Auto merge of #61995 - eddyb:hir-sep-ptr, r=petrochenkov

rustc: use a separate copy of P for HIR than for AST.

Note: this currently includes/is based on top of #61987.

Like #61968, but goes one step further and uses a separate `P<...>` for the HIR, with no `Clone`, or the ability to mutate after allocation.
There is still `into_inner`/`into_iter`, but they're only exposed for `hir::lowering`, and they would take more work to untangle.

r? @petrochenkov cc @rust-lang/compiler

src/librustc/cfg/construct.rs

@@ -1,11 +1,11 @@
 use crate::cfg::*;
 use crate::middle::region;
 use rustc_data_structures::graph::implementation as graph;
-use syntax::ptr::P;
 use crate::ty::{self, TyCtxt};
 
 use crate::hir::{self, PatKind};
 use crate::hir::def_id::DefId;
+use crate::hir::ptr::P;
 
 struct CFGBuilder<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,

src/librustc/hir/lowering.rs

@@ -39,6 +39,7 @@ use crate::hir::map::{DefKey, DefPathData, Definitions};
 use crate::hir::def_id::{DefId, DefIndex, CRATE_DEF_INDEX};
 use crate::hir::def::{Res, DefKind, PartialRes, PerNS};
 use crate::hir::{GenericArg, ConstArg};
+use crate::hir::ptr::P;
 use crate::lint::builtin::{self, PARENTHESIZED_PARAMS_IN_TYPES_AND_MODULES,
                     ELIDED_LIFETIMES_IN_PATHS};
 use crate::middle::cstore::CrateStore;
@@ -61,7 +62,6 @@ use syntax::ast::*;
 use syntax::errors;
 use syntax::ext::hygiene::{Mark, SyntaxContext};
 use syntax::print::pprust;
-use syntax::ptr::P;
 use syntax::source_map::{self, respan, ExpnInfo, CompilerDesugaringKind, Spanned};
 use syntax::source_map::CompilerDesugaringKind::IfTemporary;
 use syntax::std_inject;
@@ -1111,7 +1111,7 @@ impl<'a> LoweringContext<'a> {
             },
         );
 
-        lowered_generics.params = lowered_generics
+        let mut lowered_params: Vec<_> = lowered_generics
             .params
             .into_iter()
             .chain(in_band_defs)
@@ -1121,7 +1121,7 @@ impl<'a> LoweringContext<'a> {
         // unsorted generic parameters at the moment, so we make sure
         // that they're ordered correctly here for now. (When we chain
         // the `in_band_defs`, we might make the order unsorted.)
-        lowered_generics.params.sort_by_key(|param| {
+        lowered_params.sort_by_key(|param| {
             match param.kind {
                 hir::GenericParamKind::Lifetime { .. } => ParamKindOrd::Lifetime,
                 hir::GenericParamKind::Type { .. } => ParamKindOrd::Type,
@@ -1129,6 +1129,8 @@ impl<'a> LoweringContext<'a> {
             }
         });
 
+        lowered_generics.params = lowered_params.into();
+
         (lowered_generics, res)
     }
 
@@ -1155,13 +1157,13 @@ impl<'a> LoweringContext<'a> {
         &mut self,
         capture_clause: CaptureBy,
         closure_node_id: NodeId,
-        ret_ty: Option<&Ty>,
+        ret_ty: Option<syntax::ptr::P<Ty>>,
         span: Span,
         body: impl FnOnce(&mut LoweringContext<'_>) -> hir::Expr,
     ) -> hir::ExprKind {
         let capture_clause = self.lower_capture_clause(capture_clause);
         let output = match ret_ty {
-            Some(ty) => FunctionRetTy::Ty(P(ty.clone())),
+            Some(ty) => FunctionRetTy::Ty(ty),
             None => FunctionRetTy::Default(span),
         };
         let ast_decl = FnDecl {
@@ -2725,7 +2727,7 @@ impl<'a> LoweringContext<'a> {
 
         // ::std::future::Future<future_params>
         let future_path =
-            self.std_path(span, &[sym::future, sym::Future], Some(future_params), false);
+            P(self.std_path(span, &[sym::future, sym::Future], Some(future_params), false));
 
         hir::GenericBound::Trait(
             hir::PolyTraitRef {
@@ -3094,7 +3096,7 @@ impl<'a> LoweringContext<'a> {
 
     fn lower_trait_ref(&mut self, p: &TraitRef, itctx: ImplTraitContext<'_>) -> hir::TraitRef {
         let path = match self.lower_qpath(p.ref_id, &None, &p.path, ParamMode::Explicit, itctx) {
-            hir::QPath::Resolved(None, path) => path.and_then(|path| path),
+            hir::QPath::Resolved(None, path) => path,
             qpath => bug!("lower_trait_ref: unexpected QPath `{:?}`", qpath),
         };
         hir::TraitRef {
@@ -3620,7 +3622,7 @@ impl<'a> LoweringContext<'a> {
                             hir::Item {
                                 hir_id: new_id,
                                 ident,
-                                attrs: attrs.clone(),
+                                attrs: attrs.into_iter().cloned().collect(),
                                 node: item,
                                 vis,
                                 span,
@@ -3705,7 +3707,7 @@ impl<'a> LoweringContext<'a> {
                             hir::Item {
                                 hir_id: new_hir_id,
                                 ident,
-                                attrs: attrs.clone(),
+                                attrs: attrs.into_iter().cloned().collect(),
                                 node: item,
                                 vis,
                                 span: use_tree.span,
@@ -4567,7 +4569,7 @@ impl<'a> LoweringContext<'a> {
                         // `|x: u8| future_from_generator(|| -> X { ... })`.
                         let body_id = this.lower_fn_body(&outer_decl, |this| {
                             let async_ret_ty = if let FunctionRetTy::Ty(ty) = &decl.output {
-                                Some(&**ty)
+                                Some(ty.clone())
                             } else { None };
                             let async_body = this.make_async_expr(
                                 capture_clause, closure_id, async_ret_ty, body.span,
@@ -5577,7 +5579,7 @@ impl<'a> LoweringContext<'a> {
                         let principal = hir::PolyTraitRef {
                             bound_generic_params: hir::HirVec::new(),
                             trait_ref: hir::TraitRef {
-                                path: path.and_then(|path| path),
+                                path,
                                 hir_ref_id: hir_id,
                             },
                             span,

src/librustc/hir/mod.rs

@@ -12,6 +12,7 @@ pub use self::UnsafeSource::*;
 
 use crate::hir::def::{Res, DefKind};
 use crate::hir::def_id::{DefId, DefIndex, LocalDefId, CRATE_DEF_INDEX};
+use crate::hir::ptr::P;
 use crate::util::nodemap::{NodeMap, FxHashSet};
 use crate::mir::mono::Linkage;
 
@@ -23,7 +24,6 @@ use syntax::ast::{self, CrateSugar, Ident, Name, NodeId, AsmDialect};
 use syntax::ast::{Attribute, Label, LitKind, StrStyle, FloatTy, IntTy, UintTy};
 use syntax::attr::{InlineAttr, OptimizeAttr};
 use syntax::ext::hygiene::SyntaxContext;
-use syntax::ptr::P;
 use syntax::symbol::{Symbol, kw};
 use syntax::tokenstream::TokenStream;
 use syntax::util::parser::ExprPrecedence;
@@ -63,6 +63,7 @@ pub mod lowering;
 pub mod map;
 pub mod pat_util;
 pub mod print;
+pub mod ptr;
 pub mod upvars;
 
 /// Uniquely identifies a node in the HIR of the current crate. It is
@@ -1979,13 +1980,15 @@ pub struct InlineAsmOutput {
     pub span: Span,
 }
 
+// NOTE(eddyb) This is used within MIR as well, so unlike the rest of the HIR,
+// it needs to be `Clone` and use plain `Vec<T>` instead of `HirVec<T>`.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
 pub struct InlineAsm {
     pub asm: Symbol,
     pub asm_str_style: StrStyle,
-    pub outputs: HirVec<InlineAsmOutput>,
-    pub inputs: HirVec<Symbol>,
-    pub clobbers: HirVec<Symbol>,
+    pub outputs: Vec<InlineAsmOutput>,
+    pub inputs: Vec<Symbol>,
+    pub clobbers: Vec<Symbol>,
     pub volatile: bool,
     pub alignstack: bool,
     pub dialect: AsmDialect,
@@ -2217,7 +2220,7 @@ pub enum UseKind {
 /// within the resolution map.
 #[derive(RustcEncodable, RustcDecodable, Debug, HashStable)]
 pub struct TraitRef {
-    pub path: Path,
+    pub path: P<Path>,
     // Don't hash the ref_id. It is tracked via the thing it is used to access
     #[stable_hasher(ignore)]
     pub hir_ref_id: HirId,

src/librustc/hir/print.rs

@@ -6,7 +6,6 @@ use syntax::parse::lexer::comments;
 use syntax::print::pp::{self, Breaks};
 use syntax::print::pp::Breaks::{Consistent, Inconsistent};
 use syntax::print::pprust::{self, PrintState};
-use syntax::ptr::P;
 use syntax::symbol::kw;
 use syntax::util::parser::{self, AssocOp, Fixity};
 use syntax_pos::{self, BytePos, FileName};
@@ -14,6 +13,7 @@ use syntax_pos::{self, BytePos, FileName};
 use crate::hir;
 use crate::hir::{PatKind, GenericBound, TraitBoundModifier, RangeEnd};
 use crate::hir::{GenericParam, GenericParamKind, GenericArg};
+use crate::hir::ptr::P;
 
 use std::borrow::Cow;
 use std::cell::Cell;

src/librustc/hir/ptr.rs

@@ -0,0 +1,141 @@
+// HACK(eddyb) this is a copy of `syntax::ptr`, minus the mutation (the HIR is
+// frozen anyway). The only reason for doing this instead of replacing `P<T>`
+// with `Box<T>` in HIR, is that `&Box<[T]>` doesn't implement `IntoIterator`.
+
+use std::fmt::{self, Display, Debug};
+use std::iter::FromIterator;
+use std::ops::Deref;
+use std::{slice, vec};
+
+use serialize::{Encodable, Decodable, Encoder, Decoder};
+
+use rustc_data_structures::stable_hasher::{StableHasher, StableHasherResult,
+                                           HashStable};
+/// An owned smart pointer.
+#[derive(Hash, PartialEq, Eq)]
+pub struct P<T: ?Sized> {
+    ptr: Box<T>
+}
+
+/// Construct a `P<T>` from a `T` value.
+#[allow(non_snake_case)]
+pub fn P<T: 'static>(value: T) -> P<T> {
+    P {
+        ptr: box value
+    }
+}
+
+impl<T: 'static> P<T> {
+    // HACK(eddyb) used by HIR lowering in a few places still.
+    // NOTE: do not make this more public than `pub(super)`.
+    pub(super) fn into_inner(self) -> T {
+        *self.ptr
+    }
+}
+
+impl<T: ?Sized> Deref for P<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.ptr
+    }
+}
+
+impl<T: ?Sized + Debug> Debug for P<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Debug::fmt(&self.ptr, f)
+    }
+}
+
+impl<T: Display> Display for P<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Display::fmt(&**self, f)
+    }
+}
+
+impl<T: 'static + Decodable> Decodable for P<T> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<P<T>, D::Error> {
+        Decodable::decode(d).map(P)
+    }
+}
+
+impl<T: Encodable> Encodable for P<T> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
+        (**self).encode(s)
+    }
+}
+
+impl<T> P<[T]> {
+    pub const fn new() -> P<[T]> {
+        // HACK(eddyb) bypass the lack of a `const fn` to create an empty `Box<[T]>`
+        // (as trait methods, `default` in this case, can't be `const fn` yet).
+        P {
+            ptr: unsafe {
+                use std::ptr::NonNull;
+                std::mem::transmute(NonNull::<[T; 0]>::dangling() as NonNull<[T]>)
+            },
+        }
+    }
+
+    #[inline(never)]
+    pub fn from_vec(v: Vec<T>) -> P<[T]> {
+        P { ptr: v.into_boxed_slice() }
+    }
+
+    // HACK(eddyb) used by HIR lowering in a few places still.
+    // NOTE: do not make this more public than `pub(super)`,
+    // and do not make this into an `IntoIterator` impl.
+    pub(super) fn into_iter(self) -> vec::IntoIter<T> {
+        self.ptr.into_vec().into_iter()
+    }
+}
+
+
+impl<T> Default for P<[T]> {
+    /// Creates an empty `P<[T]>`.
+    fn default() -> P<[T]> {
+        P::new()
+    }
+}
+
+impl<T> From<Vec<T>> for P<[T]> {
+    fn from(v: Vec<T>) -> Self {
+        P::from_vec(v)
+    }
+}
+
+impl<T> FromIterator<T> for P<[T]> {
+    fn from_iter<I: IntoIterator<Item=T>>(iter: I) -> P<[T]> {
+        P::from_vec(iter.into_iter().collect())
+    }
+}
+
+impl<'a, T> IntoIterator for &'a P<[T]> {
+    type Item = &'a T;
+    type IntoIter = slice::Iter<'a, T>;
+    fn into_iter(self) -> Self::IntoIter {
+        self.ptr.into_iter()
+    }
+}
+
+impl<T: Encodable> Encodable for P<[T]> {
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
+        Encodable::encode(&**self, s)
+    }
+}
+
+impl<T: Decodable> Decodable for P<[T]> {
+    fn decode<D: Decoder>(d: &mut D) -> Result<P<[T]>, D::Error> {
+        Ok(P::from_vec(Decodable::decode(d)?))
+    }
+}
+
+impl<CTX, T> HashStable<CTX> for P<T>
+    where T: ?Sized + HashStable<CTX>
+{
+    fn hash_stable<W: StableHasherResult>(&self,
+                                          hcx: &mut CTX,
+                                          hasher: &mut StableHasher<W>) {
+        (**self).hash_stable(hcx, hasher);
+    }
+}

src/librustc/lib.rs

@@ -35,6 +35,8 @@
 #![feature(arbitrary_self_types)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
+#![feature(const_fn)]
+#![feature(const_transmute)]
 #![feature(core_intrinsics)]
 #![feature(drain_filter)]
 #![feature(inner_deref)]

src/librustc/middle/expr_use_visitor.rs

@@ -11,6 +11,7 @@ use self::OverloadedCallType::*;
 
 use crate::hir::def::{CtorOf, Res, DefKind};
 use crate::hir::def_id::DefId;
+use crate::hir::ptr::P;
 use crate::infer::InferCtxt;
 use crate::middle::mem_categorization as mc;
 use crate::middle::region;
@@ -18,7 +19,6 @@ use crate::ty::{self, DefIdTree, TyCtxt, adjustment};
 
 use crate::hir::{self, PatKind};
 use std::rc::Rc;
-use syntax::ptr::P;
 use syntax_pos::Span;
 use crate::util::nodemap::ItemLocalSet;
 

src/librustc/middle/liveness.rs

@@ -99,6 +99,7 @@ use self::VarKind::*;
 
 use crate::hir::def::*;
 use crate::hir::Node;
+use crate::hir::ptr::P;
 use crate::ty::{self, TyCtxt};
 use crate::ty::query::Providers;
 use crate::lint;
@@ -111,7 +112,6 @@ use std::io::prelude::*;
 use std::io;
 use std::rc::Rc;
 use syntax::ast;
-use syntax::ptr::P;
 use syntax::symbol::{kw, sym};
 use syntax_pos::Span;
 

src/librustc/middle/resolve_lifetime.rs

@@ -8,6 +8,7 @@
 use crate::hir::def::{Res, DefKind};
 use crate::hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
 use crate::hir::map::Map;
+use crate::hir::ptr::P;
 use crate::hir::{GenericArg, GenericParam, ItemLocalId, LifetimeName, Node, ParamName};
 use crate::ty::{self, DefIdTree, GenericParamDefKind, TyCtxt};
 
@@ -21,7 +22,6 @@ use std::cell::Cell;
 use std::mem::replace;
 use syntax::ast;
 use syntax::attr;
-use syntax::ptr::P;
 use syntax::symbol::{kw, sym};
 use syntax_pos::Span;
 

src/librustc_mir/hair/cx/to_ref.rs

@@ -1,7 +1,7 @@
 use crate::hair::*;
 
 use rustc::hir;
-use syntax::ptr::P;
+use rustc::hir::ptr::P;
 
 pub trait ToRef {
     type Output;

src/librustc_mir/hair/pattern/check_match.rs

@@ -18,12 +18,12 @@ use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc::hir::def::*;
 use rustc::hir::def_id::DefId;
 use rustc::hir::intravisit::{self, Visitor, NestedVisitorMap};
+use rustc::hir::ptr::P;
 use rustc::hir::{self, Pat, PatKind};
 
 use smallvec::smallvec;
 use std::slice;
 
-use syntax::ptr::P;
 use syntax_pos::{Span, DUMMY_SP, MultiSpan};
 
 pub(crate) fn check_match<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) {

src/librustc_mir/hair/pattern/mod.rs

@@ -20,13 +20,13 @@ use rustc::ty::layout::{VariantIdx, Size};
 use rustc::hir::{self, PatKind, RangeEnd};
 use rustc::hir::def::{CtorOf, Res, DefKind, CtorKind};
 use rustc::hir::pat_util::EnumerateAndAdjustIterator;
+use rustc::hir::ptr::P;
 
 use rustc_data_structures::indexed_vec::Idx;
 
 use std::cmp::Ordering;
 use std::fmt;
 use syntax::ast;
-use syntax::ptr::P;
 use syntax::symbol::sym;
 use syntax_pos::Span;
 

src/librustc_typeck/astconv.rs

@@ -7,6 +7,7 @@ use crate::hir::{self, GenericArg, GenericArgs, ExprKind};
 use crate::hir::def::{CtorOf, Res, DefKind};
 use crate::hir::def_id::DefId;
 use crate::hir::HirVec;
+use crate::hir::ptr::P;
 use crate::lint;
 use crate::middle::lang_items::SizedTraitLangItem;
 use crate::middle::resolve_lifetime as rl;
@@ -23,7 +24,6 @@ use crate::require_c_abi_if_c_variadic;
 use smallvec::SmallVec;
 use syntax::ast;
 use syntax::feature_gate::{GateIssue, emit_feature_err};
-use syntax::ptr::P;
 use syntax::util::lev_distance::find_best_match_for_name;
 use syntax::symbol::sym;
 use syntax_pos::{DUMMY_SP, Span, MultiSpan};

src/librustc_typeck/check/_match.rs

@@ -5,6 +5,7 @@ use errors::{Applicability, DiagnosticBuilder};
 use rustc::hir::{self, PatKind, Pat, ExprKind};
 use rustc::hir::def::{Res, DefKind, CtorKind};
 use rustc::hir::pat_util::EnumerateAndAdjustIterator;
+use rustc::hir::ptr::P;
 use rustc::infer;
 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc::traits::{ObligationCause, ObligationCauseCode};
@@ -12,7 +13,6 @@ use rustc::ty::{self, Ty, TypeFoldable};
 use rustc::ty::subst::Kind;
 use syntax::ast;
 use syntax::source_map::Spanned;
-use syntax::ptr::P;
 use syntax::util::lev_distance::find_best_match_for_name;
 use syntax_pos::Span;
 use syntax_pos::hygiene::CompilerDesugaringKind;

src/librustc_typeck/check/coercion.rs

@@ -54,6 +54,7 @@ use crate::check::{FnCtxt, Needs};
 use errors::DiagnosticBuilder;
 use rustc::hir;
 use rustc::hir::def_id::DefId;
+use rustc::hir::ptr::P;
 use rustc::infer::{Coercion, InferResult, InferOk};
 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc::traits::{self, ObligationCause, ObligationCauseCode};
@@ -67,7 +68,6 @@ use rustc::ty::relate::RelateResult;
 use smallvec::{smallvec, SmallVec};
 use std::ops::Deref;
 use syntax::feature_gate;
-use syntax::ptr::P;
 use syntax::symbol::sym;
 use syntax_pos;
 

src/librustc_typeck/check/expr.rs

@@ -20,13 +20,13 @@ use crate::astconv::AstConv as _;
 
 use errors::{Applicability, DiagnosticBuilder};
 use syntax::ast;
-use syntax::ptr::P;
 use syntax::symbol::{Symbol, LocalInternedString, kw, sym};
 use syntax::source_map::Span;
 use syntax::util::lev_distance::find_best_match_for_name;
 use rustc::hir;
 use rustc::hir::{ExprKind, QPath};
 use rustc::hir::def::{CtorKind, Res, DefKind};
+use rustc::hir::ptr::P;
 use rustc::infer;
 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc::mir::interpret::GlobalId;

src/librustc_typeck/check/mod.rs

@@ -93,6 +93,7 @@ use rustc::hir::def::{CtorOf, Res, DefKind};
 use rustc::hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
 use rustc::hir::intravisit::{self, Visitor, NestedVisitorMap};
 use rustc::hir::itemlikevisit::ItemLikeVisitor;
+use rustc::hir::ptr::P;
 use crate::middle::lang_items;
 use crate::namespace::Namespace;
 use rustc::infer::{self, InferCtxt, InferOk, InferResult};
@@ -122,7 +123,6 @@ use syntax_pos::hygiene::CompilerDesugaringKind;
 use syntax::ast;
 use syntax::attr;
 use syntax::feature_gate::{GateIssue, emit_feature_err};
-use syntax::ptr::P;
 use syntax::source_map::{DUMMY_SP, original_sp};
 use syntax::symbol::{kw, sym};
 

src/librustc_typeck/collect.rs

@@ -1353,7 +1353,7 @@ pub fn checked_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, fail: bool) -> Op
                                 None
                             }
                         }
-                        Node::TraitRef(&hir::TraitRef { ref path, .. }) => Some(path),
+                        Node::TraitRef(&hir::TraitRef { ref path, .. }) => Some(&**path),
                         _ => None,
                     };
 

src/librustdoc/clean/mod.rs

@@ -20,6 +20,7 @@ use rustc::mir::interpret::{GlobalId, ConstValue};
 use rustc::hir;
 use rustc::hir::def::{CtorKind, DefKind, Res};
 use rustc::hir::def_id::{CrateNum, DefId, CRATE_DEF_INDEX, LOCAL_CRATE};
+use rustc::hir::ptr::P;
 use rustc::ty::subst::{InternalSubsts, SubstsRef, UnpackedKind};
 use rustc::ty::{self, DefIdTree, TyCtxt, Region, RegionVid, Ty, AdtKind};
 use rustc::ty::fold::TypeFolder;
@@ -29,7 +30,6 @@ use syntax::ast::{self, AttrStyle, Ident};
 use syntax::attr;
 use syntax::ext::base::MacroKind;
 use syntax::source_map::{dummy_spanned, Spanned};
-use syntax::ptr::P;
 use syntax::symbol::{Symbol, kw, sym};
 use syntax::symbol::InternedString;
 use syntax_pos::{self, Pos, FileName};

src/librustdoc/doctree.rs

@@ -6,11 +6,11 @@ use syntax::ast;
 use syntax::ast::{Name, NodeId};
 use syntax::attr;
 use syntax::ext::base::MacroKind;
-use syntax::ptr::P;
 use syntax_pos::{self, Span};
 
 use rustc::hir;
 use rustc::hir::def_id::CrateNum;
+use rustc::hir::ptr::P;
 
 pub struct Module<'hir> {
     pub name: Option<Name>,

src/libsyntax/lib.rs

@@ -12,6 +12,7 @@
 #![deny(unused_lifetimes)]
 
 #![feature(bind_by_move_pattern_guards)]
+#![feature(box_syntax)]
 #![feature(const_fn)]
 #![feature(const_transmute)]
 #![feature(crate_visibility_modifier)]

src/libsyntax/ptr.rs

@@ -41,11 +41,11 @@ pub struct P<T: ?Sized> {
     ptr: Box<T>
 }
 
-#[allow(non_snake_case)]
 /// Construct a `P<T>` from a `T` value.
+#[allow(non_snake_case)]
 pub fn P<T: 'static>(value: T) -> P<T> {
     P {
-        ptr: Box::new(value)
+        ptr: box value
     }
 }