Implement Guard Patterns down to THIR

compiler/rustc_ast/src/ast.rs

@@ -607,7 +607,7 @@ impl Pat {
     /// Walk top-down and call `it` in each place where a pattern occurs
     /// starting with the root pattern `walk` is called on. If `it` returns
     /// false then we will descend no further but siblings will be processed.
-    pub fn walk(&self, it: &mut impl FnMut(&Pat) -> bool) {
+    pub fn walk<'s>(&'s self, it: &mut impl FnMut(&'s Pat) -> bool) {
         if !it(self) {
             return;
         }
@@ -626,9 +626,11 @@ impl Pat {
             | PatKind::Or(s) => s.iter().for_each(|p| p.walk(it)),
 
             // Trivial wrappers over inner patterns.
-            PatKind::Box(s) | PatKind::Deref(s) | PatKind::Ref(s, _) | PatKind::Paren(s) => {
-                s.walk(it)
-            }
+            PatKind::Box(s)
+            | PatKind::Deref(s)
+            | PatKind::Ref(s, _)
+            | PatKind::Paren(s)
+            | PatKind::Guard(s, _) => s.walk(it),
 
             // These patterns do not contain subpatterns, skip.
             PatKind::Wild
@@ -838,6 +840,9 @@ pub enum PatKind {
     // A never pattern `!`.
     Never,
 
+    /// A guard pattern (e.g., `x if guard(x)`).
+    Guard(P<Pat>, P<Expr>),
+
     /// Parentheses in patterns used for grouping (i.e., `(PAT)`).
     Paren(P<Pat>),
 

compiler/rustc_ast/src/mut_visit.rs

@@ -1366,6 +1366,10 @@ pub fn walk_pat<T: MutVisitor>(vis: &mut T, pat: &mut P<Pat>) {
             visit_opt(e2, |e| vis.visit_expr(e));
             vis.visit_span(span);
         }
+        PatKind::Guard(p, e) => {
+            vis.visit_pat(p);
+            vis.visit_expr(e);
+        }
         PatKind::Tuple(elems) | PatKind::Slice(elems) | PatKind::Or(elems) => {
             visit_thin_vec(elems, |elem| vis.visit_pat(elem))
         }

compiler/rustc_ast/src/visit.rs

@@ -670,6 +670,10 @@ pub fn walk_pat<'a, V: Visitor<'a>>(visitor: &mut V, pattern: &'a Pat) -> V::Res
             visit_opt!(visitor, visit_expr, lower_bound);
             visit_opt!(visitor, visit_expr, upper_bound);
         }
+        PatKind::Guard(subpattern, guard_condition) => {
+            try_visit!(visitor.visit_pat(subpattern));
+            try_visit!(visitor.visit_expr(guard_condition));
+        }
         PatKind::Wild | PatKind::Rest | PatKind::Never => {}
         PatKind::Err(_guar) => {}
         PatKind::Tuple(elems) | PatKind::Slice(elems) | PatKind::Or(elems) => {

compiler/rustc_ast_lowering/src/pat.rs

@@ -114,6 +114,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             self.lower_range_end(end, e2.is_some()),
                         );
                     }
+                    PatKind::Guard(inner, cond) => {
+                        break hir::PatKind::Guard(self.lower_pat(inner), self.lower_expr(cond));
+                    }
                     PatKind::Slice(pats) => break self.lower_pat_slice(pats),
                     PatKind::Rest => {
                         // If we reach here the `..` pattern is not semantically allowed.

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -540,6 +540,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(const_closures, "const closures are experimental");
     gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
     gate_all!(explicit_tail_calls, "`become` expression is experimental");
+    gate_all!(guard_patterns, "guard patterns are experimental", "consider using match arm guards");
     gate_all!(generic_const_items, "generic const items are experimental");
     gate_all!(fn_delegation, "functions delegation is not yet fully implemented");
     gate_all!(postfix_match, "postfix match is experimental");

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1710,6 +1710,12 @@ impl<'a> State<'a> {
                     self.print_expr(e, FixupContext::default());
                 }
             }
+            PatKind::Guard(subpat, condition) => {
+                self.print_pat(subpat);
+                self.space();
+                self.word_space("if");
+                self.print_expr(condition, FixupContext::default());
+            }
             PatKind::Slice(elts) => {
                 self.word("[");
                 self.commasep(Inconsistent, elts, |s, p| s.print_pat(p));

compiler/rustc_expand/src/expand.rs

@@ -989,7 +989,7 @@ pub fn parse_ast_fragment<'a>(
             }
         }
         AstFragmentKind::Ty => AstFragment::Ty(this.parse_ty()?),
-        AstFragmentKind::Pat => AstFragment::Pat(this.parse_pat_allow_top_alt(
+        AstFragmentKind::Pat => AstFragment::Pat(this.parse_pat_allow_top_guard(
             None,
             RecoverComma::No,
             RecoverColon::Yes,

compiler/rustc_feature/src/unstable.rs

@@ -501,6 +501,8 @@ declare_features! (
     (incomplete, generic_const_items, "1.73.0", Some(113521)),
     /// Allows registering static items globally, possibly across crates, to iterate over at runtime.
     (unstable, global_registration, "1.80.0", Some(125119)),
+    /// Allows using guards in patterns.
+    (incomplete, guard_patterns, "CURRENT_RUSTC_VERSION", Some(129967)),
     /// Allows using `..=X` as a patterns in slices.
     (unstable, half_open_range_patterns_in_slices, "1.66.0", Some(67264)),
     /// Allows `if let` guard in match arms.

compiler/rustc_hir/src/hir.rs

@@ -1139,7 +1139,7 @@ impl<'hir> Pat<'hir> {
         use PatKind::*;
         match self.kind {
             Wild | Never | Lit(_) | Range(..) | Binding(.., None) | Path(_) | Err(_) => true,
-            Box(s) | Deref(s) | Ref(s, _) | Binding(.., Some(s)) => s.walk_short_(it),
+            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)),
             TupleStruct(_, s, _) | Tuple(s, _) | Or(s) => s.iter().all(|p| p.walk_short_(it)),
             Slice(before, slice, after) => {
@@ -1166,7 +1166,7 @@ impl<'hir> Pat<'hir> {
         use PatKind::*;
         match self.kind {
             Wild | Never | Lit(_) | Range(..) | Binding(.., None) | Path(_) | Err(_) => {}
-            Box(s) | Deref(s) | Ref(s, _) | Binding(.., Some(s)) => s.walk_(it),
+            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)),
             Slice(before, slice, after) => {
@@ -1318,6 +1318,9 @@ pub enum PatKind<'hir> {
     /// A literal.
     Lit(&'hir Expr<'hir>),
 
+    /// A guard pattern (e.g., `x if guard(x)`).
+    Guard(&'hir Pat<'hir>, &'hir Expr<'hir>),
+
     /// A range pattern (e.g., `1..=2` or `1..2`).
     Range(Option<&'hir Expr<'hir>>, Option<&'hir Expr<'hir>>, RangeEnd),
 

compiler/rustc_hir/src/intravisit.rs

@@ -700,6 +700,10 @@ pub fn walk_pat<'v, V: Visitor<'v>>(visitor: &mut V, pattern: &'v Pat<'v>) -> V:
             visit_opt!(visitor, visit_pat, slice_pattern);
             walk_list!(visitor, visit_pat, postpatterns);
         }
+        PatKind::Guard(subpat, condition) => {
+            try_visit!(visitor.visit_pat(subpat));
+            try_visit!(visitor.visit_expr(condition));
+        }
     }
     V::Result::output()
 }

compiler/rustc_hir_analysis/src/check/region.rs

@@ -678,7 +678,9 @@ fn resolve_local<'tcx>(
             | PatKind::TupleStruct(_, subpats, _)
             | PatKind::Tuple(subpats, _) => subpats.iter().any(|p| is_binding_pat(p)),
 
-            PatKind::Box(subpat) | PatKind::Deref(subpat) => is_binding_pat(subpat),
+            PatKind::Box(subpat) | PatKind::Deref(subpat) | PatKind::Guard(subpat, _) => {
+                is_binding_pat(subpat)
+            }
 
             PatKind::Ref(_, _)
             | PatKind::Binding(hir::BindingMode(hir::ByRef::No, _), ..)

compiler/rustc_hir_pretty/src/lib.rs

@@ -1870,6 +1870,12 @@ impl<'a> State<'a> {
                 self.commasep(Inconsistent, after, |s, p| s.print_pat(p));
                 self.word("]");
             }
+            PatKind::Guard(inner, cond) => {
+                self.print_pat(inner);
+                self.space();
+                self.word_space("if");
+                self.print_expr(cond);
+            }
             PatKind::Err(_) => {
                 self.popen();
                 self.word("/*ERROR*/");

compiler/rustc_hir_typeck/src/expr.rs

@@ -422,6 +422,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 subpats.iter().all(|pat| self.pat_guaranteed_to_constitute_read_for_never(pat))
             }
 
+            // Passes through to the sub-pattern.
+            hir::PatKind::Guard(subpat, _) => {
+                self.pat_guaranteed_to_constitute_read_for_never(subpat)
+            }
+
             // Does constitute a read, since it is equivalent to a discriminant read.
             hir::PatKind::Never => true,
 

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -612,6 +612,7 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                     | PatKind::Box(_)
                     | PatKind::Deref(_)
                     | PatKind::Ref(..)
+                    | PatKind::Guard(..)
                     | PatKind::Wild
                     | PatKind::Err(_) => {
                         // If the PatKind is Or, Box, or Ref, the decision is made later
@@ -1722,7 +1723,7 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                 }
             }
 
-            PatKind::Binding(.., Some(subpat)) => {
+            PatKind::Binding(.., Some(subpat)) | PatKind::Guard(subpat, _) => {
                 self.cat_pattern(place_with_id, subpat, op)?;
             }
 

compiler/rustc_hir_typeck/src/pat.rs

@@ -259,6 +259,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             PatKind::Struct(ref qpath, fields, has_rest_pat) => {
                 self.check_pat_struct(pat, qpath, fields, has_rest_pat, expected, pat_info)
             }
+            PatKind::Guard(pat, cond) => {
+                self.check_pat(pat, expected, pat_info);
+                self.check_expr_has_type_or_error(cond, self.tcx.types.bool, |_| {});
+                expected
+            }
             PatKind::Or(pats) => {
                 for pat in pats {
                     self.check_pat(pat, expected, pat_info);
@@ -397,7 +402,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // An OR-pattern just propagates to each individual alternative.
             // This is maximally flexible, allowing e.g., `Some(mut x) | &Some(mut x)`.
             // In that example, `Some(mut x)` results in `Peel` whereas `&Some(mut x)` in `Reset`.
-            | PatKind::Or(_) => AdjustMode::Pass,
+            | PatKind::Or(_)
+            // Like or-patterns, guard patterns just propogate to their subpatterns.
+            | PatKind::Guard(..) => AdjustMode::Pass,
         }
     }
 
@@ -876,6 +883,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         PatKind::Struct(..)
                         | PatKind::TupleStruct(..)
                         | PatKind::Or(..)
+                        | PatKind::Guard(..)
                         | PatKind::Tuple(..)
                         | PatKind::Slice(..) => "binding",
 

compiler/rustc_lint/src/unused.rs

@@ -1226,7 +1226,7 @@ impl EarlyLintPass for UnusedParens {
                 self.check_unused_parens_pat(cx, &f.pat, false, false, keep_space);
             },
             // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
-            Ident(.., Some(p)) | Box(p) | Deref(p) => self.check_unused_parens_pat(cx, p, true, false, keep_space),
+            Ident(.., Some(p)) | Box(p) | Deref(p) | Guard(p, _) => self.check_unused_parens_pat(cx, p, true, false, keep_space),
             // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
             // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
             Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Not, keep_space),

compiler/rustc_middle/src/thir.rs

@@ -645,7 +645,8 @@ impl<'tcx> Pat<'tcx> {
             | Binding { subpattern: Some(subpattern), .. }
             | Deref { subpattern }
             | DerefPattern { subpattern, .. }
-            | InlineConstant { subpattern, .. } => subpattern.walk_(it),
+            | InlineConstant { subpattern, .. }
+            | Guard { subpattern, .. } => subpattern.walk_(it),
             Leaf { subpatterns } | Variant { subpatterns, .. } => {
                 subpatterns.iter().for_each(|field| field.pattern.walk_(it))
             }
@@ -828,6 +829,13 @@ pub enum PatKind<'tcx> {
         pats: Box<[Box<Pat<'tcx>>]>,
     },
 
+    /// A guard pattern, e.g. `x if guard(x)`.
+    Guard {
+        subpattern: Box<Pat<'tcx>>,
+        #[type_visitable(ignore)]
+        condition: ExprId,
+    },
+
     /// A never pattern `!`.
     Never,
 

compiler/rustc_middle/src/thir/visit.rs

@@ -265,5 +265,9 @@ pub fn walk_pat<'thir, 'tcx: 'thir, V: Visitor<'thir, 'tcx>>(
                 visitor.visit_pat(pat);
             }
         }
+        Guard { subpattern, condition } => {
+            visitor.visit_pat(subpattern);
+            visitor.visit_expr(&visitor.thir()[*condition]);
+        }
     };
 }

compiler/rustc_mir_build/src/build/matches/match_pair.rs

@@ -1,4 +1,5 @@
 use rustc_middle::mir::*;
+use rustc_middle::span_bug;
 use rustc_middle::thir::{self, *};
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 
@@ -250,6 +251,10 @@ impl<'pat, 'tcx> MatchPairTree<'pat, 'tcx> {
             }
 
             PatKind::Never => TestCase::Never,
+
+            PatKind::Guard { .. } => {
+                span_bug!(pattern.span, "MIR lowering is not yet implemented for guard patterns")
+            }
         };
 
         MatchPairTree { place, test_case, subpairs, pattern }

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -921,6 +921,10 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 self.visit_primary_bindings(subpattern, pattern_user_ty, f)
             }
 
+            PatKind::Guard { ref subpattern, .. } => {
+                self.visit_primary_bindings(subpattern, pattern_user_ty, f);
+            }
+
             PatKind::Leaf { ref subpatterns } => {
                 for subpattern in subpatterns {
                     let subpattern_user_ty = pattern_user_ty.clone().leaf(subpattern.field);

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -334,6 +334,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                 PatKind::Or { .. } |
                 PatKind::InlineConstant { .. } |
                 PatKind::AscribeUserType { .. } |
+                PatKind::Guard { .. } |
                 PatKind::Error(_) => {}
             }
         };

compiler/rustc_mir_build/src/lib.rs

@@ -22,7 +22,7 @@ use rustc_middle::util::Providers;
 rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 pub fn provide(providers: &mut Providers) {
-    providers.check_match = thir::pattern::check_match;
+    providers.check_match = thir::check_match::check_match;
     providers.lit_to_const = thir::constant::lit_to_const;
     providers.hooks.build_mir = build::mir_build;
     providers.closure_saved_names_of_captured_variables =

compiler/rustc_mir_build/src/thir/cx/mod.rs

@@ -2,6 +2,10 @@
 //! structures into the THIR. The `builder` is generally ignorant of the tcx,
 //! etc., and instead goes through the `Cx` for most of its work.
 
+mod block;
+mod expr;
+mod pattern;
+
 use rustc_data_structures::steal::Steal;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
@@ -13,9 +17,7 @@ use rustc_middle::bug;
 use rustc_middle::middle::region;
 use rustc_middle::thir::*;
 use rustc_middle::ty::{self, RvalueScopes, TyCtxt};
-use tracing::instrument;
 
-use crate::thir::pattern::pat_from_hir;
 use crate::thir::util::UserAnnotatedTyHelpers;
 
 pub(crate) fn thir_body(
@@ -109,11 +111,6 @@ impl<'tcx> Cx<'tcx> {
         }
     }
 
-    #[instrument(level = "debug", skip(self))]
-    fn pattern_from_hir(&mut self, p: &'tcx hir::Pat<'tcx>) -> Box<Pat<'tcx>> {
-        pat_from_hir(self.tcx, self.param_env, self.typeck_results(), p)
-    }
-
     fn closure_env_param(&self, owner_def: LocalDefId, expr_id: HirId) -> Option<Param<'tcx>> {
         if self.tcx.def_kind(owner_def) != DefKind::Closure {
             return None;
@@ -206,6 +203,3 @@ impl<'tcx> UserAnnotatedTyHelpers<'tcx> for Cx<'tcx> {
         self.typeck_results
     }
 }
-
-mod block;
-mod expr;