Add REDUNDANT_IMPORTS lint for new redundant import detection

[compiler-errors]

Defaults to Allow for now. Stacked on #123744 to avoid merge conflict, but much easier to review all as one.

r? petrochenkov

[compiler-errors]

nominating for T-lang to see if we'd like to keep this as allow or make it warn, etc., and perhaps to follow-up on the policy.

for the record, I think this lint is perhaps noisy but also very useful, so I'd like to see it at Warn level eventually. just not curious what the best rollout strategy is for that.

[rust-log-analyzer]

The job x86_64-gnu-llvm-17 failed! Check out the build log: (web) (plain)

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#16 exporting to docker image format
#16 sending tarball 29.7s done
#16 DONE 33.0s
##[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-17]
---
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-17', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--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', '--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-17/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999
---
  Downloaded boml v0.3.1
   Compiling boml v0.3.1
   Compiling y v0.1.0 (/checkout/compiler/rustc_codegen_gcc/build_system)
    Finished `release` profile [optimized] target(s) in 3.50s
     Running `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-codegen/x86_64-unknown-linux-gnu/release/y test --use-system-gcc --use-backend gcc --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/cg_gcc --release --mini-tests --std-tests`
Using system GCC
[BUILD] example
[AOT] mini_core_hello_world
/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/cg_gcc/mini_core_hello_world
abc
---
 finished in 1.465 seconds
##[endgroup]
Generating lint docs (x86_64-unknown-linux-gnu)
##[group]Running stage2 lint-docs (x86_64-unknown-linux-gnu)
error: failed to test example in lint docs for `redundant_imports` in /checkout/compiler/rustc_lint_defs/src/builtin.rs:423: lint docs should start with the text "The `redundant_imports` lint" to introduce the lint
This error was generated by the lint-docs tool.
This tool extracts documentation for lints from the source code and places
This tool extracts documentation for lints from the source code and places
them in the rustc book. See the declare_lint! documentation
https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint_defs/macro.declare_lint.html


To re-run these tests, run: ./x.py test --keep-stage=0 src/tools/lint-docs
The --keep-stage flag should be used if you have already built the compiler

Build completed unsuccessfully in 0:34:26
  local time: Thu Apr 11 19:16:43 UTC 2024
  network time: Thu, 11 Apr 2024 19:16:44 GMT

[kadiwa4]

*redundant_imports

[traviscross]

@rustbot labels -I-lang-nominated

We discussed this in the lang meeting today. We came to the consensus that we want to split out into a separate allow-by-default lint (or, alternatively, not lint at all) the case of an import that is redundant due to a glob import.

There are two reasons for this.

One, and most importantly, is that if we were to not do this, a warn-by-default lint here would make adding functions, statics, or consts to a prelude (including the one in the standard library) so disruptive as to have to be treated almost like a breaking change.

Two is that not doing this would make it annoying to migrate away from a glob import incrementally.

Once this case is separated out, we're OK with the remainder of this lint going directly to warn-by-default.