nicer errors

Author: RalfJung

Diff for: compiler/rustc_monomorphize/src/collector.rs

@@ -735,14 +735,16 @@ impl<'a, 'tcx> MirUsedCollector<'a, 'tcx> {
 
 impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
     fn visit_body(&mut self, body: &mir::Body<'tcx>) {
+        self.super_body(body);
+
+        // Go over `required_items` *after* body, so if an item appears in both we reach it the
+        // "normal" way, via the body.
         for item in &body.required_items {
             // All these also need monomorphization to ensure that if that leads to error, we find
             // those errors.
             let item = self.monomorphize(*item);
             visit_required_item(self.tcx, item, self.output);
         }
-
-        self.super_body(body);
     }
 
     fn visit_rvalue(&mut self, rvalue: &mir::Rvalue<'tcx>, location: Location) {

Diff for: tests/ui/consts/required-consts/dead-code-in-called-fn.no-opt.stderr

@@ -7,6 +7,10 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn called::<i32>`
+  --> $DIR/dead-code-in-called-fn.rs:23:5
+   |
+LL |     called::<i32>();
+   |     ^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

Diff for: tests/ui/consts/required-consts/dead-code-in-called-fn.opt.stderr

@@ -7,6 +7,10 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn called::<i32>`
+  --> $DIR/dead-code-in-called-fn.rs:23:5
+   |
+LL |     called::<i32>();
+   |     ^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

Diff for: tests/ui/consts/required-consts/dead-code-in-dead-fn.no-opt.stderr

@@ -7,6 +7,10 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn not_called::<i32>`
+  --> $DIR/dead-code-in-dead-fn.rs:26:9
+   |
+LL |         not_called::<T>();
+   |         ^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error
 

Diff for: tests/ui/generics/post_monomorphization_error_backtrace.stderr

@@ -7,6 +7,10 @@ LL |         const V: () = assert!(std::mem::size_of::<T>() == 0);
    = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn assert_zst::<u32>`
+  --> $DIR/post_monomorphization_error_backtrace.rs:18:5
+   |
+LL |     assert_zst::<U>()
+   |     ^^^^^^^^^^^^^^^^^
 
 error[E0080]: evaluation of `assert_zst::F::<i32>::V` failed
   --> $DIR/post_monomorphization_error_backtrace.rs:6:23
@@ -17,6 +21,10 @@ LL |         const V: () = assert!(std::mem::size_of::<T>() == 0);
    = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: the above error was encountered while instantiating `fn assert_zst::<i32>`
+  --> $DIR/post_monomorphization_error_backtrace.rs:18:5
+   |
+LL |     assert_zst::<U>()
+   |     ^^^^^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors