Auto merge of #122225 - DianQK:nits-120268, r=cjgillot

 Rename `UninhabitedEnumBranching` to `UnreachableEnumBranching`

Per [#120268](https://github.com/rust-lang/rust/pull/120268#discussion_r1517492060), I rename `UninhabitedEnumBranching` to `UnreachableEnumBranching` .

I solved some nits to add some comments.

I adjusted the workaround restrictions. This should be useful for `a <= b` and `if let Some/Ok(v)`. For enum with few variants, `early-tailduplication` should not cause compile time overhead.

r? RalfJung

2 files changed, 43 insertions, 24 deletions

diff --git a/tests/codegen/enum/uninhabited_enum_default_branch.rs b/tests/codegen/enum/uninhabited_enum_default_branch.rs
deleted file mode 100644
index 5f318f18dec..00000000000
--- a/tests/codegen/enum/uninhabited_enum_default_branch.rs
+++ /dev/null
@@ -1,24 +0,0 @@
-//@ compile-flags: -O
-
-#![crate_type = "lib"]
-
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
-pub struct Int(u32);
-
-const A: Int = Int(201);
-const B: Int = Int(270);
-const C: Int = Int(153);
-
-// CHECK-LABEL: @foo(
-// CHECK-SAME: [[TMP0:%.*]])
-// CHECK-NEXT:  start:
-// CHECK-NEXT:    [[TMP1:%.*]] = add i32 [[TMP0]], -201
-// CHECK-NEXT:    icmp ult i32 [[TMP1]], 70
-// CHECK-NEXT:    icmp eq i32 [[TMP0]], 153
-// CHECK-NEXT:    [[SPEC_SELECT:%.*]] = or i1
-// CHECK-NEXT:    ret i1 [[SPEC_SELECT]]
-#[no_mangle]
-pub fn foo(x: Int) -> bool {
-    (x >= A && x <= B)
-        || x == C
-}
diff --git a/tests/codegen/enum/unreachable_enum_default_branch.rs b/tests/codegen/enum/unreachable_enum_default_branch.rs
new file mode 100644
index 00000000000..dae01cfb055
--- /dev/null
+++ b/tests/codegen/enum/unreachable_enum_default_branch.rs
@@ -0,0 +1,43 @@
+//@ compile-flags: -O
+
+#![crate_type = "lib"]
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct Int(u32);
+
+const A: Int = Int(201);
+const B: Int = Int(270);
+const C: Int = Int(153);
+
+// The code is from https://github.com/rust-lang/rust/issues/119520.
+// This code will basically turn into `matches!(x.partial_cmp(&A), Some(Greater | Equal))`.
+// The otherwise branch must be `Less`.
+// CHECK-LABEL: @implicit_match(
+// CHECK-SAME: [[TMP0:%.*]])
+// CHECK-NEXT:  start:
+// CHECK-NEXT:    [[TMP1:%.*]] = add i32 [[TMP0]], -201
+// CHECK-NEXT:    icmp ult i32 [[TMP1]], 70
+// CHECK-NEXT:    icmp eq i32 [[TMP0]], 153
+// CHECK-NEXT:    [[SPEC_SELECT:%.*]] = or i1
+// CHECK-NEXT:    ret i1 [[SPEC_SELECT]]
+#[no_mangle]
+pub fn implicit_match(x: Int) -> bool {
+    (x >= A && x <= B)
+        || x == C
+}
+
+// The code is from https://github.com/rust-lang/rust/issues/110097.
+// We expect it to generate the same optimized code as a full match.
+// CHECK-LABEL: @if_let(
+// CHECK-NEXT:  start:
+// CHECK-NEXT: insertvalue
+// CHECK-NEXT: insertvalue
+// CHECK-NEXT: ret
+#[no_mangle]
+pub fn if_let(val: Result<i32, ()>) -> Result<i32, ()> {
+    if let Ok(x) = val {
+        Ok(x)
+    } else {
+        Err(())
+    }
+}