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`.

src/librustdoc/clean/inline.rs

@@ -703,7 +703,7 @@ fn build_module_items(
 pub(crate) fn print_inlined_const(tcx: TyCtxt<'_>, did: DefId) -> String {
     if let Some(did) = did.as_local() {
         let hir_id = tcx.local_def_id_to_hir_id(did);
-        rustc_hir_pretty::id_to_string(&tcx.hir(), hir_id)
+        rustc_hir_pretty::id_to_string(&tcx, hir_id)
     } else {
         tcx.rendered_const(did).clone()
     }

src/librustdoc/core.rs

@@ -464,10 +464,10 @@ impl<'tcx> EmitIgnoredResolutionErrors<'tcx> {
 impl<'tcx> Visitor<'tcx> for EmitIgnoredResolutionErrors<'tcx> {
     type NestedFilter = nested_filter::OnlyBodies;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
         // We need to recurse into nested closures,
         // since those will fallback to the parent for type checking.
-        self.tcx.hir()
+        self.tcx
     }
 
     fn visit_path(&mut self, path: &Path<'tcx>, _id: HirId) {

src/librustdoc/doctest/rust.rs

@@ -147,14 +147,14 @@ impl HirCollector<'_> {
 impl<'tcx> intravisit::Visitor<'tcx> for HirCollector<'tcx> {
     type NestedFilter = nested_filter::All;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tcx
     }
 
     fn visit_item(&mut self, item: &'tcx hir::Item<'_>) {
         let name = match &item.kind {
             hir::ItemKind::Impl(impl_) => {
-                rustc_hir_pretty::id_to_string(&self.tcx.hir(), impl_.self_ty.hir_id)
+                rustc_hir_pretty::id_to_string(&self.tcx, impl_.self_ty.hir_id)
             }
             _ => item.ident.to_string(),
         };

src/librustdoc/html/render/span_map.rs

@@ -221,8 +221,8 @@ impl SpanMapVisitor<'_> {
 impl<'tcx> Visitor<'tcx> for SpanMapVisitor<'tcx> {
     type NestedFilter = nested_filter::All;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.tcx
     }
 
     fn visit_path(&mut self, path: &rustc_hir::Path<'tcx>, _id: HirId) {

src/librustdoc/scrape_examples.rs

@@ -123,8 +123,8 @@ where
 {
     type NestedFilter = nested_filter::OnlyBodies;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.cx.tcx().hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx()
     }
 
     fn visit_expr(&mut self, ex: &'tcx hir::Expr<'tcx>) {

src/librustdoc/visit_ast.rs

@@ -563,8 +563,8 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
 impl<'tcx> Visitor<'tcx> for RustdocVisitor<'_, 'tcx> {
     type NestedFilter = nested_filter::All;
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.cx.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx
     }
 
     fn visit_item(&mut self, i: &'tcx hir::Item<'tcx>) {