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`.

Author: nnethercote

Diff for: 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 };
 

Diff for: 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)

Diff for: 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,

Diff for: compiler/rustc_hir/src/intravisit.rs

@@ -18,7 +18,7 @@
 //!    within one another.
 //!    - Example: Examine each expression to look for its type and do some check or other.
 //!    - How: Implement `intravisit::Visitor` and override the `NestedFilter` type to
-//!      `nested_filter::OnlyBodies` (and implement `nested_visit_map`), and use
+//!      `nested_filter::OnlyBodies` (and implement `maybe_tcx`), and use
 //!      `tcx.hir().visit_all_item_likes_in_crate(&mut visitor)`. Within your
 //!      `intravisit::Visitor` impl, implement methods like `visit_expr()` (don't forget to invoke
 //!      `intravisit::walk_expr()` to keep walking the subparts).
@@ -30,7 +30,7 @@
 //!    - Example: Lifetime resolution, which wants to bring lifetimes declared on the
 //!      impl into scope while visiting the impl-items, and then back out again.
 //!    - How: Implement `intravisit::Visitor` and override the `NestedFilter` type to
-//!      `nested_filter::All` (and implement `nested_visit_map`). Walk your crate with
+//!      `nested_filter::All` (and implement `maybe_tcx`). Walk your crate with
 //!      `tcx.hir().walk_toplevel_module(visitor)` invoked on `tcx.hir().krate()`.
 //!    - Pro: Visitor methods for any kind of HIR node, not just item-like things.
 //!    - Pro: Preserves nesting information
@@ -106,41 +106,43 @@ impl<'a> FnKind<'a> {
     }
 }
 
-/// An abstract representation of the HIR `rustc_middle::hir::map::Map`.
-pub trait Map<'hir> {
+/// HIR things retrievable from `TyCtxt`, avoiding an explicit dependence on
+/// `TyCtxt`. The only impls are for `!` (where these functions are never
+/// called) and `TyCtxt` (in `rustc_middle`).
+pub trait HirTyCtxt<'hir> {
     /// Retrieves the `Node` corresponding to `id`.
     fn hir_node(&self, hir_id: HirId) -> Node<'hir>;
-    fn body(&self, id: BodyId) -> &'hir Body<'hir>;
-    fn item(&self, id: ItemId) -> &'hir Item<'hir>;
-    fn trait_item(&self, id: TraitItemId) -> &'hir TraitItem<'hir>;
-    fn impl_item(&self, id: ImplItemId) -> &'hir ImplItem<'hir>;
-    fn foreign_item(&self, id: ForeignItemId) -> &'hir ForeignItem<'hir>;
+    fn hir_body(&self, id: BodyId) -> &'hir Body<'hir>;
+    fn hir_item(&self, id: ItemId) -> &'hir Item<'hir>;
+    fn hir_trait_item(&self, id: TraitItemId) -> &'hir TraitItem<'hir>;
+    fn hir_impl_item(&self, id: ImplItemId) -> &'hir ImplItem<'hir>;
+    fn hir_foreign_item(&self, id: ForeignItemId) -> &'hir ForeignItem<'hir>;
 }
 
-// Used when no map is actually available, forcing manual implementation of nested visitors.
-impl<'hir> Map<'hir> for ! {
+// Used when no tcx is actually available, forcing manual implementation of nested visitors.
+impl<'hir> HirTyCtxt<'hir> for ! {
     fn hir_node(&self, _: HirId) -> Node<'hir> {
         unreachable!();
     }
-    fn body(&self, _: BodyId) -> &'hir Body<'hir> {
+    fn hir_body(&self, _: BodyId) -> &'hir Body<'hir> {
         unreachable!();
     }
-    fn item(&self, _: ItemId) -> &'hir Item<'hir> {
+    fn hir_item(&self, _: ItemId) -> &'hir Item<'hir> {
         unreachable!();
     }
-    fn trait_item(&self, _: TraitItemId) -> &'hir TraitItem<'hir> {
+    fn hir_trait_item(&self, _: TraitItemId) -> &'hir TraitItem<'hir> {
         unreachable!();
     }
-    fn impl_item(&self, _: ImplItemId) -> &'hir ImplItem<'hir> {
+    fn hir_impl_item(&self, _: ImplItemId) -> &'hir ImplItem<'hir> {
         unreachable!();
     }
-    fn foreign_item(&self, _: ForeignItemId) -> &'hir ForeignItem<'hir> {
+    fn hir_foreign_item(&self, _: ForeignItemId) -> &'hir ForeignItem<'hir> {
         unreachable!();
     }
 }
 
 pub mod nested_filter {
-    use super::Map;
+    use super::HirTyCtxt;
 
     /// Specifies what nested things a visitor wants to visit. By "nested
     /// things", we are referring to bits of HIR that are not directly embedded
@@ -155,7 +157,7 @@ pub mod nested_filter {
     /// See the comments at [`rustc_hir::intravisit`] for more details on the overall
     /// visit strategy.
     pub trait NestedFilter<'hir> {
-        type Map: Map<'hir>;
+        type MaybeTyCtxt: HirTyCtxt<'hir>;
 
         /// Whether the visitor visits nested "item-like" things.
         /// E.g., item, impl-item.
@@ -171,10 +173,10 @@ pub mod nested_filter {
     ///
     /// Use this if you are only walking some particular kind of tree
     /// (i.e., a type, or fn signature) and you don't want to thread a
-    /// HIR map around.
+    /// `tcx` around.
     pub struct None(());
     impl NestedFilter<'_> for None {
-        type Map = !;
+        type MaybeTyCtxt = !;
         const INTER: bool = false;
         const INTRA: bool = false;
     }
@@ -199,18 +201,18 @@ use nested_filter::NestedFilter;
 /// to monitor future changes to `Visitor` in case a new method with a
 /// new default implementation gets introduced.)
 pub trait Visitor<'v>: Sized {
-    // This type should not be overridden, it exists for convenient usage as `Self::Map`.
-    type Map: Map<'v> = <Self::NestedFilter as NestedFilter<'v>>::Map;
+    // This type should not be overridden, it exists for convenient usage as `Self::MaybeTyCtxt`.
+    type MaybeTyCtxt: HirTyCtxt<'v> = <Self::NestedFilter as NestedFilter<'v>>::MaybeTyCtxt;
 
     ///////////////////////////////////////////////////////////////////////////
     // Nested items.
 
     /// Override this type to control which nested HIR are visited; see
     /// [`NestedFilter`] for details. If you override this type, you
-    /// must also override [`nested_visit_map`](Self::nested_visit_map).
+    /// must also override [`maybe_tcx`](Self::maybe_tcx).
     ///
     /// **If for some reason you want the nested behavior, but don't
-    /// have a `Map` at your disposal:** then override the
+    /// have a `tcx` at your disposal:** then override the
     /// `visit_nested_XXX` methods. If a new `visit_nested_XXX` variant is
     /// added in the future, it will cause a panic which can be detected
     /// and fixed appropriately.
@@ -222,9 +224,9 @@ pub trait Visitor<'v>: Sized {
 
     /// If `type NestedFilter` is set to visit nested items, this method
     /// must also be overridden to provide a map to retrieve nested items.
-    fn nested_visit_map(&mut self) -> Self::Map {
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
         panic!(
-            "nested_visit_map must be implemented or consider using \
+            "maybe_tcx must be implemented or consider using \
             `type NestedFilter = nested_filter::None` (the default)"
         );
     }
@@ -236,10 +238,10 @@ pub trait Visitor<'v>: Sized {
     /// "deep" visit patterns described at
     /// [`rustc_hir::intravisit`]. The only reason to override
     /// this method is if you want a nested pattern but cannot supply a
-    /// [`Map`]; see `nested_visit_map` for advice.
+    /// [`TyCtxt`]; see `maybe_tcx` for advice.
     fn visit_nested_item(&mut self, id: ItemId) -> Self::Result {
         if Self::NestedFilter::INTER {
-            let item = self.nested_visit_map().item(id);
+            let item = self.maybe_tcx().hir_item(id);
             try_visit!(self.visit_item(item));
         }
         Self::Result::output()
@@ -250,7 +252,7 @@ pub trait Visitor<'v>: Sized {
     /// method.
     fn visit_nested_trait_item(&mut self, id: TraitItemId) -> Self::Result {
         if Self::NestedFilter::INTER {
-            let item = self.nested_visit_map().trait_item(id);
+            let item = self.maybe_tcx().hir_trait_item(id);
             try_visit!(self.visit_trait_item(item));
         }
         Self::Result::output()
@@ -261,7 +263,7 @@ pub trait Visitor<'v>: Sized {
     /// method.
     fn visit_nested_impl_item(&mut self, id: ImplItemId) -> Self::Result {
         if Self::NestedFilter::INTER {
-            let item = self.nested_visit_map().impl_item(id);
+            let item = self.maybe_tcx().hir_impl_item(id);
             try_visit!(self.visit_impl_item(item));
         }
         Self::Result::output()
@@ -272,7 +274,7 @@ pub trait Visitor<'v>: Sized {
     /// method.
     fn visit_nested_foreign_item(&mut self, id: ForeignItemId) -> Self::Result {
         if Self::NestedFilter::INTER {
-            let item = self.nested_visit_map().foreign_item(id);
+            let item = self.maybe_tcx().hir_foreign_item(id);
             try_visit!(self.visit_foreign_item(item));
         }
         Self::Result::output()
@@ -283,7 +285,7 @@ pub trait Visitor<'v>: Sized {
     /// `Self::NestedFilter`.
     fn visit_nested_body(&mut self, id: BodyId) -> Self::Result {
         if Self::NestedFilter::INTRA {
-            let body = self.nested_visit_map().body(id);
+            let body = self.maybe_tcx().hir_body(id);
             try_visit!(self.visit_body(body));
         }
         Self::Result::output()

Diff for: compiler/rustc_hir_analysis/src/check/check.rs

@@ -468,8 +468,8 @@ fn best_definition_site_of_opaque<'tcx>(
     impl<'tcx> intravisit::Visitor<'tcx> for TaitConstraintLocator<'tcx> {
         type NestedFilter = nested_filter::All;
         type Result = ControlFlow<(Span, LocalDefId)>;
-        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: &'tcx hir::Expr<'tcx>) -> Self::Result {
             if let hir::ExprKind::Closure(closure) = ex.kind {

Diff for: compiler/rustc_hir_analysis/src/collect.rs

@@ -277,8 +277,8 @@ fn reject_placeholder_type_signatures_in_item<'tcx>(
 impl<'tcx> Visitor<'tcx> for CollectItemTypesVisitor<'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_item(&mut self, item: &'tcx hir::Item<'tcx>) {

Diff for: compiler/rustc_hir_analysis/src/collect/dump.rs

@@ -54,8 +54,8 @@ pub(crate) fn def_parents(tcx: TyCtxt<'_>) {
             impl<'tcx> intravisit::Visitor<'tcx> for AnonConstFinder<'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_anon_const(&mut self, c: &'tcx rustc_hir::AnonConst) {

Diff for: compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -422,8 +422,8 @@ enum NonLifetimeBinderAllowed {
 impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, '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_nested_body(&mut self, body: hir::BodyId) {

Diff for: compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

@@ -298,8 +298,8 @@ impl TaitConstraintLocator<'_> {
 impl<'tcx> intravisit::Visitor<'tcx> for TaitConstraintLocator<'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_expr(&mut self, ex: &'tcx Expr<'tcx>) {
         if let hir::ExprKind::Closure(closure) = ex.kind {
@@ -441,8 +441,8 @@ impl RpitConstraintChecker<'_> {
 impl<'tcx> intravisit::Visitor<'tcx> for RpitConstraintChecker<'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_expr(&mut self, ex: &'tcx Expr<'tcx>) {
         if let hir::ExprKind::Closure(closure) = ex.kind {

Diff for: compiler/rustc_hir_pretty/src/lib.rs

@@ -24,8 +24,8 @@ use rustc_span::source_map::SourceMap;
 use rustc_span::{FileName, Ident, Span, Symbol, kw};
 use {rustc_ast as ast, rustc_hir as hir};
 
-pub fn id_to_string(map: &dyn rustc_hir::intravisit::Map<'_>, hir_id: HirId) -> String {
-    to_string(&map, |s| s.print_node(map.hir_node(hir_id)))
+pub fn id_to_string(cx: &dyn rustc_hir::intravisit::HirTyCtxt<'_>, hir_id: HirId) -> String {
+    to_string(&cx, |s| s.print_node(cx.hir_node(hir_id)))
 }
 
 pub enum AnnNode<'a> {
@@ -54,15 +54,15 @@ pub trait PpAnn {
     fn post(&self, _state: &mut State<'_>, _node: AnnNode<'_>) {}
 }
 
-impl PpAnn for &dyn rustc_hir::intravisit::Map<'_> {
+impl PpAnn for &dyn rustc_hir::intravisit::HirTyCtxt<'_> {
     fn nested(&self, state: &mut State<'_>, nested: Nested) {
         match nested {
-            Nested::Item(id) => state.print_item(self.item(id)),
-            Nested::TraitItem(id) => state.print_trait_item(self.trait_item(id)),
-            Nested::ImplItem(id) => state.print_impl_item(self.impl_item(id)),
-            Nested::ForeignItem(id) => state.print_foreign_item(self.foreign_item(id)),
-            Nested::Body(id) => state.print_expr(self.body(id).value),
-            Nested::BodyParamPat(id, i) => state.print_pat(self.body(id).params[i].pat),
+            Nested::Item(id) => state.print_item(self.hir_item(id)),
+            Nested::TraitItem(id) => state.print_trait_item(self.hir_trait_item(id)),
+            Nested::ImplItem(id) => state.print_impl_item(self.hir_impl_item(id)),
+            Nested::ForeignItem(id) => state.print_foreign_item(self.hir_foreign_item(id)),
+            Nested::Body(id) => state.print_expr(self.hir_body(id).value),
+            Nested::BodyParamPat(id, i) => state.print_pat(self.hir_body(id).params[i].pat),
         }
     }
 }

Diff for: compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -2735,8 +2735,8 @@ struct FindClosureArg<'tcx> {
 impl<'tcx> Visitor<'tcx> for FindClosureArg<'tcx> {
     type NestedFilter = rustc_middle::hir::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_expr(&mut self, ex: &'tcx hir::Expr<'tcx>) {