From d0b9e866e3e8c6bf0a2876ce8b4f6503ec20ef8c Mon Sep 17 00:00:00 2001
From: Chayim Refael Friedman <chayimfr@gmail.com>
Date: Wed, 19 Nov 2025 05:23:48 +0200
Subject: [PATCH] Allow inferring array sizes

---
 .../rust-analyzer/crates/hir-ty/src/infer/expr.rs   | 12 ++++++++----
 .../rust-analyzer/crates/hir-ty/src/tests/simple.rs | 13 +++++++++++++
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/src/tools/rust-analyzer/crates/hir-ty/src/infer/expr.rs b/src/tools/rust-analyzer/crates/hir-ty/src/infer/expr.rs
index db3303607bad..50bc70f4ee50 100644
--- a/src/tools/rust-analyzer/crates/hir-ty/src/infer/expr.rs
+++ b/src/tools/rust-analyzer/crates/hir-ty/src/infer/expr.rs
@@ -1336,14 +1336,18 @@ fn infer_expr_array(&mut self, array: &Array, expected: &Expectation<'db>) -> Ty
                     ExprIsRead::Yes,
                 );
                 let usize = self.types.usize;
-                match self.body[repeat] {
+                let len = match self.body[repeat] {
                     Expr::Underscore => {
                         self.write_expr_ty(repeat, usize);
+                        self.table.next_const_var()
                     }
-                    _ => _ = self.infer_expr(repeat, &Expectation::HasType(usize), ExprIsRead::Yes),
-                }
+                    _ => {
+                        self.infer_expr(repeat, &Expectation::HasType(usize), ExprIsRead::Yes);
+                        consteval::eval_to_const(repeat, self)
+                    }
+                };
 
-                (elem_ty, consteval::eval_to_const(repeat, self))
+                (elem_ty, len)
             }
         };
         // Try to evaluate unevaluated constant, and insert variable if is not possible.
diff --git a/src/tools/rust-analyzer/crates/hir-ty/src/tests/simple.rs b/src/tools/rust-analyzer/crates/hir-ty/src/tests/simple.rs
index 478411f5a84f..2e107b2c5941 100644
--- a/src/tools/rust-analyzer/crates/hir-ty/src/tests/simple.rs
+++ b/src/tools/rust-analyzer/crates/hir-ty/src/tests/simple.rs
@@ -3943,3 +3943,16 @@ fn foo() {
         "#]],
     );
 }
+
+#[test]
+fn infer_array_size() {
+    check_no_mismatches(
+        r#"
+fn foo(a: [u8; 3]) {}
+
+fn bar() {
+    foo([0; _]);
+}
+    "#,
+    );
+}