Previously there wasn't any documentation to show what the type of
`Item` was inside `std::env::SplitPaths`. Now, in the same format as
other examples of docs in `srd` for `Iterator#Item`, we mention the
type.

This fixes rust-lang#59543.

It looks like the `OutputType::Metadata` kind in the compiler was
misclassified in rust-lang#38571 long ago by accident as incompatible with
codegen units and a single output file. This means that if you emit both
a linkable artifact and metadata it silently turns off multiple codegen
units unintentionally!

This commit corrects the situation to ensure that if `--emit metadata`
is used it doesn't implicitly disable multiple codegen units. This will
ensure we don't accidentally regress compiler performance when striving
to implement pipelined compilation!

Document `Item` type in `std::env::SplitPaths` iterator.

Previously there wasn't any documentation to show what the type of
`Item` was inside `std::env::SplitPaths`. Now, in the same format as
other examples of docs in `std` for `Iterator#Item`, we mention the
type.

This fixes rust-lang#59543.

r? @steveklabnik

…oli-obk

rustc: Flag metadata compatible with multiple CGUs

It looks like the `OutputType::Metadata` kind in the compiler was
misclassified in rust-lang#38571 long ago by accident as incompatible with
codegen units and a single output file. This means that if you emit both
a linkable artifact and metadata it silently turns off multiple codegen
units unintentionally!

This commit corrects the situation to ensure that if `--emit metadata`
is used it doesn't implicitly disable multiple codegen units. This will
ensure we don't accidentally regress compiler performance when striving
to implement pipelined compilation!

…i-obk

Document ast::ExprKind::Type

r? @oli-obk

…sper

Remove `-Z two-phase-borrows` and `-Z two-phase-beyond-autoref`

fixes rust-lang#60331