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.

Author: nnethercote

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<P<Ty>> {
         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<String> {
         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,
 

compiler/rustc_ast/src/mut_visit.rs

@@ -1572,7 +1572,7 @@ pub fn walk_pat<T: MutVisitor>(vis: &mut T, pat: &mut P<Pat>) {
     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));

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);

compiler/rustc_ast_lowering/src/lib.rs

@@ -1495,6 +1495,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
     fn lower_fn_params_to_names(&mut self, decl: &FnDecl) -> &'hir [Option<Ident>] {
         self.arena.alloc_from_iter(decl.inputs.iter().map(|param| match param.pat.kind {
+            PatKind::Missing => None,
             PatKind::Ident(_, ident, _) => {
                 if ident.name != kw::Empty {
                     Some(self.lower_ident(ident))
@@ -1506,7 +1507,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             _ => {
                 self.dcx().span_delayed_bug(
                     param.pat.span,
-                    "non-ident/wild param pat must trigger an error",
+                    "non-missing/ident/wild param pat must trigger an error",
                 );
                 None
             }

compiler/rustc_ast_lowering/src/pat.rs

@@ -26,6 +26,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             let pat_hir_id = self.lower_node_id(pattern.id);
             let node = loop {
                 match &pattern.kind {
+                    PatKind::Missing => break hir::PatKind::Missing,
                     PatKind::Wild => break hir::PatKind::Wild,
                     PatKind::Never => break hir::PatKind::Never,
                     PatKind::Ident(binding_mode, ident, sub) => {

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -239,7 +239,7 @@ impl<'a> AstValidator<'a> {
     fn check_decl_no_pat(decl: &FnDecl, mut report_err: impl FnMut(Span, Option<Ident>, bool)) {
         for Param { pat, .. } in &decl.inputs {
             match pat.kind {
-                PatKind::Ident(BindingMode::NONE, _, None) | PatKind::Wild => {}
+                PatKind::Missing | PatKind::Ident(BindingMode::NONE, _, None) | PatKind::Wild => {}
                 PatKind::Ident(BindingMode::MUT, ident, None) => {
                     report_err(pat.span, Some(ident), true)
                 }

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1622,9 +1622,9 @@ impl<'a> State<'a> {
     fn print_pat(&mut self, pat: &ast::Pat) {
         self.maybe_print_comment(pat.span.lo());
         self.ann.pre(self, AnnNode::Pat(pat));
-        /* Pat isn't normalized, but the beauty of it
-        is that it doesn't matter */
+        /* Pat isn't normalized, but the beauty of it is that it doesn't matter */
         match &pat.kind {
+            PatKind::Missing => unreachable!(),
             PatKind::Wild => self.word("_"),
             PatKind::Never => self.word("!"),
             PatKind::Ident(BindingMode(by_ref, mutbl), ident, sub) => {
@@ -1946,12 +1946,7 @@ impl<'a> State<'a> {
                 if let Some(eself) = input.to_self() {
                     self.print_explicit_self(&eself);
                 } else {
-                    let invalid = if let PatKind::Ident(_, ident, _) = input.pat.kind {
-                        ident.name == kw::Empty
-                    } else {
-                        false
-                    };
-                    if !invalid {
+                    if !matches!(input.pat.kind, PatKind::Missing) {
                         self.print_pat(&input.pat);
                         self.word(":");
                         self.space();

compiler/rustc_hir/src/hir.rs

@@ -1516,6 +1516,7 @@ impl<'hir> Pat<'hir> {
 
         use PatKind::*;
         match self.kind {
+            Missing => unreachable!(),
             Wild | Never | Expr(_) | Range(..) | Binding(.., None) | Err(_) => true,
             Box(s) | Deref(s) | Ref(s, _) | Binding(.., Some(s)) | Guard(s, _) => s.walk_short_(it),
             Struct(_, fields, _) => fields.iter().all(|field| field.pat.walk_short_(it)),
@@ -1543,7 +1544,7 @@ impl<'hir> Pat<'hir> {
 
         use PatKind::*;
         match self.kind {
-            Wild | Never | Expr(_) | Range(..) | Binding(.., None) | Err(_) => {}
+            Missing | Wild | Never | Expr(_) | Range(..) | Binding(.., None) | Err(_) => {}
             Box(s) | Deref(s) | Ref(s, _) | Binding(.., Some(s)) | Guard(s, _) => s.walk_(it),
             Struct(_, fields, _) => fields.iter().for_each(|field| field.pat.walk_(it)),
             TupleStruct(_, s, _) | Tuple(s, _) | Or(s) => s.iter().for_each(|p| p.walk_(it)),
@@ -1681,6 +1682,9 @@ pub enum TyPatKind<'hir> {
 
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
 pub enum PatKind<'hir> {
+    /// A missing pattern, e.g. for an anonymous param in a bare fn like `fn f(u32)`.
+    Missing,
+
     /// Represents a wildcard pattern (i.e., `_`).
     Wild,
 

compiler/rustc_hir/src/intravisit.rs

@@ -744,7 +744,7 @@ pub fn walk_pat<'v, V: Visitor<'v>>(visitor: &mut V, pattern: &'v Pat<'v>) -> V:
             visit_opt!(visitor, visit_pat_expr, lower_bound);
             visit_opt!(visitor, visit_pat_expr, upper_bound);
         }
-        PatKind::Never | PatKind::Wild | PatKind::Err(_) => (),
+        PatKind::Missing | PatKind::Never | PatKind::Wild | PatKind::Err(_) => (),
         PatKind::Slice(prepatterns, ref slice_pattern, postpatterns) => {
             walk_list!(visitor, visit_pat, prepatterns);
             visit_opt!(visitor, visit_pat, slice_pattern);

compiler/rustc_hir_analysis/src/check/region.rs

@@ -701,6 +701,7 @@ fn resolve_local<'tcx>(
 
             PatKind::Ref(_, _)
             | PatKind::Binding(hir::BindingMode(hir::ByRef::No, _), ..)
+            | PatKind::Missing
             | PatKind::Wild
             | PatKind::Never
             | PatKind::Expr(_)