The phrasing is from the commit description of 395ee0b by @matklad.

…o tracking issue

…ods into one method `unescape_literal` with a mode argument.

…o one method `validate_literal_escape` with a mode argument.

This enables simplifying the `match` in `cook_lexer_literal()`
and it eliminates 90 lines of repetition :)

…nd tests.

The value passed in already has backticks surrounding the text.

…ochenkov

Incomplete features can also be unsound

Some incomplete features do not just ICE, they are also currently unsound (e.g. rust-lang#72029, and also `specialization` -- which is not yet marked incomplete but [should be](rust-lang#71420)). This makes the message reflect that.

While at it I also added a link to the tracking issue, which hopefully should explain what is incomplete/unsound about the feature.

…reporting_cleanup, r=petrochenkov

Literal error reporting cleanup

While doing some performance work, I noticed some code duplication in `librustc_parser/lexer/mod.rs`, so I cleaned it up.

This PR is probably best reviewed commit by commit.

I'm not sure what the API stability practices for `librustc_lexer` are. Four public methods in `unescape.rs` can be removed, but two are used by clippy, so I left them in for now.
I could open a PR for Rust-Analyzer when this one lands.

But how do I open a PR for clippy? (Git submodules are frustrating to work with)

move `ty::List` into a new submodule

`rustc_middle/ty` is currently right below the 3000 lines tidy file length limit.

Moves `rustc_middle::ty::List` to the private module `rustc_middle::ty::list` and adds
a `pub use self::list::List` at its previous location.

cmdline: Make target features individually overridable

Fixes rust-lang#56527

Previously `-C target-feature=+avx2 -C target-feature=+fma` was equivalent to `-C target-feature=+fma` because the later `-C target-feature` option fully overridden previous `-C target-feature`.
With this PR `-C target-feature=+avx2 -C target-feature=+fma` is equivalent to `-C target-feature=+avx2,+fma` and the options are combined.

I'm not sure where the comma-separated features in a single option came from (clang uses a scheme with single feature per-option), but logically these features are entirely independent options.
So they should be overridable individually as well to be more useful in hierarchical build system, and more consistent with other rustc options and clang behavior as well.

Target feature options have a few other issues (rust-lang#44815), but fixing those is going to be a bit more invasive.

Remove redundant backtick in error message.

The value passed in already has backticks surrounding the text.