do not promote comparing function pointers

Author: RalfJung

src/librustc_mir/transform/qualify_consts.rs

@@ -750,7 +750,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
             }
 
             Rvalue::BinaryOp(op, ref lhs, _) => {
-                if let ty::RawPtr(_) = lhs.ty(self.mir, self.tcx).sty {
+                if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.mir, self.tcx).sty {
                     assert!(op == BinOp::Eq || op == BinOp::Ne ||
                             op == BinOp::Le || op == BinOp::Lt ||
                             op == BinOp::Ge || op == BinOp::Gt ||

src/librustc_passes/rvalue_promotion.rs

@@ -348,7 +348,7 @@ fn check_expr_kind<'a, 'tcx>(
                 return NotPromotable;
             }
             match v.tables.node_id_to_type(lhs.hir_id).sty {
-                ty::RawPtr(_) => {
+                ty::RawPtr(_) | ty::FnPtr(..) => {
                     assert!(op.node == hir::BinOpKind::Eq || op.node == hir::BinOpKind::Ne ||
                             op.node == hir::BinOpKind::Le || op.node == hir::BinOpKind::Lt ||
                             op.node == hir::BinOpKind::Ge || op.node == hir::BinOpKind::Gt);

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()));
+}