bjoernager/rust
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Add {ast,hir,thir}::PatKind::Missing variants.

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"Missing" patterns are possible in bare fn types (`fn f(u32)`) and
similar places. Currently these are represented in the AST with
`ast::PatKind::Ident` with no `by_ref`, no `mut`, an empty ident, and no
sub-pattern. This flows through to `{hir,thir}::PatKind::Binding` for
HIR and THIR.

This is a bit nasty. It's very non-obvious, and easy to forget to check
for the exceptional empty identifier case.

This commit adds a new variant, `PatKind::Missing`, to do it properly.

The process I followed:
- Add a `Missing` variant to `{ast,hir,thir}::PatKind`.
- Chang `parse_param_general` to produce `ast::PatKind::Missing`
  instead of `ast::PatKind::Missing`.
- Look through `kw::Empty` occurrences to find functions where an
  existing empty ident check needs replacing with a `PatKind::Missing`
  check: `print_param`, `check_trait_item`, `is_named_param`.
- Add a `PatKind::Missing => unreachable!(),` arm to every exhaustive
  match identified by the compiler.
- Find which arms are actually reachable by running the test suite,
  changing them to something appropriate, usually by looking at what
  would happen to a `PatKind::Ident`/`PatKind::Binding` with no ref, no
  `mut`, an empty ident, and no subpattern.

Quite a few of the `unreachable!()` arms were never reached. This makes
sense because `PatKind::Missing` can't happen in every pattern, only
in places like bare fn tys and trait fn decls.

I also tried an alternative approach: modifying `ast::Param::pat` to
hold an `Option<P<Pat>>` instead of a `P<Pat>`, but that quickly turned
into a very large and painful change. Adding `PatKind::Missing` is much
easier.
This commit is contained in:
Nicholas Nethercote 2025-03-27 09:33:02 +11:00
parent 217693a1f0
commit 9f089e080c
40 changed files with 86 additions and 46 deletions
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26
compiler/rustc_lint/src/builtin.rs
View file

@ -778,21 +778,19 @@ impl EarlyLintPass for AnonymousParameters {
}
if let ast::AssocItemKind::Fn(box Fn { ref sig, .. }) = it.kind {
for arg in sig.decl.inputs.iter() {
if let ast::PatKind::Ident(_, ident, None) = arg.pat.kind {
if ident.name == kw::Empty {
let ty_snip = cx.sess().source_map().span_to_snippet(arg.ty.span);
if let ast::PatKind::Missing = arg.pat.kind {
let ty_snip = cx.sess().source_map().span_to_snippet(arg.ty.span);
let (ty_snip, appl) = if let Ok(ref snip) = ty_snip {
(snip.as_str(), Applicability::MachineApplicable)
} else {
("<type>", Applicability::HasPlaceholders)
};
cx.emit_span_lint(
ANONYMOUS_PARAMETERS,
arg.pat.span,
BuiltinAnonymousParams { suggestion: (arg.pat.span, appl), ty_snip },
);
}
let (ty_snip, appl) = if let Ok(ref snip) = ty_snip {
(snip.as_str(), Applicability::MachineApplicable)
} else {
("<type>", Applicability::HasPlaceholders)
};
cx.emit_span_lint(
ANONYMOUS_PARAMETERS,
arg.pat.span,
BuiltinAnonymousParams { suggestion: (arg.pat.span, appl), ty_snip },
);
}
}
}

3
compiler/rustc_lint/src/unused.rs
View file

@ -1201,7 +1201,8 @@ impl EarlyLintPass for UnusedParens {
// Do not lint on `(..)` as that will result in the other arms being useless.
Paren(_)
// The other cases do not contain sub-patterns.
| Wild | Never | Rest | Expr(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) | Err(_) => {},
| Missing | Wild | Never | Rest | Expr(..) | MacCall(..) | Range(..) | Ident(.., None)
| Path(..) | Err(_) => {},
// These are list-like patterns; parens can always be removed.
TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
self.check_unused_parens_pat(cx, p, false, false, keep_space);