Consider unfulfilled obligations in binop errors

When encountering a binop where the types would have been accepted, if
all the predicates had been fulfilled, include information about the
predicates and suggest appropriate `#[derive]`s if possible.

Point at trait(s) that needs to be `impl`emented.

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -2171,10 +2171,26 @@ impl fmt::Debug for TraitRefPrintOnlyTraitPath<'tcx> {
     }
 }
 
+/// Wrapper type for `ty::TraitRef` which opts-in to pretty printing only
+/// the trait name. That is, it will print `Trait` instead of
+/// `<T as Trait<U>>`.
+#[derive(Copy, Clone, TypeFoldable, Lift)]
+pub struct TraitRefPrintOnlyTraitName<'tcx>(ty::TraitRef<'tcx>);
+
+impl fmt::Debug for TraitRefPrintOnlyTraitName<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self, f)
+    }
+}
+
 impl ty::TraitRef<'tcx> {
     pub fn print_only_trait_path(self) -> TraitRefPrintOnlyTraitPath<'tcx> {
         TraitRefPrintOnlyTraitPath(self)
     }
+
+    pub fn print_only_trait_name(self) -> TraitRefPrintOnlyTraitName<'tcx> {
+        TraitRefPrintOnlyTraitName(self)
+    }
 }
 
 impl ty::Binder<'tcx, ty::TraitRef<'tcx>> {
@@ -2193,6 +2209,7 @@ forward_display_to_print! {
     ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>,
     ty::Binder<'tcx, ty::TraitRef<'tcx>>,
     ty::Binder<'tcx, TraitRefPrintOnlyTraitPath<'tcx>>,
+    ty::Binder<'tcx, TraitRefPrintOnlyTraitName<'tcx>>,
     ty::Binder<'tcx, ty::FnSig<'tcx>>,
     ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
     ty::Binder<'tcx, ty::SubtypePredicate<'tcx>>,
@@ -2255,6 +2272,10 @@ define_print_and_forward_display! {
         p!(print_def_path(self.0.def_id, self.0.substs));
     }
 
+    TraitRefPrintOnlyTraitName<'tcx> {
+        p!(print_def_path(self.0.def_id, &[]));
+    }
+
     ty::ParamTy {
         p!(write("{}", self.name))
     }