Auto merge of #78134 - bugadani:arena-nodrop, r=lcnr

Use `DroplessArena` where we know the type doesn't need drop

This PR uses a single `DroplessArena` in resolve instead of three separate `TypedArena`s.

`DroplessArena` checks that the type indeed doesn't need drop, so in case the types change, this will result in visible failures.

compiler/rustc_resolve/src/lib.rs

@@ -19,7 +19,7 @@ pub use rustc_hir::def::{Namespace, PerNS};
 
 use Determinacy::*;
 
-use rustc_arena::TypedArena;
+use rustc_arena::{DroplessArena, TypedArena};
 use rustc_ast::node_id::NodeMap;
 use rustc_ast::unwrap_or;
 use rustc_ast::visit::{self, Visitor};
@@ -1035,12 +1035,10 @@ pub struct Resolver<'a> {
 pub struct ResolverArenas<'a> {
     modules: TypedArena<ModuleData<'a>>,
     local_modules: RefCell<Vec<Module<'a>>>,
-    name_bindings: TypedArena<NameBinding<'a>>,
     imports: TypedArena<Import<'a>>,
     name_resolutions: TypedArena<RefCell<NameResolution<'a>>>,
-    macro_rules_bindings: TypedArena<MacroRulesBinding<'a>>,
     ast_paths: TypedArena<ast::Path>,
-    pattern_spans: TypedArena<Span>,
+    dropless: DroplessArena,
 }
 
 impl<'a> ResolverArenas<'a> {
@@ -1055,7 +1053,7 @@ impl<'a> ResolverArenas<'a> {
         self.local_modules.borrow()
     }
     fn alloc_name_binding(&'a self, name_binding: NameBinding<'a>) -> &'a NameBinding<'a> {
-        self.name_bindings.alloc(name_binding)
+        self.dropless.alloc(name_binding)
     }
     fn alloc_import(&'a self, import: Import<'a>) -> &'a Import<'_> {
         self.imports.alloc(import)
@@ -1067,13 +1065,13 @@ impl<'a> ResolverArenas<'a> {
         &'a self,
         binding: MacroRulesBinding<'a>,
     ) -> &'a MacroRulesBinding<'a> {
-        self.macro_rules_bindings.alloc(binding)
+        self.dropless.alloc(binding)
     }
     fn alloc_ast_paths(&'a self, paths: &[ast::Path]) -> &'a [ast::Path] {
         self.ast_paths.alloc_from_iter(paths.iter().cloned())
     }
     fn alloc_pattern_spans(&'a self, spans: impl Iterator<Item = Span>) -> &'a [Span] {
-        self.pattern_spans.alloc_from_iter(spans)
+        self.dropless.alloc_from_iter(spans)
     }
 }
 

compiler/rustc_typeck/src/variance/mod.rs

@@ -4,7 +4,7 @@
 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/variance.html
 
 use hir::Node;
-use rustc_arena::TypedArena;
+use rustc_arena::DroplessArena;
 use rustc_hir as hir;
 use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
 use rustc_middle::ty::query::Providers;
@@ -32,8 +32,8 @@ pub fn provide(providers: &mut Providers) {
 
 fn crate_variances(tcx: TyCtxt<'_>, crate_num: CrateNum) -> CrateVariancesMap<'_> {
     assert_eq!(crate_num, LOCAL_CRATE);
-    let mut arena = TypedArena::default();
+    let arena = DroplessArena::default();
-    let terms_cx = terms::determine_parameters_to_be_inferred(tcx, &mut arena);
+    let terms_cx = terms::determine_parameters_to_be_inferred(tcx, &arena);
     let constraints_cx = constraints::add_constraints_from_crate(terms_cx);
     solve::solve_constraints(constraints_cx)
 }

compiler/rustc_typeck/src/variance/terms.rs

@@ -9,7 +9,7 @@
 // `InferredIndex` is a newtype'd int representing the index of such
 // a variable.
 
-use rustc_arena::TypedArena;
+use rustc_arena::DroplessArena;
 use rustc_hir as hir;
 use rustc_hir::itemlikevisit::ItemLikeVisitor;
 use rustc_hir::HirIdMap;
@@ -47,7 +47,7 @@ impl<'a> fmt::Debug for VarianceTerm<'a> {
 
 pub struct TermsContext<'a, 'tcx> {
     pub tcx: TyCtxt<'tcx>,
-    pub arena: &'a TypedArena<VarianceTerm<'a>>,
+    pub arena: &'a DroplessArena,
 
     // For marker types, UnsafeCell, and other lang items where
     // variance is hardcoded, records the item-id and the hardcoded
@@ -64,7 +64,7 @@ pub struct TermsContext<'a, 'tcx> {
 
 pub fn determine_parameters_to_be_inferred<'a, 'tcx>(
     tcx: TyCtxt<'tcx>,
-    arena: &'a mut TypedArena<VarianceTerm<'a>>,
+    arena: &'a DroplessArena,
 ) -> TermsContext<'a, 'tcx> {
     let mut terms_cx = TermsContext {
         tcx,