Implement RFC 3624 supertrait_item_shadowing (v2)

[compiler-errors]

Implements RFC 3624 and the associated lint in the RFC.

Tracking:

• Tracking Issue for RFC: Supertrait Item Shadowing #89151

cc @Amanieu and rust-lang/rfcs#3624 and #89151

[bors]

☔ The latest upstream changes (presumably #125443) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

OK, this is ready pending RFC approval.

[compiler-errors]

The RFC is now in FCP, so this could probably get reviewed preliminarily but should probably not land until the RFC is merged.

r? compiler

[bors]

☔ The latest upstream changes (presumably #124895) made this pull request unmergeable. Please resolve the merge conflicts.

[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 26.9s done
#16 DONE 39.3s
##[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
---
diff of stderr:

20   --> $DIR/false-subtrait-after-inference.rs:19:7
21    |
22 LL |     x.hello();
-    |       ^^^^^ the trait `Foo` is not implemented for `i32`, which is required by `W<_>: Downstream`
24    |
25 help: this trait has no implementations, consider adding one
26   --> $DIR/false-subtrait-after-inference.rs:15:1



The actual stderr differed from the expected stderr.
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args methods/supertrait-shadowing/false-subtrait-after-inference.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/methods/supertrait-shadowing/false-subtrait-after-inference.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "--target=x86_64-unknown-linux-gnu" "--check-cfg" "cfg(FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/methods/supertrait-shadowing/false-subtrait-after-inference" "-A" "unused" "-A" "internal_features" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
--- stderr -------------------------------
--- stderr -------------------------------
warning: trait method `hello` from `Downstream` shadows identically named method from supertrait
##[warning]  --> /checkout/tests/ui/methods/supertrait-shadowing/false-subtrait-after-inference.rs:19:7
   |
LL |     x.hello();
   |
note: method from `Downstream` shadows a supertrait method
  --> /checkout/tests/ui/methods/supertrait-shadowing/false-subtrait-after-inference.rs:11:5
   |
---

error[E0277]: the trait bound `i32: Foo` is not satisfied
##[error]  --> /checkout/tests/ui/methods/supertrait-shadowing/false-subtrait-after-inference.rs:19:7
   |
LL |     x.hello();
   |
help: this trait has no implementations, consider adding one
  --> /checkout/tests/ui/methods/supertrait-shadowing/false-subtrait-after-inference.rs:15:1
   |
   |
LL | trait Foo {}
   | ^^^^^^^^^
note: required for `W<i32>` to implement `Downstream`
  --> /checkout/tests/ui/methods/supertrait-shadowing/false-subtrait-after-inference.rs:13:9
   |
LL | impl<T> Downstream for W<T> where T: Foo {}
   |         ^^^^^^^^^^     ^^^^          --- unsatisfied trait bound introduced here
error: aborting due to 1 previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0277`.
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