compiler: allow transmute of ZST arrays with generics

Extend the `SizeSkeleton` evaluator to shortcut zero-sized arrays, thus considering `[T; 0]` to have a compile-time fixed-size of 0.

The existing evaluator already deals with generic arrays under the feature-guard `transmute_const_generics`. However, it merely allows comparing fixed-size types with fixed-size types, and generic types with generic types. For generic types, it merely compares whether their arguments match (ordering them first). Even if their exact sizes are not known at compile time, it can ensure that they will eventually be the same.

This patch extends this by shortcutting the size-evaluation of zero sized arrays and thus allowing size comparisons of `()` with `[T; 0]`, where one contains generics and the other does not.

This code is guarded by `transmute_const_generics` (rust-lang#109929), even though it is unclear whether it should be. However, this assumes that a separate stabilization PR is required to move this out of the feature guard.

Initially reported in rust-lang#98104.

…kingjubilee

Rollup of 13 pull requests

Successful merges:

 - rust-lang#113525 (Dynamically size sigaltstk in std)
 - rust-lang#114009 (compiler: allow transmute of ZST arrays with generics)
 - rust-lang#116793 (Allow targets to override default codegen backend)
 - rust-lang#118623 (Improve std::fs::read_to_string example)
 - rust-lang#120504 (Vec::try_with_capacity)
 - rust-lang#121089 (Remove `feed_local_def_id`)
 - rust-lang#121280 (Implement MaybeUninit::fill{,_with,_from})
 - rust-lang#122087 (Add missing background color for top-level rust documentation page and increase contrast by setting text color to black)
 - rust-lang#122104 (Rust is a proper name: rust → Rust)
 - rust-lang#122110 (Make `x t miri` respect `MIRI_TEMP`)
 - rust-lang#122114 (Make not finding core a fatal error)
 - rust-lang#122115 (Cancel parsing ever made during recovery)
 - rust-lang#122126 (Fix `tidy --bless` on  ̶X̶e̶n̶i̶x̶ Windows)

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

…iaskrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#114009 (compiler: allow transmute of ZST arrays with generics)
 - rust-lang#122195 (Note that the caller chooses a type for type param)
 - rust-lang#122651 (Suggest `_` for missing generic arguments in turbofish)
 - rust-lang#122784 (Add `tag_for_variant` query)
 - rust-lang#122839 (Split out `PredicatePolarity` from `ImplPolarity`)
 - rust-lang#122873 (Merge my contributor emails into one using mailmap)
 - rust-lang#122885 (Adjust better spastorino membership to triagebot's adhoc_groups)
 - rust-lang#122888 (add a couple more tests)

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

…iaskrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#114009 (compiler: allow transmute of ZST arrays with generics)
 - rust-lang#122195 (Note that the caller chooses a type for type param)
 - rust-lang#122651 (Suggest `_` for missing generic arguments in turbofish)
 - rust-lang#122784 (Add `tag_for_variant` query)
 - rust-lang#122839 (Split out `PredicatePolarity` from `ImplPolarity`)
 - rust-lang#122873 (Merge my contributor emails into one using mailmap)
 - rust-lang#122885 (Adjust better spastorino membership to triagebot's adhoc_groups)
 - rust-lang#122888 (add a couple more tests)

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

Rollup merge of rust-lang#114009 - dvdhrm:pr/transmzst, r=pnkfelix

compiler: allow transmute of ZST arrays with generics

Extend the `SizeSkeleton` evaluator to shortcut zero-sized arrays, thus considering `[T; 0]` to have a compile-time fixed-size of 0.

The existing evaluator already deals with generic arrays under the feature-guard `transmute_const_generics`. However, it merely allows comparing fixed-size types with fixed-size types, and generic types with generic types. For generic types, it merely compares whether their arguments match (ordering them first). Even if their exact sizes are not known at compile time, it can ensure that they will eventually be the same.

This patch extends this by shortcutting the size-evaluation of zero sized arrays and thus allowing size comparisons of `()` with `[T; 0]`, where one contains generics and the other does not.

This code is guarded by `transmute_const_generics` (rust-lang#109929), even though it is unclear whether it should be. However, this assumes that a separate stabilization PR is required to move this out of the feature guard.

Initially reported in rust-lang#98104.

…iaskrgr

Rollup of 8 pull requests

Successful merges:

 - rust-lang#114009 (compiler: allow transmute of ZST arrays with generics)
 - rust-lang#122195 (Note that the caller chooses a type for type param)
 - rust-lang#122651 (Suggest `_` for missing generic arguments in turbofish)
 - rust-lang#122784 (Add `tag_for_variant` query)
 - rust-lang#122839 (Split out `PredicatePolarity` from `ImplPolarity`)
 - rust-lang#122873 (Merge my contributor emails into one using mailmap)
 - rust-lang#122885 (Adjust better spastorino membership to triagebot's adhoc_groups)
 - rust-lang#122888 (add a couple more tests)

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