Remove ?Sized bound from Idx in Index and IndexMut

[benw]

Idx: ?Sized is useless because the trait methods pass the index by value, so it's impossible to impl with an unsized Idx type.

Removing this makes it a little easier for learners to understand the intent of the Index and IndexMut traits.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @jhpratt (or someone else) some time within the next two weeks.

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• Owners of files modified in this PR: libs
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[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

- error: aborting due to 2 previous errors
+ error[E0277]: the size for values of type `str` cannot be known at compilation time
+   --> $DIR/issue-113808-invalid-unused-qualifications-suggestion.rs:16:5
+    |
+ LL |     fn index(&self, _: str) -> &Self::Output {
+    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+    |
+    = help: the trait `Sized` is not implemented for `str`
+ note: required by an implicit `Sized` bound in `Index`
+   --> $SRC_DIR/core/src/ops/index.rs:LL:COL
---
+ 
+ error[E0277]: the size for values of type `str` cannot be known at compilation time
+   --> $DIR/issue-113808-invalid-unused-qualifications-suggestion.rs:26:5
+    |
+ LL |     fn index(&self, _: str) -> &Self::Output {
+    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+    |
+    = help: the trait `Sized` is not implemented for `str`
+ note: required by an implicit `Sized` bound in `Index`
+   --> $SRC_DIR/core/src/ops/index.rs:LL:COL
---
-   --> /rustc/FAKE_PREFIX/library/core/src/ops/index.rs:59:0
+ error[E0277]: the size for values of type `str` cannot be known at compilation time
+   --> $DIR/issue-113808-invalid-unused-qualifications-suggestion.rs:16:5
+    |
+ LL |     fn index(&self, _: str) -> &Self::Output {
+    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+    |
+    = help: the trait `Sized` is not implemented for `str`
+ note: required by an implicit `Sized` bound in `Index`
+   --> $SRC_DIR/core/src/ops/index.rs:LL:COL
---
+ 
+ error[E0277]: the size for values of type `str` cannot be known at compilation time
+   --> $DIR/issue-113808-invalid-unused-qualifications-suggestion.rs:26:5
+    |
+ LL |     fn index(&self, _: str) -> &Self::Output {
+    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+    |
+    = help: the trait `Sized` is not implemented for `str`
+ note: required by an implicit `Sized` bound in `Index`
+   --> $SRC_DIR/core/src/ops/index.rs:LL:COL
---
To only update this specific test, also pass `--test-args resolve/issue-113808-invalid-unused-qualifications-suggestion.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/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/resolve/issue-113808-invalid-unused-qualifications-suggestion.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/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,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/aarch64-unknown-linux-gnu/test/ui/resolve/issue-113808-invalid-unused-qualifications-suggestion" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error: unnecessary qualification
##[error]  --> /checkout/tests/ui/resolve/issue-113808-invalid-unused-qualifications-suggestion.rs:13:6
   |
---
---- [ui] tests/ui/unsized-locals/unsized-index.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/unsized-locals/unsized-index.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/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/unsized-locals/unsized-index/a" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0277]: the size for values of type `str` cannot be known at compilation time
##[error]  --> /checkout/tests/ui/unsized-locals/unsized-index.rs:12:5
   |
---

error[E0277]: the size for values of type `str` cannot be known at compilation time
##[error]  --> /checkout/tests/ui/unsized-locals/unsized-index.rs:17:6
   |
LL | impl ops::IndexMut<str> for A {
   |      ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `str`
note: required by an implicit `Sized` bound in `IndexMut`
  --> /rustc/FAKE_PREFIX/library/core/src/ops/index.rs:170:0

error[E0277]: the size for values of type `str` cannot be known at compilation time
##[error]  --> /checkout/tests/ui/unsized-locals/unsized-index.rs:26:29
   |
LL |     assert_eq!(&(), a.index(*s));
   |                       ----- ^^ doesn't have a size known at compile-time
   |                       |
   |                       required by a bound introduced by this call
   |
   = help: the trait `Sized` is not implemented for `str`
note: required by a bound in `std::ops::Index::index`

[jhpratt]

See #48055 for supporting unsized function parameters. While there is RFC 3829 that may remove that (it's not clear whether it's only locals or also arguments), that RFC is not yet accepted. Because unsized locals are still formally accepted, I'll be closing this. With that said, don't be discouraged! You're absolutely right that on stable the relaxed bound is unnecessary.