Add better explanation for `rustc_index::IndexVec`

I feel like I didn't do a great job explaining what this does in rust-lang#119800, so this PR tries to give an example of why and how you would use it.

Addresses rust-lang#93792.

…iaskrgr

Rollup of 9 pull requests

Successful merges:

 - rust-lang#113518 (bootstrap/libtest: print test name eagerly on failure even with `verbose-tests=false` / `--quiet`)
 - rust-lang#119888 (Stabilize the `#[diagnostic]` namespace and `#[diagnostic::on_unimplemented]` attribute)
 - rust-lang#121752 (Detect unused struct impls pub trait)
 - rust-lang#121885 (Move generic `NonZero` `rustc_layout_scalar_valid_range_start` attribute to inner type.)
 - rust-lang#121926 (`f16` and `f128` step 3: compiler support & feature gate)
 - rust-lang#121959 (Removing absolute path in proc-macro)
 - rust-lang#122015 (Add better explanation for `rustc_index::IndexVec`)
 - rust-lang#122027 (Uplift some feeding out of `associated_type_for_impl_trait_in_impl` and into queries)
 - rust-lang#122038 (Fix linting paths with qself in `unused_qualifications`)

r? `@ghost`
`@rustbot` modify labels: rollup

Add better explanation for `rustc_index::IndexVec`

I feel like I didn't do a great job explaining what this does in rust-lang#119800, so this PR tries to give an example of why and how you would use it.

Addresses rust-lang#93792.

Add better explanation for `rustc_index::IndexVec`

I feel like I didn't do a great job explaining what this does in rust-lang#119800, so this PR tries to give an example of why and how you would use it.

Addresses rust-lang#93792.

…iaskrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#113518 (bootstrap/libtest: print test name eagerly on failure even with `verbose-tests=false` / `--quiet`)
 - rust-lang#119888 (Stabilize the `#[diagnostic]` namespace and `#[diagnostic::on_unimplemented]` attribute)
 - rust-lang#121089 (Remove `feed_local_def_id`)
 - rust-lang#121926 (`f16` and `f128` step 3: compiler support & feature gate)
 - rust-lang#121959 (Removing absolute path in proc-macro)
 - rust-lang#122015 (Add better explanation for `rustc_index::IndexVec`)
 - rust-lang#122027 (Uplift some feeding out of `associated_type_for_impl_trait_in_impl` and into queries)
 - rust-lang#122038 (Fix linting paths with qself in `unused_qualifications`)

r? `@ghost`
`@rustbot` modify labels: rollup

…iaskrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#122015 (Add better explanation for `rustc_index::IndexVec`)
 - rust-lang#122061 (Clarify FatalErrorHandler)
 - rust-lang#122062 (Explicitly assign constructed C++ classes)
 - rust-lang#122072 (Refer to "slice" instead of "vector" in Ord and PartialOrd trait impl of slices)
 - rust-lang#122088 (Remove unnecessary fixme on new thread stack size)
 - rust-lang#122094 (Remove outdated footnote "missing-stack-probe" in platform-support)
 - rust-lang#122107 (Temporarily make allow-by-default the `non_local_definitions` lint)
 - rust-lang#122109 (compiletest: Add a `//@ needs-threads` directive)

Failed merges:

 - rust-lang#122104 (Rust is a proper name: rust → Rust)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of rust-lang#122015 - dev-ardi:master, r=nnethercote

Add better explanation for `rustc_index::IndexVec`

I feel like I didn't do a great job explaining what this does in rust-lang#119800, so this PR tries to give an example of why and how you would use it.

Addresses rust-lang#93792.