From 8a57f3d8441a62ecda9aa3e537b2ca82e8b40e23 Mon Sep 17 00:00:00 2001
From: human9000 <human9000.2021@gmail.com>
Date: Fri, 13 Mar 2026 18:22:43 +0500
Subject: [PATCH] Introduce guard pattern to THIR

Co-authored-by: Max Niederman <max@maxniederman.com>
---
 compiler/rustc_middle/src/thir.rs                      |  7 +++++++
 compiler/rustc_middle/src/thir/visit.rs                | 10 ++++++++++
 .../rustc_mir_build/src/builder/matches/match_pair.rs  |  5 +++++
 compiler/rustc_mir_build/src/builder/matches/mod.rs    |  3 +++
 compiler/rustc_mir_build/src/check_unsafety.rs         |  1 +
 compiler/rustc_mir_build/src/thir/print.rs             |  8 ++++++++
 compiler/rustc_pattern_analysis/src/rustc.rs           |  3 ++-
 7 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/compiler/rustc_middle/src/thir.rs b/compiler/rustc_middle/src/thir.rs
index 881165706a03..51f33691bb7d 100644
--- a/compiler/rustc_middle/src/thir.rs
+++ b/compiler/rustc_middle/src/thir.rs
@@ -866,6 +866,13 @@ pub enum PatKind<'tcx> {
         pats: 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,
 
diff --git a/compiler/rustc_middle/src/thir/visit.rs b/compiler/rustc_middle/src/thir/visit.rs
index c17e15513cdf..aa1b6b1663bf 100644
--- a/compiler/rustc_middle/src/thir/visit.rs
+++ b/compiler/rustc_middle/src/thir/visit.rs
@@ -247,6 +247,12 @@ pub fn walk_pat<'thir, 'tcx: 'thir, V: Visitor<'thir, 'tcx>>(
     visitor: &mut V,
     pat: &'thir Pat<'tcx>,
 ) {
+    if let PatKind::Guard { subpattern, condition } = &pat.kind {
+        visitor.visit_pat(subpattern);
+        visitor.visit_expr(&visitor.thir()[*condition]);
+        return;
+    };
+
     for_each_immediate_subpat(pat, |p| visitor.visit_pat(p));
 }
 
@@ -287,5 +293,9 @@ pub(crate) fn for_each_immediate_subpat<'a, 'tcx>(
                 callback(pat);
             }
         }
+
+        PatKind::Guard { subpattern, .. } => {
+            callback(subpattern);
+        }
     }
 }
diff --git a/compiler/rustc_mir_build/src/builder/matches/match_pair.rs b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
index 7698af4cd38b..cbe31fd7d132 100644
--- a/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
@@ -360,6 +360,11 @@ pub(super) fn for_pattern(
                 Some(TestableCase::Deref { temp, mutability })
             }
 
+            PatKind::Guard { .. } => {
+                // FIXME(guard_patterns)
+                None
+            }
+
             PatKind::Never => Some(TestableCase::Never),
         };
 
diff --git a/compiler/rustc_mir_build/src/builder/matches/mod.rs b/compiler/rustc_mir_build/src/builder/matches/mod.rs
index d7edf82ae4af..d4f5fe84e0ff 100644
--- a/compiler/rustc_mir_build/src/builder/matches/mod.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/mod.rs
@@ -944,6 +944,9 @@ fn visit_primary_bindings_special(
                     visit_subpat(self, subpattern, user_tys, f);
                 }
             }
+            PatKind::Guard { ref subpattern, .. } => {
+                visit_subpat(self, subpattern, user_tys, f);
+            }
         }
     }
 }
diff --git a/compiler/rustc_mir_build/src/check_unsafety.rs b/compiler/rustc_mir_build/src/check_unsafety.rs
index ee35539221e6..aed0c6e6085d 100644
--- a/compiler/rustc_mir_build/src/check_unsafety.rs
+++ b/compiler/rustc_mir_build/src/check_unsafety.rs
@@ -315,6 +315,7 @@ fn visit_pat(&mut self, pat: &'a Pat<'tcx>) {
                 | PatKind::Range { .. }
                 | PatKind::Slice { .. }
                 | PatKind::Array { .. }
+                | PatKind::Guard { .. }
                 // Never constitutes a witness of uninhabitedness.
                 | PatKind::Never => {
                     self.requires_unsafe(pat.span, AccessToUnionField);
diff --git a/compiler/rustc_mir_build/src/thir/print.rs b/compiler/rustc_mir_build/src/thir/print.rs
index 27dec99696f5..ea34e5f4d97d 100644
--- a/compiler/rustc_mir_build/src/thir/print.rs
+++ b/compiler/rustc_mir_build/src/thir/print.rs
@@ -844,6 +844,14 @@ fn print_pat_kind(&mut self, pat_kind: &PatKind<'tcx>, depth_lvl: usize) {
                 print_indented!(self, "]", depth_lvl + 2);
                 print_indented!(self, "}", depth_lvl + 1);
             }
+            PatKind::Guard { subpattern, condition } => {
+                print_indented!(self, "Guard pattern: {", depth_lvl + 1);
+                print_indented!(self, "subpattern: ", depth_lvl + 2);
+                self.print_pat(subpattern, depth_lvl + 3);
+                print_indented!(self, "guard: ", depth_lvl + 2);
+                self.print_expr(*condition, depth_lvl + 3);
+                print_indented!(self, "}", depth_lvl + 1);
+            }
             PatKind::Error(_) => {
                 print_indented!(self, "Error", depth_lvl + 1);
             }
diff --git a/compiler/rustc_pattern_analysis/src/rustc.rs b/compiler/rustc_pattern_analysis/src/rustc.rs
index dc38f2d8bc70..0226c54db58c 100644
--- a/compiler/rustc_pattern_analysis/src/rustc.rs
+++ b/compiler/rustc_pattern_analysis/src/rustc.rs
@@ -462,7 +462,8 @@ pub fn lower_pat(&self, pat: &'p Pat<'tcx>) -> DeconstructedPat<'p, 'tcx> {
         let arity;
         let fields: Vec<_>;
         match &pat.kind {
-            PatKind::Binding { subpattern: Some(subpat), .. } => return self.lower_pat(subpat),
+            PatKind::Binding { subpattern: Some(subpat), .. }
+            | PatKind::Guard { subpattern: subpat, .. } => return self.lower_pat(subpat),
             PatKind::Missing | PatKind::Binding { subpattern: None, .. } | PatKind::Wild => {
                 ctor = Wildcard;
                 fields = vec![];