Add initial implementation of HIR-based WF checking for diagnostics

During well-formed checking, we walk through all types 'nested' in
generic arguments. For example, WF-checking `Option<MyStruct<u8>>`
will cause us to check `MyStruct<u8>` and `u8`. However, this is done
on a `rustc_middle::ty::Ty`, which has no span information. As a result,
any errors that occur will have a very general span (e.g. the
definintion of an associated item).

This becomes a problem when macros are involved. In general, an
associated type like `type MyType = Option<MyStruct<u8>>;` may
have completely different spans for each nested type in the HIR. Using
the span of the entire associated item might end up pointing to a macro
invocation, even though a user-provided span is available in one of the
nested types.

This PR adds a framework for HIR-based well formed checking. This check
is only run during error reporting, and is used to obtain a more precise
span for an existing error. This is accomplished by individually
checking each 'nested' type in the HIR for the type, allowing us to
find the most-specific type (and span) that produces a given error.

The majority of the changes are to the error-reporting code. However,
some of the general trait code is modified to pass through more
information.

Since this has no soundness implications, I've implemented a minimal
version to begin with, which can be extended over time. In particular,
this only works for HIR items with a corresponding `DefId` (e.g. it will
not work for WF-checking performed within function bodies).

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

@@ -28,6 +28,7 @@ use crate::traits::error_reporting::InferCtxtExt;
 use crate::traits::project::ProjectionCacheKeyExt;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::stack::ensure_sufficient_stack;
+use rustc_data_structures::sync::Lrc;
 use rustc_errors::ErrorReported;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
@@ -48,7 +49,6 @@ use std::cell::{Cell, RefCell};
 use std::cmp;
 use std::fmt::{self, Display};
 use std::iter;
-use std::rc::Rc;
 
 pub use rustc_middle::traits::select::*;
 
@@ -2168,7 +2168,7 @@ impl<'tcx> TraitObligationExt<'tcx> for TraitObligation<'tcx> {
         // by using -Z verbose or just a CLI argument.
         let derived_cause = DerivedObligationCause {
             parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
-            parent_code: Rc::new(obligation.cause.code.clone()),
+            parent_code: Lrc::new(obligation.cause.code.clone()),
         };
         let derived_code = variant(derived_cause);
         ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)