Various style improvements to `rustc_lint::levels`

While reading this file, I noticed a few opportunities to make things a little nicer:

- Replace some nested if-let with let-chains
- Tweak a match pattern to allow shorthand struct syntax
- Fuse an `is_empty` check with getting the last element
- Merge some common code that emits `MalformedAttribute` and continues
- Format `"{tool}::{name}"` in a way that's consistent with other match arms
- Replace if-let-else-panic with let-else
- Use early-exit to flatten a method body

Some of these changes cause indentation churn, so ignoring whitespace is recommended.

…iaskrgr

Rollup of 12 pull requests

Successful merges:

 - rust-lang#104353 (Add CStr::bytes iterator)
 - rust-lang#120699 (Document `TRACK_DIAGNOSTIC` calls.)
 - rust-lang#121207 (Add `-Z external-clangrt`)
 - rust-lang#122397 (Various cleanups around the const eval query providers)
 - rust-lang#122416 (Various style improvements to `rustc_lint::levels`)
 - rust-lang#122422 (compiletest: Allow `only-unix` in test headers)
 - rust-lang#122424 (fix: typos)
 - rust-lang#122425 (Increase timeout for new bors bot)
 - rust-lang#122426 (Fix StableMIR `WrappingRange::is_full` computation)
 - rust-lang#122430 (Generate link to `Local` in `hir::Let` documentation)
 - rust-lang#122434 (pattern analysis: rename a few types)
 - rust-lang#122437 (pattern analysis: remove `MaybeInfiniteInt::JustAfterMax`)

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

Various style improvements to `rustc_lint::levels`

While reading this file, I noticed a few opportunities to make things a little nicer:

- Replace some nested if-let with let-chains
- Tweak a match pattern to allow shorthand struct syntax
- Fuse an `is_empty` check with getting the last element
- Merge some common code that emits `MalformedAttribute` and continues
- Format `"{tool}::{name}"` in a way that's consistent with other match arms
- Replace if-let-else-panic with let-else
- Use early-exit to flatten a method body

Some of these changes cause indentation churn, so ignoring whitespace is recommended.

Various style improvements to `rustc_lint::levels`

While reading this file, I noticed a few opportunities to make things a little nicer:

- Replace some nested if-let with let-chains
- Tweak a match pattern to allow shorthand struct syntax
- Fuse an `is_empty` check with getting the last element
- Merge some common code that emits `MalformedAttribute` and continues
- Format `"{tool}::{name}"` in a way that's consistent with other match arms
- Replace if-let-else-panic with let-else
- Use early-exit to flatten a method body

Some of these changes cause indentation churn, so ignoring whitespace is recommended.

…iaskrgr

Rollup of 9 pull requests

Successful merges:

 - rust-lang#104353 (Add CStr::bytes iterator)
 - rust-lang#120699 (Document `TRACK_DIAGNOSTIC` calls.)
 - rust-lang#120943 (Create some minimal HIR for associated opaque types)
 - rust-lang#121764 (Make incremental sessions identity no longer depend on the crate names provided by source code)
 - rust-lang#122375 (CFI: Break tests into smaller files)
 - rust-lang#122397 (Various cleanups around the const eval query providers)
 - rust-lang#122405 (Add methods to create StableMIR constant)
 - rust-lang#122416 (Various style improvements to `rustc_lint::levels`)
 - rust-lang#122440 (const-eval: organize and extend tests for required-consts)

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

Various style improvements to `rustc_lint::levels`

While reading this file, I noticed a few opportunities to make things a little nicer:

- Replace some nested if-let with let-chains
- Tweak a match pattern to allow shorthand struct syntax
- Fuse an `is_empty` check with getting the last element
- Merge some common code that emits `MalformedAttribute` and continues
- Format `"{tool}::{name}"` in a way that's consistent with other match arms
- Replace if-let-else-panic with let-else
- Use early-exit to flatten a method body

Some of these changes cause indentation churn, so ignoring whitespace is recommended.

…iaskrgr

Rollup of 11 pull requests

Successful merges:

 - rust-lang#104353 (Add CStr::bytes iterator)
 - rust-lang#114038 (unix time module now return result)
 - rust-lang#119676 (rustdoc-search: search types by higher-order functions)
 - rust-lang#120699 (Document `TRACK_DIAGNOSTIC` calls.)
 - rust-lang#121899 (Document how removing a type's field can be bad and what to do instead)
 - rust-lang#121940 (Mention Register Size in `#[warn(asm_sub_register)]`)
 - rust-lang#122397 (Various cleanups around the const eval query providers)
 - rust-lang#122405 (Add methods to create StableMIR constant)
 - rust-lang#122416 (Various style improvements to `rustc_lint::levels`)
 - rust-lang#122440 (const-eval: organize and extend tests for required-consts)
 - rust-lang#122461 (fix unsoundness in Step::forward_unchecked for signed integers)

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

Various style improvements to `rustc_lint::levels`

While reading this file, I noticed a few opportunities to make things a little nicer:

- Replace some nested if-let with let-chains
- Tweak a match pattern to allow shorthand struct syntax
- Fuse an `is_empty` check with getting the last element
- Merge some common code that emits `MalformedAttribute` and continues
- Format `"{tool}::{name}"` in a way that's consistent with other match arms
- Replace if-let-else-panic with let-else
- Use early-exit to flatten a method body

Some of these changes cause indentation churn, so ignoring whitespace is recommended.

…iaskrgr

Rollup of 9 pull requests

Successful merges:

 - rust-lang#104353 (Add CStr::bytes iterator)
 - rust-lang#119676 (rustdoc-search: search types by higher-order functions)
 - rust-lang#120699 (Document `TRACK_DIAGNOSTIC` calls.)
 - rust-lang#121899 (Document how removing a type's field can be bad and what to do instead)
 - rust-lang#122405 (Add methods to create StableMIR constant)
 - rust-lang#122416 (Various style improvements to `rustc_lint::levels`)
 - rust-lang#122421 (Improve `Step` docs)
 - rust-lang#122440 (const-eval: organize and extend tests for required-consts)
 - rust-lang#122461 (fix unsoundness in Step::forward_unchecked for signed integers)

Failed merges:

 - rust-lang#122397 (Various cleanups around the const eval query providers)

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

Rollup merge of rust-lang#122416 - Zalathar:levels, r=petrochenkov

Various style improvements to `rustc_lint::levels`

While reading this file, I noticed a few opportunities to make things a little nicer:

- Replace some nested if-let with let-chains
- Tweak a match pattern to allow shorthand struct syntax
- Fuse an `is_empty` check with getting the last element
- Merge some common code that emits `MalformedAttribute` and continues
- Format `"{tool}::{name}"` in a way that's consistent with other match arms
- Replace if-let-else-panic with let-else
- Use early-exit to flatten a method body

Some of these changes cause indentation churn, so ignoring whitespace is recommended.