function's output type contain the type to be replaced. Note that some unnecessary_to_owned tests contain a function whose output type is changed by the lint's suggestion:

https://github.com/rust-lang/rust-clippy/blob/14dfc0359717c1576f6b7060614f9bbd331fb6a7/tests/ui/unnecessary_to_owned.rs#L460
https://github.com/rust-lang/rust-clippy/blob/14dfc0359717c1576f6b7060614f9bbd331fb6a7/tests/ui/unnecessary_to_owned.rs#L604

Also, unnecessary_to_owned will not eliminate a call to ToString::to_string unless the receiver implements Deref<Target = str> or AsRef<str>. needless_conversion_for_trait incorporates this restriction as well.

`check_inherent_functions`

The lint includes a check_crate_post check that runs when Clippy is compiled in debug mode. The check looks for functions the lint should either warn about, or should explicitly ignore. For example, while preparing this PR, the check flagged Vec::into_chunks, which was added to the list of explicitly ignored functions.

False positive in `clippy_utils`

There are three false positives in clippy_utils that I do not know how best to resolve. They concern the implementations of the three functions for DiagExt: https://github.com/rust-lang/rust-clippy/blob/14dfc0359717c1576f6b7060614f9bbd331fb6a7/clippy_utils/src/sugg.rs#L702-L715

needless_conversion_for_trait reports that msg.to_string() could be replaced with msg. However, making this change causes the borrow checker to think msg's reference flows into &mut self. I have allowed the lint for the trait implementation, but I am open to suggestions for eliminating this class of false positives.

changelog: Add needless_conversion_for_trait lint

Summary Notes

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Aug 10 '25 12:08 smoelius

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Aug 10 '25 12:08 rustbot

Lintcheck changes for 33a368e26a4052fba4304dcc614c7e9c89541090

Lint	Added	Removed	Changed
`clippy::needless_conversion_for_trait`	42	0	0
`clippy::unnecessary_to_owned`	0	2	0

This comment will be updated if you push new changes

Aug 10 '25 12:08 github-actions[bot]

:umbrella: The latest upstream changes (possibly 73b5a59c224c189735aa628f85c874849e52cabf) made this pull request unmergeable. Please resolve the merge conflicts.

Aug 18 '25 15:08 rustbot

:umbrella: The latest upstream changes (possibly 368b2355797b13511776f0a35ec4864468b87ece) made this pull request unmergeable. Please resolve the merge conflicts.

Sep 08 '25 08:09 rustbot