From 9f089e080c47bc282aa98f1e8c72ff44076afbb9 Mon Sep 17 00:00:00 2001 From: Nicholas Nethercote Date: Thu, 27 Mar 2025 09:33:02 +1100 Subject: 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>` instead of a `P`, but that quickly turned into a very large and painful change. Adding `PatKind::Missing` is much easier. --- compiler/rustc_ast/src/ast.rs | 8 +++++++- compiler/rustc_ast/src/mut_visit.rs | 2 +- compiler/rustc_ast/src/visit.rs | 2 +- 3 files changed, 9 insertions(+), 3 deletions(-) (limited to 'compiler/rustc_ast/src') diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs index 064f05ef1f3..944ae65ffc6 100644 --- a/compiler/rustc_ast/src/ast.rs +++ b/compiler/rustc_ast/src/ast.rs @@ -563,6 +563,7 @@ impl Pat { /// This is intended for use by diagnostics. pub fn to_ty(&self) -> Option> { let kind = match &self.kind { + PatKind::Missing => unreachable!(), // In a type expression `_` is an inference variable. PatKind::Wild => TyKind::Infer, // An IDENT pattern with no binding mode would be valid as path to a type. E.g. `u32`. @@ -625,7 +626,8 @@ impl Pat { | PatKind::Guard(s, _) => s.walk(it), // These patterns do not contain subpatterns, skip. - PatKind::Wild + PatKind::Missing + | PatKind::Wild | PatKind::Rest | PatKind::Never | PatKind::Expr(_) @@ -676,6 +678,7 @@ impl Pat { /// Return a name suitable for diagnostics. pub fn descr(&self) -> Option { match &self.kind { + PatKind::Missing => unreachable!(), PatKind::Wild => Some("_".to_string()), PatKind::Ident(BindingMode::NONE, ident, None) => Some(format!("{ident}")), PatKind::Ref(pat, mutbl) => pat.descr().map(|d| format!("&{}{d}", mutbl.prefix_str())), @@ -769,6 +772,9 @@ pub enum RangeSyntax { // Adding a new variant? Please update `test_pat` in `tests/ui/macros/stringify.rs`. #[derive(Clone, Encodable, Decodable, Debug)] pub enum PatKind { + /// A missing pattern, e.g. for an anonymous param in a bare fn like `fn f(u32)`. + Missing, + /// Represents a wildcard pattern (`_`). Wild, diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs index 274fe312f7f..8c6ddf2a69c 100644 --- a/compiler/rustc_ast/src/mut_visit.rs +++ b/compiler/rustc_ast/src/mut_visit.rs @@ -1572,7 +1572,7 @@ pub fn walk_pat(vis: &mut T, pat: &mut P) { vis.visit_id(id); match kind { PatKind::Err(_guar) => {} - PatKind::Wild | PatKind::Rest | PatKind::Never => {} + PatKind::Missing | PatKind::Wild | PatKind::Rest | PatKind::Never => {} PatKind::Ident(_binding_mode, ident, sub) => { vis.visit_ident(ident); visit_opt(sub, |sub| vis.visit_pat(sub)); diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs index 2716601ca4f..3d78f65f634 100644 --- a/compiler/rustc_ast/src/visit.rs +++ b/compiler/rustc_ast/src/visit.rs @@ -727,7 +727,7 @@ pub fn walk_pat<'a, V: Visitor<'a>>(visitor: &mut V, pattern: &'a Pat) -> V::Res try_visit!(visitor.visit_pat(subpattern)); try_visit!(visitor.visit_expr(guard_condition)); } - PatKind::Wild | PatKind::Rest | PatKind::Never => {} + PatKind::Missing | PatKind::Wild | PatKind::Rest | PatKind::Never => {} PatKind::Err(_guar) => {} PatKind::Tuple(elems) | PatKind::Slice(elems) | PatKind::Or(elems) => { walk_list!(visitor, visit_pat, elems); -- cgit 1.4.1-3-g733a5