Rollup merge of #38150 - estebank:fix-23286, r=nikomatsakis

Point out the known type when field doesn't satisfy bound

For file

```rust
use std::path::Path;

fn f(p: Path) { }
```

provide the following error

```nocode
error[E0277]: the trait bound `[u8]: std::marker::Sized` is not satisfied in `std::path::Path`
 --> file.rs:3:6
  |
3 | fn f(p: Path) { }
  |      ^ within `std::path::Path`, the trait `std::marker::Sized` is not implemented for `[u8]`
  |
  = note: `[u8]` does not have a constant size known at compile-time
  = note: required because it appears within the type `std::path::Path`
  = note: all local variables must have a statically known size
```

Fix #23286.

src/librustc/traits/error_reporting.rs

@@ -458,11 +458,28 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         err
     }
 
+
+    /// Get the parent trait chain start
+    fn get_parent_trait_ref(&self, code: &ObligationCauseCode<'tcx>) -> Option<String> {
+        match code {
+            &ObligationCauseCode::BuiltinDerivedObligation(ref data) => {
+                let parent_trait_ref = self.resolve_type_vars_if_possible(
+                    &data.parent_trait_ref);
+                match self.get_parent_trait_ref(&data.parent_code) {
+                    Some(t) => Some(t),
+                    None => Some(format!("{}", parent_trait_ref.0.self_ty())),
+                }
+            }
+            _ => None,
+        }
+    }
+
     pub fn report_selection_error(&self,
                                   obligation: &PredicateObligation<'tcx>,
                                   error: &SelectionError<'tcx>)
     {
         let span = obligation.cause.span;
+
         let mut err = match *error {
             SelectionError::Unimplemented => {
                 if let ObligationCauseCode::CompareImplMethodObligation {
@@ -487,14 +504,27 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                                 return;
                             } else {
                                 let trait_ref = trait_predicate.to_poly_trait_ref();
-
-                                let mut err = struct_span_err!(self.tcx.sess, span, E0277,
-                                    "the trait bound `{}` is not satisfied",
-                                    trait_ref.to_predicate());
-                                err.span_label(span, &format!("the trait `{}` is not implemented \
-                                                               for `{}`",
-                                                              trait_ref,
-                                                              trait_ref.self_ty()));
+                                let (post_message, pre_message) = match self.get_parent_trait_ref(
+                                    &obligation.cause.code)
+                                {
+                                    Some(t) => {
+                                        (format!(" in `{}`", t), format!("within `{}`, ", t))
+                                    }
+                                    None => (String::new(), String::new()),
+                                };
+                                let mut err = struct_span_err!(
+                                    self.tcx.sess,
+                                    span,
+                                    E0277,
+                                    "the trait bound `{}` is not satisfied{}",
+                                    trait_ref.to_predicate(),
+                                    post_message);
+                                err.span_label(span,
+                                               &format!("{}the trait `{}` is not \
+                                                         implemented for `{}`",
+                                                        pre_message,
+                                                        trait_ref,
+                                                        trait_ref.self_ty()));
 
                                 // Try to report a help message