Have the spans of TAIT type conflict errors point to the actual site instead of the owning function

compiler/rustc_middle/src/ty/mod.rs

@@ -1081,6 +1081,38 @@ pub struct OpaqueTypeKey<'tcx> {
     pub substs: SubstsRef<'tcx>,
 }
 
+#[derive(Copy, Clone, Debug, TypeFoldable, HashStable, TyEncodable, TyDecodable)]
+pub struct OpaqueHiddenType<'tcx> {
+    /// The span of this particular definition of the opaque type. So
+    /// for example:
+    ///
+    /// ```ignore (incomplete snippet)
+    /// type Foo = impl Baz;
+    /// fn bar() -> Foo {
+    /// //          ^^^ This is the span we are looking for!
+    /// }
+    /// ```
+    ///
+    /// In cases where the fn returns `(impl Trait, impl Trait)` or
+    /// other such combinations, the result is currently
+    /// over-approximated, but better than nothing.
+    pub span: Span,
+
+    /// The type variable that represents the value of the opaque type
+    /// that we require. In other words, after we compile this function,
+    /// we will be created a constraint like:
+    ///
+    ///     Foo<'a, T> = ?C
+    ///
+    /// where `?C` is the value of this type variable. =) It may
+    /// naturally refer to the type and lifetime parameters in scope
+    /// in this function, though ultimately it should only reference
+    /// those that are arguments to `Foo` in the constraint above. (In
+    /// other words, `?C` should not include `'b`, even though it's a
+    /// lifetime parameter on `foo`.)
+    pub ty: Ty<'tcx>,
+}
+
 rustc_index::newtype_index! {
     /// "Universes" are used during type- and trait-checking in the
     /// presence of `for<..>` binders to control what sets of names are