It makes it clearer that the symbol is unused and doesn't matter.

To use one `kw::Empty` instead of two. It's a little more direct this
way, and avoids `kw::Empty` being used for both "no string" and "empty
string".

This is part of the implementation of `#[doc(keyword = "match")]`
attributes used by `std` to provide documentation for keywords.

`is_doc_keyword` currently does a crude keyword range test that's
intended to catch all keywords but misses `kw::Yeet`. This commit
changes it to use `Symbol` methods, including the new `is_weak` method
(required for `union`). `Symbol` methods are much less prone to falling
out of date if new keywords are added.

By changing two of the fields to use `Option<Ident>` instead of `Ident`.
As a result, `None` now means "no identifier", which is much clearer
than using an empty identifier.

Fix `armv7-sony-vita-newlibeabihf` LLVM target triple

It was previously normalized by LLVM to `thumbv7a-vita-unknown-eabihf` (can be seen with `clang -target thumbv7a-vita-eabihf -v`), which seems wrong, as Vita is the OS name.

Motivation: To make it easier to verify that [`cc-rs`' conversion from `rustc` to Clang/LLVM triples](rust-lang/cc-rs#1431) is correct.

CC target maintainers ``@nikarh,`` ``@pheki`` and ``@ZetaNumbers.``
r? jieyouxu

…iser

rustc_resolve: Test the order that preludes are resolved

This test is exhaustive. See attached truth table:
![image](https://github.com/user-attachments/assets/11fe703c-e114-48df-84f8-426b63395784)

Companion PR to rust-lang/reference#1765.

…rochenkov

Reduce kw::Empty usage, part 4

Another step towards rust-lang#137978.

r? `@petrochenkov`

…zkan

tidy: properly check for orphaned unstable_book pages

This also recommends using underscores - something that took me a little bit too long to figure out.

Note: this PR deletes the page `src/doc/unstable-book/src/library-features/c-variadic.md`. The page `src/doc/unstable-book/src/lang-features/c-variadic.md` remains.

Remove fragile equal-pointers-unequal/*/print3.rs tests.

These tests were added in rust-lang#127003

The print3.rs tests stop working when I change implementation details of format_args!(). (For example, in rust-lang#139175 and rust-lang#139135). These tests shouldn't rely on such implementation details. It gets in the way for format_args!() improvements.

If they test anything that aren't already covered by the other tests in this directory, they should be expressed in a less fragile way that doesn't rely on internal details of format_args!().

cc ``@GrigorenkoPV,`` author of these tests.

Remove me from vacation

Fix invalid link in docs