Various notes on match lowering

This is an assortment of comments for things that I found unclear or confusing when I was learning how match lowering works.

This PR only adds/modifies comments, so there are no functional changes.

I have tried to avoid touching code that would conflict with rust-lang#127159.

r? `@Nadrieril`

Rollup merge of rust-lang#128085 - Zalathar:notes, r=Nadrieril

Various notes on match lowering

This is an assortment of comments for things that I found unclear or confusing when I was learning how match lowering works.

This PR only adds/modifies comments, so there are no functional changes.

I have tried to avoid touching code that would conflict with rust-lang#127159.

r? `@Nadrieril`

…wjasper

match lowering: Hide `Candidate` from outside the lowering algorithm

The internals of `Candidate` are tricky and a source of confusion. This PR makes it so we don't expose `Candidate`s outside the lowering algorithm. Now:
- false edges are handled in `lower_match_tree`;
- `lower_match_tree` takes a list of patterns as input;
- `lower_match_tree` returns a flat datastructure that contains only the necessary information.

r? `@matthewjasper`

…iaskrgr

Rollup of 7 pull requests

Successful merges:

 - rust-lang#123813 (Add `REDUNDANT_IMPORTS` lint for new redundant import detection)
 - rust-lang#127159 (match lowering: Hide `Candidate` from outside the lowering algorithm)
 - rust-lang#128162 (Cleanup sys module to match house style)
 - rust-lang#128296 (Update target-spec metadata for loongarch64 targets)
 - rust-lang#128417 (Add `f16` and `f128` math functions)
 - rust-lang#128431 (Add myself as VxWorks target maintainer for reference)
 - rust-lang#128437 (improve bootstrap to allow selecting llvm tools individually)

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

…wjasper

match lowering: Hide `Candidate` from outside the lowering algorithm

The internals of `Candidate` are tricky and a source of confusion. This PR makes it so we don't expose `Candidate`s outside the lowering algorithm. Now:
- false edges are handled in `lower_match_tree`;
- `lower_match_tree` takes a list of patterns as input;
- `lower_match_tree` returns a flat datastructure that contains only the necessary information.

r? ``@matthewjasper``

Rollup of 7 pull requests

Successful merges:

 - rust-lang#117468 (Stabilize Wasm relaxed SIMD)
 - rust-lang#123813 (Add `REDUNDANT_IMPORTS` lint for new redundant import detection)
 - rust-lang#127060 (Migrate `symbol-visibility` `run-make` test to rmake)
 - rust-lang#127159 (match lowering: Hide `Candidate` from outside the lowering algorithm)
 - rust-lang#128296 (Update target-spec metadata for loongarch64 targets)
 - rust-lang#128416 (android: Remove libstd hacks for unsupported Android APIs)
 - rust-lang#128431 (Add myself as VxWorks target maintainer for reference)

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

…iaskrgr

Rollup of 7 pull requests

Successful merges:

 - rust-lang#123813 (Add `REDUNDANT_IMPORTS` lint for new redundant import detection)
 - rust-lang#126697 ([RFC] mbe: consider the `_` in 2024 an expression)
 - rust-lang#127159 (match lowering: Hide `Candidate` from outside the lowering algorithm)
 - rust-lang#128244 (Peel off explicit (or implicit) deref before suggesting clone on move error in borrowck, remove some hacks)
 - rust-lang#128431 (Add myself as VxWorks target maintainer for reference)
 - rust-lang#128438 (Add special-case for [T, 0] in dropck_outlives)
 - rust-lang#128457 (Fix docs for OnceLock::get_mut_or_init)

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

…iaskrgr

Rollup of 7 pull requests

Successful merges:

 - rust-lang#123813 (Add `REDUNDANT_IMPORTS` lint for new redundant import detection)
 - rust-lang#126697 ([RFC] mbe: consider the `_` in 2024 an expression)
 - rust-lang#127159 (match lowering: Hide `Candidate` from outside the lowering algorithm)
 - rust-lang#128244 (Peel off explicit (or implicit) deref before suggesting clone on move error in borrowck, remove some hacks)
 - rust-lang#128431 (Add myself as VxWorks target maintainer for reference)
 - rust-lang#128438 (Add special-case for [T, 0] in dropck_outlives)
 - rust-lang#128457 (Fix docs for OnceLock::get_mut_or_init)

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

Rollup merge of rust-lang#127159 - Nadrieril:hide-candidate, r=matthewjasper

match lowering: Hide `Candidate` from outside the lowering algorithm

The internals of `Candidate` are tricky and a source of confusion. This PR makes it so we don't expose `Candidate`s outside the lowering algorithm. Now:
- false edges are handled in `lower_match_tree`;
- `lower_match_tree` takes a list of patterns as input;
- `lower_match_tree` returns a flat datastructure that contains only the necessary information.

r? ```@matthewjasper```