Add regression test for rust-lang#86106

Signed-off-by: Yuki Okushi <[email protected]>

Author: JohnTitor

src/test/codegen/issue-86106.rs

@@ -0,0 +1,56 @@
+// min-llvm-version: 15.0
+// compile-flags: -C opt-level=3 -C target-cpu=native
+
+// The below two functions ensure that both `String::new()` and `"".to_string()`
+// produce the identical code.
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @string_new = unnamed_addr alias void (ptr), ptr @empty_to_string
+// CHECK: define void @empty_to_string
+#[no_mangle]
+pub fn string_new() -> String {
+    String::new()
+}
+
+#[no_mangle]
+pub fn empty_to_string() -> String {
+    // CHECK-NOT: load i8
+    // CHECK: store i64
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store ptr
+    // CHECK-NEXT: getelementptr
+    // CHECK-NOT: store i8
+    // CHECK-NEXT: store i64
+    // CHECK-NEXT: ret void
+    "".to_string()
+}
+
+// The below two functions ensure that both `vec![]` and `vec![].clone()`
+// produce the identical code.
+
+// CHECK-LABEL: @empty_vec
+#[no_mangle]
+pub fn empty_vec() -> Vec<u8> {
+    // CHECK: store i64
+    // CHECK-NOT: load i8
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store ptr
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store i64
+    // CHECK-NEXT: ret void
+    vec![]
+}
+
+// CHECK-LABEL: @empty_vec_clone
+#[no_mangle]
+pub fn empty_vec_clone() -> Vec<u8> {
+    // CHECK: store i64
+    // CHECK-NOT: load i8
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store ptr
+    // CHECK-NEXT: getelementptr
+    // CHECK-NEXT: store i64
+    // CHECK-NEXT: ret void
+    vec![].clone()
+}