Rollup merge of #54702 - RalfJung:fn-ptr-promotion, r=oli-obk

do not promote comparing function pointers

This *could* break existing code that relied on fn ptr comparison getting promoted to `'static` lifetime.

Fixes https://github.com/rust-lang/rust/issues/54696

src/test/run-pass/issues/issue-54696.rs

@@ -0,0 +1,8 @@
+// run-pass
+
+fn main() {
+    // We shouldn't promote this
+    &(main as fn() == main as fn());
+    // Also check nested case
+    &(&(main as fn()) == &(main as fn()));
+}

src/test/ui/consts/const-eval/promoted_raw_ptr_ops.nll.stderr

@@ -14,7 +14,7 @@ error[E0716]: temporary value dropped while borrowed
    |
 LL |     let y: &'static usize = &(&1 as *const i32 as usize + 1); //~ ERROR does not live long enough
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
-LL |     let z: &'static i32 = &(unsafe { *(42 as *const i32) }); //~ ERROR does not live long enough
+...
 LL | }
    | - temporary value is freed at the end of this statement
    |
@@ -25,11 +25,22 @@ error[E0716]: temporary value dropped while borrowed
    |
 LL |     let z: &'static i32 = &(unsafe { *(42 as *const i32) }); //~ ERROR does not live long enough
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
+LL |     let a: &'static bool = &(main as fn() == main as fn()); //~ ERROR does not live long enough
 LL | }
    | - temporary value is freed at the end of this statement
    |
    = note: borrowed value must be valid for the static lifetime...
 
-error: aborting due to 3 previous errors
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promoted_raw_ptr_ops.rs:18:29
+   |
+LL |     let a: &'static bool = &(main as fn() == main as fn()); //~ ERROR does not live long enough
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0716`.

src/test/ui/consts/const-eval/promoted_raw_ptr_ops.rs

@@ -15,4 +15,5 @@ fn main() {
     //~^ ERROR does not live long enough
     let y: &'static usize = &(&1 as *const i32 as usize + 1); //~ ERROR does not live long enough
     let z: &'static i32 = &(unsafe { *(42 as *const i32) }); //~ ERROR does not live long enough
+    let a: &'static bool = &(main as fn() == main as fn()); //~ ERROR does not live long enough
 }

src/test/ui/consts/const-eval/promoted_raw_ptr_ops.stderr

@@ -14,7 +14,7 @@ error[E0597]: borrowed value does not live long enough
    |
 LL |     let y: &'static usize = &(&1 as *const i32 as usize + 1); //~ ERROR does not live long enough
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
-LL |     let z: &'static i32 = &(unsafe { *(42 as *const i32) }); //~ ERROR does not live long enough
+...
 LL | }
    | - temporary value only lives until here
    |
@@ -25,11 +25,22 @@ error[E0597]: borrowed value does not live long enough
    |
 LL |     let z: &'static i32 = &(unsafe { *(42 as *const i32) }); //~ ERROR does not live long enough
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
+LL |     let a: &'static bool = &(main as fn() == main as fn()); //~ ERROR does not live long enough
 LL | }
    | - temporary value only lives until here
    |
    = note: borrowed value must be valid for the static lifetime...
 
-error: aborting due to 3 previous errors
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/promoted_raw_ptr_ops.rs:18:29
+   |
+LL |     let a: &'static bool = &(main as fn() == main as fn()); //~ ERROR does not live long enough
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0597`.