derive: emit intrinsics::unreachable for impls on empty enums

fixes #31574

src/libsyntax_ext/deriving/generic/mod.rs

@@ -381,6 +381,22 @@ fn find_type_parameters(ty: &ast::Ty, ty_param_names: &[ast::Name]) -> Vec<P<ast
     visitor.types
 }
 
+/// Replacement for expr_unreachable which generates intrinsics::unreachable()
+/// instead of unreachable!()
+fn expr_unreachable_intrinsic(cx: &ExtCtxt, sp: Span) -> P<Expr> {
+    let path = cx.std_path(&["intrinsics", "unreachable"]);
+    let call = cx.expr_call_global(
+        sp, path, vec![]);
+    let unreachable = cx.expr_block(P(ast::Block {
+        stmts: vec![],
+        expr: Some(call),
+        id: ast::DUMMY_NODE_ID,
+        rules: ast::BlockCheckMode::Unsafe(ast::CompilerGenerated),
+        span: sp }));
+
+    unreachable
+}
+
 impl<'a> TraitDef<'a> {
     pub fn expand(&self,
                   cx: &mut ExtCtxt,
@@ -1297,16 +1313,7 @@ impl<'a> MethodDef<'a> {
             //Since we know that all the arguments will match if we reach the match expression we
             //add the unreachable intrinsics as the result of the catch all which should help llvm
             //in optimizing it
-            let path = cx.std_path(&["intrinsics", "unreachable"]);
+            match_arms.push(cx.arm(sp, vec![cx.pat_wild(sp)], expr_unreachable_intrinsic(cx, sp)));
-            let call = cx.expr_call_global(
-                sp, path, vec![]);
-            let unreachable = cx.expr_block(P(ast::Block {
-                stmts: vec![],
-                expr: Some(call),
-                id: ast::DUMMY_NODE_ID,
-                rules: ast::BlockCheckMode::Unsafe(ast::CompilerGenerated),
-                span: sp }));
-            match_arms.push(cx.arm(sp, vec![cx.pat_wild(sp)], unreachable));
 
             // Final wrinkle: the self_args are expressions that deref
             // down to desired l-values, but we cannot actually deref
@@ -1382,7 +1389,7 @@ impl<'a> MethodDef<'a> {
             // derive Debug on such a type could here generate code
             // that needs the feature gate enabled.)
 
-            cx.expr_unreachable(sp)
+            expr_unreachable_intrinsic(cx, sp)
         }
         else {