They used to be covered by `optin_builtin_traits` but negative impls
are now applicable to all traits, not just auto traits.
This also adds docs in the unstable book for the current state of auto traits.
Expansion-driven outline module parsing
After this PR, the parser will not do any conditional compilation or loading of external module files when `mod foo;` is encountered. Instead, the parser only leaves `mod foo;` in place in the AST, with no items filled in. Expansion later kicks in and will load the actual files and do the parsing. This entails that the following is now valid:
```rust
#[cfg(FALSE)]
mod foo {
mod bar {
mod baz; // `foo/bar/baz.rs` doesn't exist, but no error!
}
}
```
Fixes https://github.com/rust-lang/rust/issues/64197.
r? @petrochenkov
Previously, attributes on 'if' expressions (e.g. #[attr] if true {})
were disallowed during parsing. This made it impossible for macros to
perform any custom handling of such attributes (e.g. stripping them
away), since a compilation error would be emitted before they ever had a
chance to run.
This PR permits attributes on 'if' expressions ('if-attrs' from here on).
Both built-in attributes (e.g. `#[allow]`, `#[cfg]`) are supported.
We still do *not* accept attributes on 'other parts' of an if-else
chain. That is, the following code snippet still fails to parse:
```rust
if true {} #[attr] else if false {} else #[attr] if false {} #[attr]
else {}
```
There is no longer a need to append the string `", ..."` to a functions
args as `...` is parsed as an argument and will appear in the functions
arguments.
Joshua Netterfield reported an ICE when the unused-parentheses lint
triggered around an async block (#54752). In order to compose an
autofixable suggestion, the lint invokes the pretty-printer on the
unnecessarily-parenthesized expression. (One wonders why the lint
doesn't just use `SourceMap::span_to_snippet` instead, to preserve the
formatting of the original source?—but for that, you'd have to ask the
author of 5c9f806d.)
But then the pretty-printer panics when trying to call `<pprust::State
as PrintState>::end` when `State.boxes` is empty. Empirically, the
problem would seem to be solved if we start some "boxes" beforehand in
the `ast::ExprKind::Async` arm of the big match in
`print_expr_outer_attr_style`, exactly like we do in the
immediately-preceding match arm for `ast::ExprKind::Block`—it would
seem pretty ("pretty") reasonable for the pretty-printing of async
blocks to work a lot like the pretty-printing of ordinary non-async
blocks, right??
Of course, it would be shamefully cargo-culty to commit code on the
basis of this kind of mere reasoning-by-analogy (in contrast to
understanding the design of the pretty-printer in such detail that the
correctness of the patch is comprehended with all the lucid certainty
of mathematical proof, rather than being merely surmised by
intuition). But maybe we care more about fixing the bug with high
probability today, than with certainty in some indefinite hypothetical
future? Maybe the effort is worth a fifth of a shirt??
Humbly resolves#54752.
