Avoid suggesting inside macro call

Specifically avoid non-sense suggestion when calling `panic!` with
something `!Send`.

Author: estebank

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -1537,6 +1537,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
                     let generics = self.tcx.generics_of(*def_id);
                     if generics.params.iter().any(|p| p.name != kw::SelfUpper)
                         && !snippet.ends_with('>')
+                        && !span.from_expansion()
                     {
                         // FIXME: To avoid spurious suggestions in functions where type arguments
                         // where already supplied, we check the snippet to make sure it doesn't

src/test/ui/issues/issue-16966.stderr

@@ -11,17 +11,6 @@ LL | pub fn begin_panic<M: Any + Send>(msg: M) -> ! {
    |
    = note: cannot satisfy `_: Send`
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
-help: consider specifying the type argument in the function call
-   |
-LL |     ($msg:expr) => ({ $crate::rt::begin_panic::<IoSlice<'a>>($msg) });
-   |                                              ^^^^^^^^^^^^^^^
-LL |     ($msg:expr) => ({ $crate::rt::begin_panic::<IoSliceMut<'a>>($msg) });
-   |                                              ^^^^^^^^^^^^^^^^^^
-LL |     ($msg:expr) => ({ $crate::rt::begin_panic::<ResumeTy>($msg) });
-   |                                              ^^^^^^^^^^^^
-LL |     ($msg:expr) => ({ $crate::rt::begin_panic::<Waker>($msg) });
-   |                                              ^^^^^^^^^
-     and 13 other candidates
 
 error: aborting due to previous error