Auto merge of #116370 - nnethercote:more-arena-stuff, r=cjgillot

Remove the `TypedArena::alloc_from_iter` specialization.

It was added in #78569. It's complicated and doesn't actually help
performance.

r? `@cjgillot`

6 files changed, 74 insertions, 128 deletions

diff --git a/compiler/rustc_arena/src/lib.rs b/compiler/rustc_arena/src/lib.rs
index 06eb8a185be..bf8a7eb293e 100644
--- a/compiler/rustc_arena/src/lib.rs
+++ b/compiler/rustc_arena/src/lib.rs
@@ -15,7 +15,6 @@
 #![feature(dropck_eyepatch)]
 #![feature(new_uninit)]
 #![feature(maybe_uninit_slice)]
-#![feature(min_specialization)]
 #![feature(decl_macro)]
 #![feature(pointer_byte_offsets)]
 #![feature(rustc_attrs)]
@@ -44,23 +43,6 @@ fn outline<F: FnOnce() -> R, R>(f: F) -> R {
     f()
 }
 
-/// An arena that can hold objects of only one type.
-pub struct TypedArena<T> {
-    /// A pointer to the next object to be allocated.
-    ptr: Cell<*mut T>,
-
-    /// A pointer to the end of the allocated area. When this pointer is
-    /// reached, a new chunk is allocated.
-    end: Cell<*mut T>,
-
-    /// A vector of arena chunks.
-    chunks: RefCell<Vec<ArenaChunk<T>>>,
-
-    /// Marker indicating that dropping the arena causes its owned
-    /// instances of `T` to be dropped.
-    _own: PhantomData<T>,
-}
-
 struct ArenaChunk<T = u8> {
     /// The raw storage for the arena chunk.
     storage: NonNull<[MaybeUninit<T>]>,
@@ -130,6 +112,23 @@ impl<T> ArenaChunk<T> {
 const PAGE: usize = 4096;
 const HUGE_PAGE: usize = 2 * 1024 * 1024;
 
+/// An arena that can hold objects of only one type.
+pub struct TypedArena<T> {
+    /// A pointer to the next object to be allocated.
+    ptr: Cell<*mut T>,
+
+    /// A pointer to the end of the allocated area. When this pointer is
+    /// reached, a new chunk is allocated.
+    end: Cell<*mut T>,
+
+    /// A vector of arena chunks.
+    chunks: RefCell<Vec<ArenaChunk<T>>>,
+
+    /// Marker indicating that dropping the arena causes its owned
+    /// instances of `T` to be dropped.
+    _own: PhantomData<T>,
+}
+
 impl<T> Default for TypedArena<T> {
     /// Creates a new `TypedArena`.
     fn default() -> TypedArena<T> {
@@ -144,77 +143,6 @@ impl<T> Default for TypedArena<T> {
     }
 }
 
-trait IterExt<T> {
-    fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T];
-}
-
-impl<I, T> IterExt<T> for I
-where
-    I: IntoIterator<Item = T>,
-{
-    // This default collects into a `SmallVec` and then allocates by copying
-    // from it. The specializations below for types like `Vec` are more
-    // efficient, copying directly without the intermediate collecting step.
-    // This default could be made more efficient, like
-    // `DroplessArena::alloc_from_iter`, but it's not hot enough to bother.
-    #[inline]
-    default fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T] {
-        let vec: SmallVec<[_; 8]> = self.into_iter().collect();
-        vec.alloc_from_iter(arena)
-    }
-}
-
-impl<T, const N: usize> IterExt<T> for std::array::IntoIter<T, N> {
-    #[inline]
-    fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T] {
-        let len = self.len();
-        if len == 0 {
-            return &mut [];
-        }
-        // Move the content to the arena by copying and then forgetting it.
-        let start_ptr = arena.alloc_raw_slice(len);
-        unsafe {
-            self.as_slice().as_ptr().copy_to_nonoverlapping(start_ptr, len);
-            mem::forget(self);
-            slice::from_raw_parts_mut(start_ptr, len)
-        }
-    }
-}
-
-impl<T> IterExt<T> for Vec<T> {
-    #[inline]
-    fn alloc_from_iter(mut self, arena: &TypedArena<T>) -> &mut [T] {
-        let len = self.len();
-        if len == 0 {
-            return &mut [];
-        }
-        // Move the content to the arena by copying and then forgetting it.
-        let start_ptr = arena.alloc_raw_slice(len);
-        unsafe {
-            self.as_ptr().copy_to_nonoverlapping(start_ptr, len);
-            self.set_len(0);
-            slice::from_raw_parts_mut(start_ptr, len)
-        }
-    }
-}
-
-impl<A: smallvec::Array> IterExt<A::Item> for SmallVec<A> {
-    #[inline]
-    fn alloc_from_iter(mut self, arena: &TypedArena<A::Item>) -> &mut [A::Item] {
-        let len = self.len();
-        if len == 0 {
-            return &mut [];
-        }
-        // Move the content to the arena by copying and then forgetting it.
-        let start_ptr = arena.alloc_raw_slice(len);
-        unsafe {
-            self.as_ptr().copy_to_nonoverlapping(start_ptr, len);
-            self.set_len(0);
-            slice::from_raw_parts_mut(start_ptr, len)
-        }
-    }
-}
-
 impl<T> TypedArena<T> {
     /// Allocates an object in the `TypedArena`, returning a reference to it.
     #[inline]
@@ -270,8 +198,35 @@ impl<T> TypedArena<T> {
 
     #[inline]
     pub fn alloc_from_iter<I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
+        // This implementation is entirely separate to
+        // `DroplessIterator::alloc_from_iter`, even though conceptually they
+        // are the same.
+        //
+        // `DroplessIterator` (in the fast case) writes elements from the
+        // iterator one at a time into the allocated memory. That's easy
+        // because the elements don't implement `Drop`. But for `TypedArena`
+        // they do implement `Drop`, which means that if the iterator panics we
+        // could end up with some allocated-but-uninitialized elements, which
+        // will then cause UB in `TypedArena::drop`.
+        //
+        // Instead we use an approach where any iterator panic will occur
+        // before the memory is allocated. This function is much less hot than
+        // `DroplessArena::alloc_from_iter`, so it doesn't need to be
+        // hyper-optimized.
         assert!(mem::size_of::<T>() != 0);
-        iter.alloc_from_iter(self)
+
+        let mut vec: SmallVec<[_; 8]> = iter.into_iter().collect();
+        if vec.is_empty() {
+            return &mut [];
+        }
+        // Move the content to the arena by copying and then forgetting it.
+        let len = vec.len();
+        let start_ptr = self.alloc_raw_slice(len);
+        unsafe {
+            vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
+            vec.set_len(0);
+            slice::from_raw_parts_mut(start_ptr, len)
+        }
     }
 
     /// Grows the arena.
diff --git a/compiler/rustc_ast_lowering/src/format.rs b/compiler/rustc_ast_lowering/src/format.rs
index afcf8b15cd8..45a9bebfcf6 100644
--- a/compiler/rustc_ast_lowering/src/format.rs
+++ b/compiler/rustc_ast_lowering/src/format.rs
@@ -410,15 +410,11 @@ fn expand_format_args<'hir>(
     let format_options = use_format_options.then(|| {
         // Generate:
         //     &[format_spec_0, format_spec_1, format_spec_2]
-        let elements: Vec<_> = fmt
-            .template
-            .iter()
-            .filter_map(|piece| {
-                let FormatArgsPiece::Placeholder(placeholder) = piece else { return None };
-                Some(make_format_spec(ctx, macsp, placeholder, &mut argmap))
-            })
-            .collect();
-        ctx.expr_array_ref(macsp, ctx.arena.alloc_from_iter(elements))
+        let elements = ctx.arena.alloc_from_iter(fmt.template.iter().filter_map(|piece| {
+            let FormatArgsPiece::Placeholder(placeholder) = piece else { return None };
+            Some(make_format_spec(ctx, macsp, placeholder, &mut argmap))
+        }));
+        ctx.expr_array_ref(macsp, elements)
     });
 
     let arguments = fmt.arguments.all_args();
@@ -477,10 +473,8 @@ fn expand_format_args<'hir>(
         //         <core::fmt::Argument>::new_debug(&arg2),
         //         …
         //     ]
-        let elements: Vec<_> = arguments
-            .iter()
-            .zip(argmap)
-            .map(|(arg, ((_, ty), placeholder_span))| {
+        let elements = ctx.arena.alloc_from_iter(arguments.iter().zip(argmap).map(
+            |(arg, ((_, ty), placeholder_span))| {
                 let placeholder_span =
                     placeholder_span.unwrap_or(arg.expr.span).with_ctxt(macsp.ctxt());
                 let arg_span = match arg.kind {
@@ -493,9 +487,9 @@ fn expand_format_args<'hir>(
                     hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, arg),
                 ));
                 make_argument(ctx, placeholder_span, ref_arg, ty)
-            })
-            .collect();
-        ctx.expr_array_ref(macsp, ctx.arena.alloc_from_iter(elements))
+            },
+        ));
+        ctx.expr_array_ref(macsp, elements)
     } else {
         // Generate:
         //     &match (&arg0, &arg1, &…) {
@@ -528,19 +522,14 @@ fn expand_format_args<'hir>(
                 make_argument(ctx, placeholder_span, arg, ty)
             },
         ));
-        let elements: Vec<_> = arguments
-            .iter()
-            .map(|arg| {
-                let arg_expr = ctx.lower_expr(&arg.expr);
-                ctx.expr(
-                    arg.expr.span.with_ctxt(macsp.ctxt()),
-                    hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, arg_expr),
-                )
-            })
-            .collect();
-        let args_tuple = ctx
-            .arena
-            .alloc(ctx.expr(macsp, hir::ExprKind::Tup(ctx.arena.alloc_from_iter(elements))));
+        let elements = ctx.arena.alloc_from_iter(arguments.iter().map(|arg| {
+            let arg_expr = ctx.lower_expr(&arg.expr);
+            ctx.expr(
+                arg.expr.span.with_ctxt(macsp.ctxt()),
+                hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, arg_expr),
+            )
+        }));
+        let args_tuple = ctx.arena.alloc(ctx.expr(macsp, hir::ExprKind::Tup(elements)));
         let array = ctx.arena.alloc(ctx.expr(macsp, hir::ExprKind::Array(args)));
         let match_arms = ctx.arena.alloc_from_iter([ctx.arm(args_pat, array)]);
         let match_expr = ctx.arena.alloc(ctx.expr_match(
diff --git a/compiler/rustc_hir_analysis/src/variance/mod.rs b/compiler/rustc_hir_analysis/src/variance/mod.rs
index 85e0000ab47..9fb39a0e93b 100644
--- a/compiler/rustc_hir_analysis/src/variance/mod.rs
+++ b/compiler/rustc_hir_analysis/src/variance/mod.rs
@@ -192,5 +192,5 @@ fn variance_of_opaque(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Varianc
             }
         }
     }
-    tcx.arena.alloc_from_iter(collector.variances.into_iter())
+    tcx.arena.alloc_from_iter(collector.variances)
 }
diff --git a/compiler/rustc_middle/src/ty/codec.rs b/compiler/rustc_middle/src/ty/codec.rs
index dff7ff8c66b..e4069e11df2 100644
--- a/compiler/rustc_middle/src/ty/codec.rs
+++ b/compiler/rustc_middle/src/ty/codec.rs
@@ -348,9 +348,10 @@ impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for ty::Const<'tcx> {
 
 impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for [ty::ValTree<'tcx>] {
     fn decode(decoder: &mut D) -> &'tcx Self {
-        decoder.interner().arena.alloc_from_iter(
-            (0..decoder.read_usize()).map(|_| Decodable::decode(decoder)).collect::<Vec<_>>(),
-        )
+        decoder
+            .interner()
+            .arena
+            .alloc_from_iter((0..decoder.read_usize()).map(|_| Decodable::decode(decoder)))
     }
 }
 
@@ -368,9 +369,10 @@ impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for AdtDef<'tcx> {
 
 impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> RefDecodable<'tcx, D> for [(ty::Clause<'tcx>, Span)] {
     fn decode(decoder: &mut D) -> &'tcx Self {
-        decoder.interner().arena.alloc_from_iter(
-            (0..decoder.read_usize()).map(|_| Decodable::decode(decoder)).collect::<Vec<_>>(),
-        )
+        decoder
+            .interner()
+            .arena
+            .alloc_from_iter((0..decoder.read_usize()).map(|_| Decodable::decode(decoder)))
     }
 }
 
diff --git a/compiler/rustc_trait_selection/src/traits/vtable.rs b/compiler/rustc_trait_selection/src/traits/vtable.rs
index e41073937be..f23c100a686 100644
--- a/compiler/rustc_trait_selection/src/traits/vtable.rs
+++ b/compiler/rustc_trait_selection/src/traits/vtable.rs
@@ -316,7 +316,7 @@ fn vtable_entries<'tcx>(
         dump_vtable_entries(tcx, sp, trait_ref, &entries);
     }
 
-    tcx.arena.alloc_from_iter(entries.into_iter())
+    tcx.arena.alloc_from_iter(entries)
 }
 
 /// Find slot base for trait methods within vtable entries of another trait
diff --git a/compiler/rustc_ty_utils/src/consts.rs b/compiler/rustc_ty_utils/src/consts.rs
index 383cc996b9e..35487d3b698 100644
--- a/compiler/rustc_ty_utils/src/consts.rs
+++ b/compiler/rustc_ty_utils/src/consts.rs
@@ -71,7 +71,7 @@ pub(crate) fn destructure_const<'tcx>(
         _ => bug!("cannot destructure constant {:?}", const_),
     };
 
-    let fields = tcx.arena.alloc_from_iter(fields.into_iter());
+    let fields = tcx.arena.alloc_from_iter(fields);
 
     ty::DestructuredConst { variant, fields }
 }