Add {ast,hir,thir}::PatKind::Missing
variants.
"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.
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40 changed files with 86 additions and 46 deletions
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@ -747,6 +747,9 @@ pub struct Ascription<'tcx> {
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#[derive(Clone, Debug, HashStable, TypeVisitable)]
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pub enum PatKind<'tcx> {
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/// A missing pattern, e.g. for an anonymous param in a bare fn like `fn f(u32)`.
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Missing,
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/// A wildcard pattern: `_`.
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Wild,
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@ -250,7 +250,8 @@ pub(crate) fn for_each_immediate_subpat<'a, 'tcx>(
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mut callback: impl FnMut(&'a Pat<'tcx>),
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) {
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match &pat.kind {
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PatKind::Wild
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PatKind::Missing
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| PatKind::Wild
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| PatKind::Binding { subpattern: None, .. }
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| PatKind::Constant { value: _ }
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| PatKind::Range(_)
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