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

clippy_lints/src/derive.rs

@@ -437,8 +437,8 @@ impl<'tcx> Visitor<'tcx> for UnsafeVisitor<'_, 'tcx> {
         walk_expr(self, expr)
     }
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.cx.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx
     }
 }
 

clippy_lints/src/doc/mod.rs

@@ -1079,8 +1079,8 @@ impl<'tcx> Visitor<'tcx> for FindPanicUnwrap<'_, 'tcx> {
     // Panics in const blocks will cause compilation to fail.
     fn visit_anon_const(&mut self, _: &'tcx AnonConst) {}
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.cx.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx
     }
 }
 

clippy_lints/src/extra_unused_type_parameters.rs

@@ -241,8 +241,8 @@ impl<'tcx> Visitor<'tcx> for TypeWalker<'_, 'tcx> {
         }
     }
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.cx.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx
     }
 }
 

clippy_lints/src/from_over_into.rs

@@ -134,8 +134,8 @@ impl<'tcx> Visitor<'tcx> for SelfFinder<'_, 'tcx> {
     type Result = ControlFlow<()>;
     type NestedFilter = 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_path(&mut self, path: &Path<'tcx>, _id: HirId) -> Self::Result {

clippy_lints/src/implicit_hasher.rs

@@ -363,7 +363,7 @@ impl<'tcx> Visitor<'tcx> for ImplicitHasherConstructorVisitor<'_, '_, 'tcx> {
         walk_expr(self, e);
     }
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.cx.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx
     }
 }

clippy_lints/src/index_refutable_slice.rs

@@ -223,8 +223,8 @@ struct SliceIndexLintingVisitor<'a, 'tcx> {
 impl<'tcx> Visitor<'tcx> for SliceIndexLintingVisitor<'_, 'tcx> {
     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, expr: &'tcx hir::Expr<'tcx>) {

clippy_lints/src/lifetimes.rs

@@ -19,8 +19,8 @@ use rustc_hir::{
     WherePredicateKind, lang_items,
 };
 use rustc_lint::{LateContext, LateLintPass, LintContext};
-use rustc_middle::hir::map::Map;
 use rustc_middle::hir::nested_filter as middle_nested_filter;
+use rustc_middle::ty::TyCtxt;
 use rustc_session::impl_lint_pass;
 use rustc_span::Span;
 use rustc_span::def_id::LocalDefId;
@@ -582,7 +582,7 @@ impl<'tcx, F> Visitor<'tcx> for LifetimeChecker<'_, 'tcx, F>
 where
     F: NestedFilter<'tcx>,
 {
-    type Map = Map<'tcx>;
+    type MaybeTyCtxt = TyCtxt<'tcx>;
     type NestedFilter = F;
 
     // for lifetimes as parameters of generics
@@ -628,8 +628,8 @@ where
         self.lifetime_elision_impossible = false;
     }
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.cx.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx
     }
 }
 

clippy_lints/src/loops/utils.rs

@@ -240,8 +240,8 @@ impl<'tcx> Visitor<'tcx> for InitializeVisitor<'_, 'tcx> {
         }
     }
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.cx.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx
     }
 }
 

clippy_lints/src/loops/while_let_on_iterator.rs

@@ -245,8 +245,8 @@ fn needs_mutable_borrow(cx: &LateContext<'_>, iter_expr: &IterExpr, loop_expr: &
     impl<'tcx> Visitor<'tcx> for AfterLoopVisitor<'_, '_, 'tcx> {
         type NestedFilter = OnlyBodies;
         type Result = ControlFlow<()>;
-        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, e: &'tcx Expr<'_>) -> Self::Result {
@@ -288,8 +288,8 @@ fn needs_mutable_borrow(cx: &LateContext<'_>, iter_expr: &IterExpr, loop_expr: &
     }
     impl<'tcx> Visitor<'tcx> for NestedLoopVisitor<'_, '_, 'tcx> {
         type NestedFilter = 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_local(&mut self, l: &'tcx LetStmt<'_>) {

clippy_lints/src/methods/needless_collect.rs

@@ -456,8 +456,8 @@ impl<'tcx> Visitor<'tcx> for UsedCountVisitor<'_, 'tcx> {
         }
     }
 
-    fn nested_visit_map(&mut self) -> Self::Map {
-        self.cx.tcx.hir()
+    fn maybe_tcx(&mut self) -> Self::MaybeTyCtxt {
+        self.cx.tcx
     }
 }