Re: [PATCH v2 5/5] rust: enable `clippy::as_underscore` lint

[Benno Lossin <benno.lossin@proton.me>]

On Wed Mar 12, 2025 at 9:41 PM CET, Tamir Duberstein wrote:
> On Wed, Mar 12, 2025 at 4:07 PM Tamir Duberstein <tamird@gmail.com> wrote:
>>
>> On Wed, Mar 12, 2025 at 3:43 PM Benno Lossin <benno.lossin@proton.me> wrote:
>> >
>> > On Wed Mar 12, 2025 at 8:19 PM CET, Tamir Duberstein wrote:
>> > > I tried using the strict provenance lints locally and I think we can't
>> > > until we properly bump MSRV due to `clippy::incompatible_msrv`:
>> > >
>> > > warning: current MSRV (Minimum Supported Rust Version) is `1.78.0` but
>> > > this item is stable since `1.84.0`
>> > >    --> ../rust/kernel/str.rs:696:22
>> > >     |
>> > > 696 |             pos: pos.expose_provenance(),
>> > >     |                      ^^^^^^^^^^^^^^^^^^^
>> > >     |
>> > >     = help: for further information visit
>> > > https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
>> >
>> > Oh this is annoying...
>> >
>> > > This is with `#![feature(strict_provenance)]`. I can file the issue
>> > > but I think it's blocked on MSRV >= 1.84.0. But maybe you know of a
>> > > path forward :)
>> >
>> > I think we should be able to just `allow(clippy::incompatible_msrv)`,
>> > since Miguel & other maintainers will test with 1.78 (or at least are
>> > supposed to :).
>>
>> Alright, you've sniped me. This is coming in v3.
>
> I just realized I only covered the kernel crate. In order to cover all
> Rust code, I need to move the lints and the features out to the root
> Makefile. I tried something like this:
>
>> diff --git a/Makefile b/Makefile
>> index 2af40bfed9ce..10af1e44370b 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -466,13 +466,21 @@ KBUILD_USERHOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
>>  KBUILD_USERCFLAGS  := $(KBUILD_USERHOSTCFLAGS) $(USERCFLAGS)
>>  KBUILD_USERLDFLAGS := $(USERLDFLAGS)
>>
>> +# Lints were moved to `strict_provenance_lints` when `strict_provenance` was stabilized.
>> +#
>> +# See https://github.com/rust-lang/rust/commit/56ee492a6e7a917b2b3f888e33dd52a13d3ecb64.
>> +KBUILD_RUST_STRICT_PROVENANCE_FEATURE = $(if $(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE),strict_provenance_lints,strict_provenance)
>> +
>>  # These flags apply to all Rust code in the tree, including the kernel and
>>  # host programs.
>>  export rust_common_flags := --edition=2021 \
>>      -Zbinary_dep_depinfo=y \
>> +     -Zcrate-attr="feature($(KBUILD_RUST_STRICT_PROVENANCE_FEATURE))" \
>>      -Astable_features \
>>      -Dnon_ascii_idents \
>>      -Dunsafe_op_in_unsafe_fn \
>> +     -Wfuzzy_provenance_casts \
>> +     -Wlossy_provenance_casts \
>>      -Wmissing_docs \
>>      -Wrust_2018_idioms \
>>      -Wunreachable_pub \
>> diff --git a/rust/Makefile b/rust/Makefile
>> index ea3849eb78f6..d7d5be741245 100644
>> --- a/rust/Makefile
>> +++ b/rust/Makefile
>> @@ -435,8 +435,10 @@ $(obj)/helpers/helpers.o: $(src)/helpers/helpers.c $(recordmcount_source) FORCE
>>  # symbol versions generated from Rust objects.
>>  $(obj)/exports.o: private skip_gendwarfksyms = 1
>>
>> +KBUILD_RUST_STRICT_PROVENANCE_FEATURE := $(if $(CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE),strict_provenance_lints,strict_provenance)
>> +
>>  $(obj)/core.o: private skip_clippy = 1
>> -$(obj)/core.o: private skip_flags = -Wunreachable_pub
>> +$(obj)/core.o: private skip_flags = -Zcrate-attr="feature($(KBUILD_RUST_STRICT_PROVENANCE_FEATURE))" -Wunreachable_pub -Wfuzzy_provenance_casts -Wlossy_provenance_casts
>>  $(obj)/core.o: private rustc_objcopy = $(foreach sym,$(redirect-intrinsics),--redefine-sym $(sym)=__rust$(sym))
>>  $(obj)/core.o: private rustc_target_flags = $(core-cfgs)
>>  $(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs \
>
> but this doesn't work because
> `CONFIG_RUSTC_HAS_STABLE_STRICT_PROVENANCE` is not yet defined when I
> read it in the root Makefile. I can read it lower down and then append
> the feature flag to `KBUILD_RUSTFLAGS` but by then the rustdoc flags
> have been copied from `rust_common_flags` and so rustdoc doesn't get
> the feature flag, resulting in unknown lint warnings in rustdoc and
> kunit tests.
>
> Any ideas?

Always enable the features, we have `allow(stable_features)` for this
reason (then you don't have to do this dance with checking if it's
already stable or not :)

---
Cheers,
Benno