Rollup merge of #145651 - borsakv:139093, r=cjgillot

Regression test for const promotion with Option<Ordering>

https://rust.godbolt.org/z/EjxqE8WcT

Fixes #139093

Add a regression test to ensure that comparing `Option<Ordering>` to
`Some(Ordering::Equal)` does not trigger unnecessary const promotion
in MIR.

Previously, inlined constants like `Some(Ordering::Equal)` would get
promoted, leading to more complex MIR and redundant LLVM IR checks.
This test verifies that both the direct form and the `let`-binding form
now generate equivalent, simplified MIR.

r? cjgillot

Author: dianqk

tests/mir-opt/pre-codegen/const_promotion_option_ordering_eq.direct.PreCodegen.after.mir

@@ -0,0 +1,46 @@
+// MIR for `direct` after PreCodegen
+
+fn direct(_1: Option<std::cmp::Ordering>) -> bool {
+    debug e => _1;
+    let mut _0: bool;
+    scope 1 (inlined <Option<std::cmp::Ordering> as PartialEq>::eq) {
+        let mut _2: isize;
+        scope 2 {
+            scope 3 (inlined <std::cmp::Ordering as PartialEq>::eq) {
+                let _3: i8;
+                scope 4 {
+                    scope 5 {
+                    }
+                }
+            }
+        }
+    }
+
+    bb0: {
+        StorageLive(_2);
+        _2 = discriminant(_1);
+        switchInt(move _2) -> [0: bb1, 1: bb2, otherwise: bb4];
+    }
+
+    bb1: {
+        _0 = const false;
+        goto -> bb3;
+    }
+
+    bb2: {
+        StorageLive(_3);
+        _3 = discriminant(((_1 as Some).0: std::cmp::Ordering));
+        _0 = Eq(copy _3, const 0_i8);
+        StorageDead(_3);
+        goto -> bb3;
+    }
+
+    bb3: {
+        StorageDead(_2);
+        return;
+    }
+
+    bb4: {
+        unreachable;
+    }
+}

tests/mir-opt/pre-codegen/const_promotion_option_ordering_eq.rs

@@ -0,0 +1,30 @@
+//@ compile-flags: -O -Zmir-opt-level=2 -Cdebuginfo=0
+
+// Check that comparing `Option<Ordering>` to a constant inlined `Some(...)`
+// does not produce unnecessarily complex MIR compared to using a local binding.
+//
+// Regression test for <https://github.com/rust-lang/rust/issues/139093>.
+// Originally, inlined constants like `Some(Ordering::Equal)` would get promoted,
+// leading to more MIR (and extra LLVM IR checks) than necessary.
+// Both cases should now generate identical MIR.
+
+use std::cmp::Ordering;
+
+// EMIT_MIR const_promotion_option_ordering_eq.direct.PreCodegen.after.mir
+pub fn direct(e: Option<Ordering>) -> bool {
+    // CHECK-LABEL: fn direct(
+    // CHECK-NOT: promoted[
+    // CHECK: switchInt(
+    // CHECK: return
+    e == Some(Ordering::Equal)
+}
+
+// EMIT_MIR const_promotion_option_ordering_eq.with_let.PreCodegen.after.mir
+pub fn with_let(e: Option<Ordering>) -> bool {
+    // CHECK-LABEL: fn with_let(
+    // CHECK-NOT: promoted[
+    // CHECK: switchInt(
+    // CHECK: return
+    let eq = Ordering::Equal;
+    e == Some(eq)
+}

tests/mir-opt/pre-codegen/const_promotion_option_ordering_eq.with_let.PreCodegen.after.mir

@@ -0,0 +1,49 @@
+// MIR for `with_let` after PreCodegen
+
+fn with_let(_1: Option<std::cmp::Ordering>) -> bool {
+    debug e => _1;
+    let mut _0: bool;
+    scope 1 {
+        debug eq => const Equal;
+        scope 2 (inlined <Option<std::cmp::Ordering> as PartialEq>::eq) {
+            let mut _2: isize;
+            scope 3 {
+                scope 4 (inlined <std::cmp::Ordering as PartialEq>::eq) {
+                    let _3: i8;
+                    scope 5 {
+                        scope 6 {
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    bb0: {
+        StorageLive(_2);
+        _2 = discriminant(_1);
+        switchInt(move _2) -> [0: bb1, 1: bb2, otherwise: bb4];
+    }
+
+    bb1: {
+        _0 = const false;
+        goto -> bb3;
+    }
+
+    bb2: {
+        StorageLive(_3);
+        _3 = discriminant(((_1 as Some).0: std::cmp::Ordering));
+        _0 = Eq(copy _3, const 0_i8);
+        StorageDead(_3);
+        goto -> bb3;
+    }
+
+    bb3: {
+        StorageDead(_2);
+        return;
+    }
+
+    bb4: {
+        unreachable;
+    }
+}