From 4a886add5c472aaa8d68c572b32566b57ee51315 Mon Sep 17 00:00:00 2001
From: yukang <moorekang@gmail.com>
Date: Wed, 15 Oct 2025 10:00:15 +0800
Subject: [PATCH] Fix ICE in pattern matching with generic const array length
 errors

---
 .../src/builder/matches/match_pair.rs         | 24 +++++++++++++------
 ...generic-const-array-pattern-ice-139815.rs} |  6 +++--
 ...eric-const-array-pattern-ice-139815.stderr | 16 +++++++++++++
 3 files changed, 37 insertions(+), 9 deletions(-)
 rename tests/{crashes/139815.rs => ui/const-generics/generic-const-array-pattern-ice-139815.rs} (61%)
 create mode 100644 tests/ui/const-generics/generic-const-array-pattern-ice-139815.stderr

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 82c769eb3d584..c3028b9b5c4ea 100644
--- a/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/match_pair.rs
@@ -43,13 +43,23 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     ) {
         let tcx = self.tcx;
         let (min_length, exact_size) = if let Some(place_resolved) = place.try_to_place(self) {
-            match place_resolved.ty(&self.local_decls, tcx).ty.kind() {
-                ty::Array(_, length) => (
-                    length
-                        .try_to_target_usize(tcx)
-                        .expect("expected len of array pat to be definite"),
-                    true,
-                ),
+            let place_ty = place_resolved.ty(&self.local_decls, tcx).ty;
+            match place_ty.kind() {
+                ty::Array(_, length) => {
+                    if let Some(length) = length.try_to_target_usize(tcx) {
+                        (length, true)
+                    } else {
+                        // This can happen when the array length is a generic const
+                        // expression that couldn't be evaluated (e.g., due to an error).
+                        // Since there's already a compilation error, we use a fallback
+                        // to avoid an ICE.
+                        tcx.dcx().span_delayed_bug(
+                            tcx.def_span(self.def_id),
+                            "array length in pattern couldn't be evaluated",
+                        );
+                        ((prefix.len() + suffix.len()).try_into().unwrap(), false)
+                    }
+                }
                 _ => ((prefix.len() + suffix.len()).try_into().unwrap(), false),
             }
         } else {
diff --git a/tests/crashes/139815.rs b/tests/ui/const-generics/generic-const-array-pattern-ice-139815.rs
similarity index 61%
rename from tests/crashes/139815.rs
rename to tests/ui/const-generics/generic-const-array-pattern-ice-139815.rs
index 9094acdc94b21..52f4b246d069f 100644
--- a/tests/crashes/139815.rs
+++ b/tests/ui/const-generics/generic-const-array-pattern-ice-139815.rs
@@ -1,8 +1,10 @@
-//@ known-bug: #139815
-
+//@ compile-flags: --crate-type=lib
+#![allow(incomplete_features)]
 #![feature(generic_const_exprs)]
+
 fn is_123<const N: usize>(
     x: [u32; {
+        //~^ ERROR overly complex generic constant
         N + 1;
         5
     }],
diff --git a/tests/ui/const-generics/generic-const-array-pattern-ice-139815.stderr b/tests/ui/const-generics/generic-const-array-pattern-ice-139815.stderr
new file mode 100644
index 0000000000000..9da973ef3a04e
--- /dev/null
+++ b/tests/ui/const-generics/generic-const-array-pattern-ice-139815.stderr
@@ -0,0 +1,16 @@
+error: overly complex generic constant
+  --> $DIR/generic-const-array-pattern-ice-139815.rs:6:14
+   |
+LL |       x: [u32; {
+   |  ______________^
+LL | |
+LL | |         N + 1;
+LL | |         5
+LL | |     }],
+   | |_____^ blocks are not supported in generic constants
+   |
+   = help: consider moving this anonymous constant into a `const` function
+   = note: this operation may be supported in the future
+
+error: aborting due to 1 previous error
+