only resolve top-level guard patterns' guards once

We resolve guard patterns' guards in `resolve_pattern_inner`, so to
avoid resolving them multiple times, we must avoid doing so earlier. To
accomplish this, `LateResolutionVisitor::visit_pat` contains a case for
guard patterns that avoids visiting their guards while walking patterns.
This fixes an ICE due to `visit::walk_pat` being used instead, which
meant guards at the top level of a pattern would be visited twice.

Author: dianne

compiler/rustc_resolve/src/late.rs

@@ -3901,7 +3901,9 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         // We walk the pattern before declaring the pattern's inner bindings,
         // so that we avoid resolving a literal expression to a binding defined
         // by the pattern.
-        visit::walk_pat(self, pat);
+        // NB: `Self::visit_pat` must be used rather than `visit::walk_pat` to avoid resolving guard
+        // patterns' guard expressions multiple times (#141265).
+        self.visit_pat(pat);
         self.resolve_pattern_inner(pat, pat_src, bindings);
         // This has to happen *after* we determine which pat_idents are variants:
         self.check_consistent_bindings(pat);

tests/ui/pattern/rfc-3637-guard-patterns/only-resolve-top-level-guard-expr-once-ice-141265.rs

@@ -0,0 +1,12 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/141265>.
+//! Make sure expressions in top-level guard patterns are only resolved once.
+
+fn main() {
+    for
+    else if b 0 {}
+    //~^ ERROR expected identifier, found keyword `else`
+    //~| ERROR missing `in` in `for` loop
+    //~| ERROR cannot find value `b` in this scope
+    //~| ERROR guard patterns are experimental
+    //~| ERROR `{integer}` is not an iterator
+}

tests/ui/pattern/rfc-3637-guard-patterns/only-resolve-top-level-guard-expr-once-ice-141265.stderr

@@ -0,0 +1,48 @@
+error: expected identifier, found keyword `else`
+  --> $DIR/only-resolve-top-level-guard-expr-once-ice-141265.rs:6:5
+   |
+LL |     else if b 0 {}
+   |     ^^^^ expected identifier, found keyword
+
+error: missing `in` in `for` loop
+  --> $DIR/only-resolve-top-level-guard-expr-once-ice-141265.rs:6:14
+   |
+LL |     else if b 0 {}
+   |              ^
+   |
+help: try adding `in` here
+   |
+LL |     else if b in 0 {}
+   |               ++
+
+error[E0425]: cannot find value `b` in this scope
+  --> $DIR/only-resolve-top-level-guard-expr-once-ice-141265.rs:6:13
+   |
+LL |     else if b 0 {}
+   |             ^ not found in this scope
+
+error[E0658]: guard patterns are experimental
+  --> $DIR/only-resolve-top-level-guard-expr-once-ice-141265.rs:6:13
+   |
+LL |     else if b 0 {}
+   |             ^
+   |
+   = note: see issue #129967 <https://github.com/rust-lang/rust/issues/129967> for more information
+   = help: add `#![feature(guard_patterns)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: consider using match arm guards
+
+error[E0277]: `{integer}` is not an iterator
+  --> $DIR/only-resolve-top-level-guard-expr-once-ice-141265.rs:6:15
+   |
+LL |     else if b 0 {}
+   |               ^ `{integer}` is not an iterator
+   |
+   = help: the trait `Iterator` is not implemented for `{integer}`
+   = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
+   = note: required for `{integer}` to implement `IntoIterator`
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0277, E0425, E0658.
+For more information about an error, try `rustc --explain E0277`.