polymorphize: visit promoted MIR

This commit makes polymorphization visited the MIR of unevaluated
constants with available promoted MIR instead of visiting the
substitutions of that constant - which will mark all of the generic
parameters as used.
Signed-off-by: NDavid Wood <david@davidtw.co>

src/librustc_mir/monomorphize/polymorphize.rs

@@ -245,6 +245,13 @@ fn visit_const(&mut self, c: &'tcx Const<'tcx>) -> bool {
                 self.unused_parameters.clear(param.index);
                 false
             }
+            ty::ConstKind::Unevaluated(_, _, Some(p)) => {
+                // If there is a promoted, don't look at the substs - since it will always contain
+                // the generic parameters, instead, traverse the promoted MIR.
+                let promoted = self.tcx.promoted_mir(self.def_id);
+                self.visit_body(&promoted[p]);
+                false
+            }
             _ => c.super_visit_with(self),
         }
     }

src/test/ui/polymorphization/promoted-function-1.rs

+// build-fail
+// compile-flags: -Zpolymorphize=on
+#![crate_type = "lib"]
+#![feature(rustc_attrs)]
+
+fn foo<'a>(_: &'a ()) {}
+
+#[rustc_polymorphize_error]
+pub fn test<T>() {
+    //~^ ERROR item has unused generic parameters
+    foo(&());
+}

src/test/ui/polymorphization/promoted-function-1.stderr

+error: item has unused generic parameters
+  --> $DIR/promoted-function-1.rs:9:8
+   |
+LL | pub fn test<T>() {
+   |        ^^^^ - generic parameter `T` is unused
+
+error: aborting due to previous error
+

src/test/ui/polymorphization/promoted-function.rs

 // run-pass
+// compile-flags:-Zpolymorphize=on
+
 fn fop<T>() {}
 
 fn bar<T>() -> &'static fn() {

src/test/ui/polymorphization/unsized_cast.rs

@@ -17,6 +17,7 @@ fn foo<T: Default>() {
 fn foo2<T: Default>() {
     let _: T = Default::default();
     (|| {
+        //~^ ERROR item has unused generic parameters
         let call: extern "rust-call" fn(_, _) = Fn::call;
         call(&|| {}, ());
         //~^ ERROR item has unused generic parameters

src/test/ui/polymorphization/unsized_cast.stderr

@@ -17,7 +17,7 @@ LL |     (|| Box::new(|| {}) as Box<dyn Fn()>)();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: item has unused generic parameters
-  --> $DIR/unsized_cast.rs:21:15
+  --> $DIR/unsized_cast.rs:22:15
    |
 LL | fn foo2<T: Default>() {
    |         - generic parameter `T` is unused
@@ -25,5 +25,19 @@ LL | fn foo2<T: Default>() {
 LL |         call(&|| {}, ());
    |               ^^^^^
 
-error: aborting due to 3 previous errors
+error: item has unused generic parameters
+  --> $DIR/unsized_cast.rs:19:5
+   |
+LL |   fn foo2<T: Default>() {
+   |           - generic parameter `T` is unused
+LL |       let _: T = Default::default();
+LL | /     (|| {
+LL | |
+LL | |         let call: extern "rust-call" fn(_, _) = Fn::call;
+LL | |         call(&|| {}, ());
+LL | |
+LL | |     })();
+   | |______^
+
+error: aborting due to 4 previous errors