Re: [PATCH 1/2] rust: allow `clippy::collapsible_match` globally

[Gary Guo]

On Sun Apr 26, 2026 at 3:42 PM BST, Miguel Ojeda wrote:
> The `clippy::collapsible_match` lint [1] can make code harder to read
> in certain cases [2], e.g.
>
>       CLIPPY P rust/libmacros.so - due to command line change
>     warning: this `if` can be collapsed into the outer `match`
>       --> rust/pin-init/internal/src/helpers.rs:91:17
>        |
>     91 | /                 if nesting == 1 {
>     92 | |                     impl_generics.push(tt.clone());
>     93 | |                     impl_generics.push(tt);
>     94 | |                     skip_until_comma = false;
>     95 | |                 }
>        | |_________________^
>        |
>        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match
>        = note: `-W clippy::collapsible-match` implied by `-W clippy::all`
>        = help: to override `-W clippy::all` add `#[allow(clippy::collapsible_match)]`
>     help: collapse nested if block
>        |
>     90 ~             TokenTree::Punct(p) if skip_until_comma && p.as_char() == ','
>     91 ~                 && nesting == 1 => {
>     92 |                     impl_generics.push(tt.clone());
>     93 |                     impl_generics.push(tt);
>     94 |                     skip_until_comma = false;
>     95 ~                 }
>        |
>
> The lint does not have much upside -- when the suggestion may be a good
> one, it would still read fine when nested anyway. And it is the kind of
> lint that may easily bias people to just apply the suggestion instead
> of allowing it.

It's also just wrong.

    pattern => {
        if x {
            ...
        }
    }

is not equal to

    pattern if x => { ... }

because the former prevents other arms later (e.g. a `_`) from being to act
while the latter doesn't.

See https://github.com/rust-lang/rust-clippy/pull/16878

>
> Thus just let developers decide on their own.
>
> Cc: stable@xxxxxxxxxxxxxxx # Needed in 6.12.y and later (Rust is pinned in older LTSs).
> Link: https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_match [1]
> Link: https://lore.kernel.org/rust-for-linux/CANiq72nWYJna_hdFxjQCQZK6yJBrr1Mb86iKavivV0U0BgufeA@xxxxxxxxxxxxxx/ [2]
> Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>

Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>

Best,
Gary

> ---
>  Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/Makefile b/Makefile
> index 54e1ae602000..e2a810bc4409 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -486,6 +486,7 @@ export rust_common_flags := --edition=2021 \
>  			    -Wclippy::as_ptr_cast_mut \
>  			    -Wclippy::as_underscore \
>  			    -Wclippy::cast_lossless \
> +			    -Aclippy::collapsible_match \
>  			    -Wclippy::ignored_unit_patterns \
>  			    -Aclippy::incompatible_msrv \
>  			    -Wclippy::mut_mut \
>
> base-commit: 897d54018cc9aa97fd1529ca08a53b429d05a566