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_borrowck/src/diagnostics/conflict_errors.rs

@@ -348,13 +348,13 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
             expr: Option<&'hir hir::Expr<'hir>>,
             pat: Option<&'hir hir::Pat<'hir>>,
             parent_pat: Option<&'hir hir::Pat<'hir>>,
-            hir: rustc_middle::hir::map::Map<'hir>,
+            tcx: TyCtxt<'hir>,
         }
         impl<'hir> Visitor<'hir> for ExpressionFinder<'hir> {
             type NestedFilter = OnlyBodies;
 
-            fn nested_visit_map(&mut self) -> Self::Map {
-                self.hir
+            fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+                self.tcx
             }
 
             fn visit_expr(&mut self, e: &'hir hir::Expr<'hir>) {
@@ -396,7 +396,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 expr: None,
                 pat: None,
                 parent_pat: None,
-                hir,
+                tcx: self.infcx.tcx,
             };
             finder.visit_expr(expr);
             if let Some(span) = span
@@ -2455,7 +2455,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         let body_expr = tcx.hir_body(body_id).value;
 
         struct ClosureFinder<'hir> {
-            hir: rustc_middle::hir::map::Map<'hir>,
+            tcx: TyCtxt<'hir>,
             borrow_span: Span,
             res: Option<(&'hir hir::Expr<'hir>, &'hir hir::Closure<'hir>)>,
             /// The path expression with the `borrow_span` span
@@ -2464,8 +2464,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         impl<'hir> Visitor<'hir> for ClosureFinder<'hir> {
             type NestedFilter = OnlyBodies;
 
-            fn nested_visit_map(&mut self) -> Self::Map {
-                self.hir
+            fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+                self.tcx
             }
 
             fn visit_expr(&mut self, ex: &'hir hir::Expr<'hir>) {
@@ -2491,7 +2491,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
 
         // Find the closure that most tightly wraps `capture_kind_span`
         let mut finder =
-            ClosureFinder { hir, borrow_span: capture_kind_span, res: None, error_path: None };
+            ClosureFinder { tcx, borrow_span: capture_kind_span, res: None, error_path: None };
         finder.visit_expr(body_expr);
         let Some((closure_expr, closure)) = finder.res else { return };
 

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -256,8 +256,8 @@ impl<'tcx> BorrowExplanation<'tcx> {
 
                         impl<'hir> rustc_hir::intravisit::Visitor<'hir> for FindLetExpr<'hir> {
                             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, expr: &'hir hir::Expr<'hir>) {
                                 if let hir::ExprKind::If(cond, _conseq, _alt)

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -7,7 +7,7 @@ use rustc_hir::intravisit::Visitor;
 use rustc_hir::{self as hir, CaptureBy, ExprKind, HirId, Node};
 use rustc_middle::bug;
 use rustc_middle::mir::*;
-use rustc_middle::ty::{self, Ty};
+use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_mir_dataflow::move_paths::{LookupResult, MovePathIndex};
 use rustc_span::{BytePos, ExpnKind, MacroKind, Span};
 use rustc_trait_selection::error_reporting::traits::FindExprBySpan;
@@ -690,7 +690,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         /// make it bind by reference instead (if possible)
         struct BindingFinder<'tcx> {
             typeck_results: &'tcx ty::TypeckResults<'tcx>,
-            hir: rustc_middle::hir::map::Map<'tcx>,
+            tcx: TyCtxt<'tcx>,
             /// Input: the span of the pattern we're finding bindings in
             pat_span: Span,
             /// Input: the spans of the bindings we're providing suggestions for
@@ -709,8 +709,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         impl<'tcx> Visitor<'tcx> for BindingFinder<'tcx> {
             type NestedFilter = rustc_middle::hir::nested_filter::OnlyBodies;
 
-            fn nested_visit_map(&mut self) -> Self::Map {
-                self.hir
+            fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+                self.tcx
             }
 
             fn visit_expr(&mut self, ex: &'tcx hir::Expr<'tcx>) -> Self::Result {
@@ -782,7 +782,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         let typeck_results = self.infcx.tcx.typeck(self.mir_def_id());
         let mut finder = BindingFinder {
             typeck_results,
-            hir,
+            tcx: self.infcx.tcx,
             pat_span,
             binding_spans,
             found_pat: false,