Optimize some alloc_from_iter call sites.

There's no need to collect an iterator into a `Vec`, or to call
`into_iter` at the call sites.

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(

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)
 }

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)))
     }
 }
 

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

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 }
 }