Overhaul the intravisit::Map trait.

First of all, note that `Map` has three different relevant meanings.
- The `intravisit::Map` trait.
- The `map::Map` struct.
- The `NestedFilter::Map` associated type.

The `intravisit::Map` trait is impl'd twice.
- For `!`, where the methods are all unreachable.
- For `map::Map`, which gets HIR stuff from the `TyCtxt`.

As part of getting rid of `map::Map`, this commit changes `impl
intravisit::Map for map::Map` to `impl intravisit::Map for TyCtxt`. It's
fairly straightforward except various things are renamed, because the
existing names would no longer have made sense.

- `trait intravisit::Map` becomes `trait intravisit::HirTyCtxt`, so named
  because it gets some HIR stuff from a `TyCtxt`.
- `NestedFilter::Map` assoc type becomes `NestedFilter::MaybeTyCtxt`,
  because it's always `!` or `TyCtxt`.
- `Visitor::nested_visit_map` becomes `Visitor::maybe_tcx`.

I deliberately made the new trait and associated type names different to
avoid the old `type Map: Map` situation, which I found confusing. We now
have `type MaybeTyCtxt: HirTyCtxt`.

compiler/rustc_trait_selection/src/error_reporting/infer/need_type_info.rs

@@ -1202,8 +1202,8 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
 impl<'a, 'tcx> Visitor<'tcx> for FindInferSourceVisitor<'a, 'tcx> {
     type NestedFilter = nested_filter::OnlyBodies;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.tecx.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tecx.tcx
     }
 
     fn visit_local(&mut self, local: &'tcx LetStmt<'tcx>) {

compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/find_anon_type.rs

@@ -3,7 +3,6 @@ use core::ops::ControlFlow;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::{self, Visitor, VisitorExt};
 use rustc_hir::{self as hir, AmbigArg};
-use rustc_middle::hir::map::Map;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::middle::resolve_bound_vars as rbv;
 use rustc_middle::ty::{self, Region, TyCtxt};
@@ -70,8 +69,8 @@ impl<'tcx> Visitor<'tcx> for FindNestedTypeVisitor<'tcx> {
     type Result = ControlFlow<&'tcx hir::Ty<'tcx>>;
     type NestedFilter = nested_filter::OnlyBodies;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tcx
     }
 
     fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx, AmbigArg>) -> Self::Result {
@@ -176,8 +175,8 @@ impl<'tcx> Visitor<'tcx> for TyPathVisitor<'tcx> {
     type Result = ControlFlow<()>;
     type NestedFilter = nested_filter::OnlyBodies;
 
-    fn nested_visit_map(&mut self) -> Map<'tcx> {
-        self.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tcx
     }
 
     fn visit_lifetime(&mut self, lifetime: &hir::Lifetime) -> Self::Result {

compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/trait_impl_difference.rs

@@ -133,8 +133,8 @@ struct TypeParamSpanVisitor<'tcx> {
 impl<'tcx> Visitor<'tcx> for TypeParamSpanVisitor<'tcx> {
     type NestedFilter = nested_filter::OnlyBodies;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tcx
     }
 
     fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx, AmbigArg>) {

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

@@ -68,8 +68,8 @@ impl<'hir> FindExprBySpan<'hir> {
 impl<'v> Visitor<'v> for FindExprBySpan<'v> {
     type NestedFilter = rustc_middle::hir::nested_filter::OnlyBodies;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tcx
     }
 
     fn visit_expr(&mut self, ex: &'v hir::Expr<'v>) {