Implement From<&NonZero<T>> for &T

[Alphare]

This is my first contribution because I basically stumbled upon this not being implemented and figured it would be a quick fix, sorry if I'm missing anything. I see that all other impls in this file have feature annotations, I'm not sure if this would need one.

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[rust-log-analyzer]

The job x86_64-gnu-llvm-18 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

#16 exporting to docker image format
#16 sending tarball 29.0s done
#16 DONE 31.7s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-18]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-18', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-18/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---
   Compiling compiler_builtins v0.1.138
error: implementation has missing stability attribute
   --> core/src/num/nonzero.rs:290:1
    |
290 | / impl<T> From<&NonZero<T>> for &T
291 | | where
292 | |     T: ZeroablePrimitive,
293 | | {
299 | |     }
300 | | }
    | |_^

[jhpratt]

Trait implementations require a stability attribute, which is why CI is failing. Unfortunately, they also cannot be unstable. For that reason, I'll have the team take a look at it as it will require FCP.

@rustbot label +I-libs-api-nominated

[joshtriplett]

Seems reasonable to me.

I wonder if (in a separate PR) we should also add impl TryFrom<&T> for &NonZero<T>.

[Amanieu]

We discussed this in the libs-api meeting: the team feels that it would be better to provide this using the AsRef trait instead of From.

[jhpratt]

Closing for that reason.