Consider lifetimes when comparing types for equality in MIR validator

compiler/rustc_const_eval/src/transform/validate.rs

@@ -315,9 +315,8 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                                     | ty::FnPtr(..)
                             )
                         }
-                        // None of the possible types have lifetimes, so we can just compare
-                        // directly
-                        if a != b {
+                        // The function pointer types can have lifetimes
+                        if !self.mir_assign_valid_types(a, b) {
                             self.fail(
                                 location,
                                 format!("Cannot compare unequal types {:?} and {:?}", a, b),
@@ -464,7 +463,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 };
                 // since CopyNonOverlapping is parametrized by 1 type,
                 // we only need to check that they are equal and not keep an extra parameter.
-                if op_src_ty != op_dst_ty {
+                if !self.mir_assign_valid_types(op_src_ty, op_dst_ty) {
                     self.fail(location, format!("bad arg ({:?} != {:?})", op_src_ty, op_dst_ty));
                 }
 

compiler/rustc_middle/src/mir/mod.rs

@@ -2518,7 +2518,8 @@ pub enum Rvalue<'tcx> {
     /// * `Offset` has the same semantics as [`offset`](pointer::offset), except that the second
     ///   parameter may be a `usize` as well.
     /// * The comparison operations accept `bool`s, `char`s, signed or unsigned integers, floats,
-    ///   raw pointers, or function pointers of matching types and return a `bool`.
+    ///   raw pointers, or function pointers and return a `bool`. The types of the operands must be
+    ///   matching, up to the usual caveat of the lifetimes in function pointers.
     /// * Left and right shift operations accept signed or unsigned integers not necessarily of the
     ///   same type and return a value of the same type as their LHS. Like in Rust, the RHS is
     ///   truncated as needed.

src/test/ui/mir/issue-95978-validator-lifetime-comparison.rs

@@ -0,0 +1,10 @@
+// check-pass
+// compile-flags: -Zvalidate-mir
+
+fn foo(_a: &str) {}
+
+fn main() {
+    let x = foo as fn(&'static str);
+
+    let _ = x == foo;
+}