delay introducing pattern bindings into scope

This splits introduction of bindings into scope
(`apply_pattern_bindings`) apart from manipulation of the pattern's
binding map (`fresh_binding`). By delaying the latter, we can keep
bindings from appearing in-scope in guards.

Since `fresh_binding` is now specifically for manipulating a pattern's
bindings map, this commit also inlines a use of `fresh_binding` that was
only adding to the innermost rib.

Author: dianne

compiler/rustc_resolve/src/late.rs

@@ -120,7 +120,9 @@ enum PatBoundCtx {
 /// - The guard expression of a guard pattern may use bindings from within the guard pattern, but
 ///   not from elsewhere in the pattern containing it. This allows us to isolate the bindings in the
 ///   subpattern to construct the scope for the guard.
-type PatternBindings = SmallVec<[(PatBoundCtx, FxHashSet<Ident>); 1]>;
+///
+/// Each identifier must map to at most one distinct [`Res`].
+type PatternBindings = SmallVec<[(PatBoundCtx, FxIndexMap<Ident, Res>); 1]>;
 
 /// Does this the item (from the item rib scope) allow generic parameters?
 #[derive(Copy, Clone, Debug)]
@@ -2308,7 +2310,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
     fn resolve_fn_params(
         &mut self,
         has_self: bool,
-        inputs: impl Iterator<Item = (Option<&'ast Pat>, &'ast Ty)>,
+        inputs: impl Iterator<Item = (Option<&'ast Pat>, &'ast Ty)> + Clone,
     ) -> Result<LifetimeRes, (Vec<MissingLifetime>, Vec<ElisionFnParameter>)> {
         enum Elision {
             /// We have not found any candidate.
@@ -2330,15 +2332,20 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         let mut parameter_info = Vec::new();
         let mut all_candidates = Vec::new();
 
+        // Resolve and apply bindings first so diagnostics can see if they're used in types.
         let mut bindings = smallvec![(PatBoundCtx::Product, Default::default())];
-        for (index, (pat, ty)) in inputs.enumerate() {
-            debug!(?pat, ?ty);
+        for (pat, _) in inputs.clone() {
+            debug!("resolving bindings in pat = {pat:?}");
             self.with_lifetime_rib(LifetimeRibKind::Elided(LifetimeRes::Infer), |this| {
                 if let Some(pat) = pat {
                     this.resolve_pattern(pat, PatternSource::FnParam, &mut bindings);
                 }
             });
+        }
+        self.apply_pattern_bindings(bindings);
 
+        for (index, (pat, ty)) in inputs.enumerate() {
+            debug!("resolving type for pat = {pat:?}, ty = {ty:?}");
             // Record elision candidates only for this parameter.
             debug_assert_matches!(self.lifetime_elision_candidates, None);
             self.lifetime_elision_candidates = Some(Default::default());
@@ -3626,16 +3633,10 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         self.visit_path(&delegation.path, delegation.id);
         let Some(body) = &delegation.body else { return };
         self.with_rib(ValueNS, RibKind::FnOrCoroutine, |this| {
-            // `PatBoundCtx` is not necessary in this context
-            let mut bindings = smallvec![(PatBoundCtx::Product, Default::default())];
-
             let span = delegation.path.segments.last().unwrap().ident.span;
-            this.fresh_binding(
-                Ident::new(kw::SelfLower, span),
-                delegation.id,
-                PatternSource::FnParam,
-                &mut bindings,
-            );
+            let ident = Ident::new(kw::SelfLower, span.normalize_to_macro_rules());
+            let res = Res::Local(delegation.id);
+            this.innermost_rib_bindings(ValueNS).insert(ident, res);
             this.visit_block(body);
         });
     }
@@ -3646,6 +3647,7 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
             for Param { pat, .. } in params {
                 this.resolve_pattern(pat, PatternSource::FnParam, &mut bindings);
             }
+            this.apply_pattern_bindings(bindings);
         });
         for Param { ty, .. } in params {
             self.visit_ty(ty);
@@ -3862,8 +3864,27 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
     fn resolve_pattern_top(&mut self, pat: &'ast Pat, pat_src: PatternSource) {
         let mut bindings = smallvec![(PatBoundCtx::Product, Default::default())];
         self.resolve_pattern(pat, pat_src, &mut bindings);
+        self.apply_pattern_bindings(bindings);
     }
 
+    /// Apply the bindings from a pattern to the innermost rib of the current scope.
+    fn apply_pattern_bindings(&mut self, mut pat_bindings: PatternBindings) {
+        let rib_bindings = self.innermost_rib_bindings(ValueNS);
+        let Some((_, pat_bindings)) = pat_bindings.pop() else {
+            bug!("tried applying nonexistent bindings from pattern");
+        };
+
+        if rib_bindings.is_empty() {
+            // Often, such as for match arms, the bindings are introduced into a new rib.
+            // In this case, we can move the bindings over directly.
+            *rib_bindings = pat_bindings;
+        } else {
+            rib_bindings.extend(pat_bindings);
+        }
+    }
+
+    /// Resolve bindings in a pattern. `apply_pattern_bindings` must be called after to introduce
+    /// the bindings into scope.
     fn resolve_pattern(
         &mut self,
         pat: &'ast Pat,
@@ -4001,18 +4022,15 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         pat_src: PatternSource,
         bindings: &mut PatternBindings,
     ) -> Res {
-        // Add the binding to the local ribs, if it doesn't already exist in the bindings map.
+        // Add the binding to the bindings map, if it doesn't already exist.
         // (We must not add it if it's in the bindings map because that breaks the assumptions
         // later passes make about or-patterns.)
         let ident = ident.normalize_to_macro_rules();
 
-        let mut bound_iter = bindings.iter().filter(|(_, set)| set.contains(&ident));
         // Already bound in a product pattern? e.g. `(a, a)` which is not allowed.
-        let already_bound_and = bound_iter.clone().any(|(ctx, _)| *ctx == PatBoundCtx::Product);
-        // Already bound in an or-pattern? e.g. `V1(a) | V2(a)`.
-        // This is *required* for consistency which is checked later.
-        let already_bound_or = bound_iter.any(|(ctx, _)| *ctx == PatBoundCtx::Or);
-
+        let already_bound_and = bindings
+            .iter()
+            .any(|(ctx, map)| *ctx == PatBoundCtx::Product && map.contains_key(&ident));
         if already_bound_and {
             // Overlap in a product pattern somewhere; report an error.
             use ResolutionError::*;
@@ -4025,19 +4043,23 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
             self.report_error(ident.span, error(ident));
         }
 
-        // Record as bound.
-        bindings.last_mut().unwrap().1.insert(ident);
-
-        if already_bound_or {
+        // Already bound in an or-pattern? e.g. `V1(a) | V2(a)`.
+        // This is *required* for consistency which is checked later.
+        let already_bound_or = bindings
+            .iter()
+            .find_map(|(ctx, map)| if *ctx == PatBoundCtx::Or { map.get(&ident) } else { None });
+        let res = if let Some(&res) = already_bound_or {
             // `Variant1(a) | Variant2(a)`, ok
             // Reuse definition from the first `a`.
-            self.innermost_rib_bindings(ValueNS)[&ident]
-        } else {
-            // A completely fresh binding is added to the set.
-            let res = Res::Local(pat_id);
-            self.innermost_rib_bindings(ValueNS).insert(ident, res);
             res
-        }
+        } else {
+            // A completely fresh binding is added to the map.
+            Res::Local(pat_id)
+        };
+
+        // Record as bound.
+        bindings.last_mut().unwrap().1.insert(ident, res);
+        res
     }
 
     fn innermost_rib_bindings(&mut self, ns: Namespace) -> &mut FxIndexMap<Ident, Res> {