Rollup merge of #140746 - dianne:guard-pat-res, r=oli-obk

name resolution for guard patterns

This PR provides an initial implementation of name resolution for guard patterns [(RFC 3637)](https://github.com/rust-lang/rfcs/blob/master/text/3637-guard-patterns.md). This does not change the requirement that the bindings on either side of an or-pattern must be the same [(proposal here)](https://github.com/rust-lang/rfcs/blob/master/text/3637-guard-patterns.md#allowing-mismatching-bindings-when-possible); the code that handles that is separate from what this PR touches, so I'm saving it for a follow-up.

On a technical level, this separates "collecting the bindings in a pattern" (which was already done for or-patterns) from "introducing those bindings into scope". I believe the approach used here can be extended straightforwardly in the future to work with `if let` guard patterns, but I haven't tried it myself since we don't allow those yet.

Tracking issue for guard patterns: #129967

cc ``@Nadrieril``

1 files changed, 99 insertions, 34 deletions

diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index faee0e7dd5f..1b682d0cf8a 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -111,6 +111,19 @@ enum PatBoundCtx {
     Or,
 }
 
+/// Tracks bindings resolved within a pattern. This serves two purposes:
+///
+/// - This tracks when identifiers are bound multiple times within a pattern. In a product context,
+///   this is an error. In an or-pattern, this lets us reuse the same resolution for each instance.
+///   See `fresh_binding` and `resolve_pattern_inner` for more information.
+///
+/// - 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.
+///
+/// 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)]
 pub(crate) enum HasGenericParams {
@@ -786,7 +799,14 @@ impl<'ra: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'_, 'ast, 'r
     fn visit_pat(&mut self, p: &'ast Pat) {
         let prev = self.diag_metadata.current_pat;
         self.diag_metadata.current_pat = Some(p);
-        visit::walk_pat(self, p);
+
+        if let PatKind::Guard(subpat, _) = &p.kind {
+            // We walk the guard expression in `resolve_pattern_inner`. Don't resolve it twice.
+            self.visit_pat(subpat);
+        } else {
+            visit::walk_pat(self, p);
+        }
+
         self.diag_metadata.current_pat = prev;
     }
     fn visit_local(&mut self, local: &'ast Local) {
@@ -2297,7 +2317,7 @@ 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.
@@ -2319,15 +2339,20 @@ 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());
@@ -3615,16 +3640,10 @@ 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);
         });
     }
@@ -3635,6 +3654,7 @@ 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);
@@ -3851,13 +3871,32 @@ 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,
         pat_src: PatternSource,
-        bindings: &mut SmallVec<[(PatBoundCtx, FxHashSet<Ident>); 1]>,
+        bindings: &mut PatternBindings,
     ) {
         // We walk the pattern before declaring the pattern's inner bindings,
         // so that we avoid resolving a literal expression to a binding defined
@@ -3890,9 +3929,9 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
     #[tracing::instrument(skip(self, bindings), level = "debug")]
     fn resolve_pattern_inner(
         &mut self,
-        pat: &Pat,
+        pat: &'ast Pat,
         pat_src: PatternSource,
-        bindings: &mut SmallVec<[(PatBoundCtx, FxHashSet<Ident>); 1]>,
+        bindings: &mut PatternBindings,
     ) {
         // Visit all direct subpatterns of this pattern.
         pat.walk(&mut |pat| {
@@ -3950,6 +3989,31 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                     // Prevent visiting `ps` as we've already done so above.
                     return false;
                 }
+                PatKind::Guard(ref subpat, ref guard) => {
+                    // Add a new set of bindings to the stack to collect bindings in `subpat`.
+                    bindings.push((PatBoundCtx::Product, Default::default()));
+                    // Resolving `subpat` adds bindings onto the newly-pushed context. After, the
+                    // total number of contexts on the stack should be the same as before.
+                    let binding_ctx_stack_len = bindings.len();
+                    self.resolve_pattern_inner(subpat, pat_src, bindings);
+                    assert_eq!(bindings.len(), binding_ctx_stack_len);
+                    // These bindings, but none from the surrounding pattern, are visible in the
+                    // guard; put them in scope and resolve `guard`.
+                    let subpat_bindings = bindings.pop().unwrap().1;
+                    self.with_rib(ValueNS, RibKind::Normal, |this| {
+                        *this.innermost_rib_bindings(ValueNS) = subpat_bindings.clone();
+                        this.resolve_expr(guard, None);
+                    });
+                    // Propagate the subpattern's bindings upwards.
+                    // FIXME(guard_patterns): For `if let` guards, we'll also need to get the
+                    // bindings introduced by the guard from its rib and propagate them upwards.
+                    // This will require checking the identifiers for overlaps with `bindings`, like
+                    // what `fresh_binding` does (ideally sharing its logic). To keep them separate
+                    // from `subpat_bindings`, we can introduce a fresh rib for the guard.
+                    bindings.last_mut().unwrap().1.extend(subpat_bindings);
+                    // Prevent visiting `subpat` as we've already done so above.
+                    return false;
+                }
                 _ => {}
             }
             true
@@ -3988,20 +4052,17 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         ident: Ident,
         pat_id: NodeId,
         pat_src: PatternSource,
-        bindings: &mut SmallVec<[(PatBoundCtx, FxHashSet<Ident>); 1]>,
+        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::*;
@@ -4014,19 +4075,23 @@ 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> {