Implement non-capturing closure to fn coercion

src/librustc/middle/expr_use_visitor.rs

@@ -715,6 +715,7 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
                 adjustment::Adjust::NeverToAny |
                 adjustment::Adjust::ReifyFnPointer |
                 adjustment::Adjust::UnsafeFnPointer |
+                adjustment::Adjust::ClosureFnPointer |
                 adjustment::Adjust::MutToConstPointer => {
                     // Creating a closure/fn-pointer or unsizing consumes
                     // the input and stores it into the resulting rvalue.

src/librustc/middle/mem_categorization.rs

@@ -464,6 +464,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
                     adjustment::Adjust::NeverToAny |
                     adjustment::Adjust::ReifyFnPointer |
                     adjustment::Adjust::UnsafeFnPointer |
+                    adjustment::Adjust::ClosureFnPointer |
                     adjustment::Adjust::MutToConstPointer |
                     adjustment::Adjust::DerefRef {..} => {
                         debug!("cat_expr({:?}): {:?}",

src/librustc/mir/mod.rs

@@ -1022,6 +1022,9 @@ pub enum CastKind {
     /// Convert unique, zero-sized type for a fn to fn()
     ReifyFnPointer,
 
+    /// Convert non capturing closure to fn()
+    ClosureFnPointer,
+
     /// Convert safe fn() to unsafe fn()
     UnsafeFnPointer,
 

src/librustc/ty/adjustment.rs

@@ -33,6 +33,9 @@ pub enum Adjust<'tcx> {
     /// Go from a safe fn pointer to an unsafe fn pointer.
     UnsafeFnPointer,
 
+    // Go from a non-capturing closure to an fn pointer.
+    ClosureFnPointer,
+
     /// Go from a mut raw pointer to a const raw pointer.
     MutToConstPointer,
 
@@ -120,6 +123,7 @@ impl<'tcx> Adjustment<'tcx> {
 
             Adjust::ReifyFnPointer |
             Adjust::UnsafeFnPointer |
+            Adjust::ClosureFnPointer |
             Adjust::MutToConstPointer |
             Adjust::DerefRef {..} => false,
         }

src/librustc_mir/build/expr/as_lvalue.rs

@@ -99,6 +99,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             ExprKind::Use { .. } |
             ExprKind::NeverToAny { .. } |
             ExprKind::ReifyFnPointer { .. } |
+            ExprKind::ClosureFnPointer { .. } |
             ExprKind::UnsafeFnPointer { .. } |
             ExprKind::Unsize { .. } |
             ExprKind::Repeat { .. } |

src/librustc_mir/build/expr/as_rvalue.rs

@@ -112,6 +112,10 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                 let source = unpack!(block = this.as_operand(block, source));
                 block.and(Rvalue::Cast(CastKind::UnsafeFnPointer, source, expr.ty))
             }
+            ExprKind::ClosureFnPointer { source } => {
+                let source = unpack!(block = this.as_operand(block, source));
+                block.and(Rvalue::Cast(CastKind::ClosureFnPointer, source, expr.ty))
+            }
             ExprKind::Unsize { source } => {
                 let source = unpack!(block = this.as_operand(block, source));
                 block.and(Rvalue::Cast(CastKind::Unsize, source, expr.ty))

src/librustc_mir/build/expr/category.rs

@@ -70,6 +70,7 @@ impl Category {
             ExprKind::Cast { .. } |
             ExprKind::Use { .. } |
             ExprKind::ReifyFnPointer { .. } |
+            ExprKind::ClosureFnPointer { .. } |
             ExprKind::UnsafeFnPointer { .. } |
             ExprKind::Unsize { .. } |
             ExprKind::Repeat { .. } |

src/librustc_mir/build/expr/into.rs

@@ -244,6 +244,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             ExprKind::Cast { .. } |
             ExprKind::Use { .. } |
             ExprKind::ReifyFnPointer { .. } |
+            ExprKind::ClosureFnPointer { .. } |
             ExprKind::UnsafeFnPointer { .. } |
             ExprKind::Unsize { .. } |
             ExprKind::Repeat { .. } |

src/librustc_mir/hair/cx/expr.rs

@@ -60,6 +60,15 @@ impl<'tcx> Mirror<'tcx> for &'tcx hir::Expr {
                     kind: ExprKind::UnsafeFnPointer { source: expr.to_ref() },
                 };
             }
+            Some((ty::adjustment::Adjust::ClosureFnPointer, adjusted_ty)) => {
+                expr = Expr {
+                    temp_lifetime: temp_lifetime,
+                    temp_lifetime_was_shrunk: was_shrunk,
+                    ty: adjusted_ty,
+                    span: self.span,
+                    kind: ExprKind::ClosureFnPointer { source: expr.to_ref() },
+                };
+            }
             Some((ty::adjustment::Adjust::NeverToAny, adjusted_ty)) => {
                 expr = Expr {
                     temp_lifetime: temp_lifetime,

src/librustc_mir/hair/mod.rs

@@ -152,6 +152,9 @@ pub enum ExprKind<'tcx> {
     ReifyFnPointer {
         source: ExprRef<'tcx>,
     },
+    ClosureFnPointer {
+        source: ExprRef<'tcx>,
+    },
     UnsafeFnPointer {
         source: ExprRef<'tcx>,
     },

src/librustc_mir/transform/qualify_consts.rs

@@ -619,6 +619,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
             Rvalue::CheckedBinaryOp(..) |
             Rvalue::Cast(CastKind::ReifyFnPointer, ..) |
             Rvalue::Cast(CastKind::UnsafeFnPointer, ..) |
+            Rvalue::Cast(CastKind::ClosureFnPointer, ..) |
             Rvalue::Cast(CastKind::Unsize, ..) => {}
 
             Rvalue::Len(_) => {

src/librustc_passes/consts.rs

@@ -447,6 +447,7 @@ fn check_adjustments<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>, e: &hir::Exp
         Some(Adjust::NeverToAny) |
         Some(Adjust::ReifyFnPointer) |
         Some(Adjust::UnsafeFnPointer) |
+        Some(Adjust::ClosureFnPointer) |
         Some(Adjust::MutToConstPointer) => {}
 
         Some(Adjust::DerefRef { autoderefs, .. }) => {

src/librustc_trans/builder.rs

@@ -1181,7 +1181,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         }
 
         assert!(fn_ty.kind() == llvm::TypeKind::Function,
-                "builder::{} not passed a function", typ);
+                "builder::{} not passed a function, but {:?}", typ, fn_ty);
 
         let param_tys = fn_ty.func_params();
 

src/librustc_trans/collector.rs

@@ -489,6 +489,20 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirNeighborCollector<'a, 'tcx> {
                                                           self.output);
                 }
             }
+            mir::Rvalue::Cast(mir::CastKind::ClosureFnPointer, ref operand, _) => {
+                let source_ty = operand.ty(self.mir, self.scx.tcx());
+                match source_ty.sty {
+                    ty::TyClosure(def_id, substs) => {
+                        let closure_trans_item =
+                            create_fn_trans_item(self.scx,
+                                                 def_id,
+                                                 substs.substs,
+                                                 self.param_substs);
+                        self.output.push(closure_trans_item);
+                    }
+                    _ => bug!(),
+                }
+            }
             mir::Rvalue::Box(..) => {
                 let exchange_malloc_fn_def_id =
                     self.scx

src/librustc_trans/mir/constant.rs

@@ -578,6 +578,35 @@ impl<'a, 'tcx> MirConstContext<'a, 'tcx> {
                             }
                         }
                     }
+                    mir::CastKind::ClosureFnPointer => {
+                        match operand.ty.sty {
+                            ty::TyClosure(def_id, substs) => {
+                                // Get the def_id for FnOnce::call_once
+                                let fn_once = tcx.lang_items.fn_once_trait().unwrap();
+                                let call_once = tcx
+                                    .global_tcx().associated_items(fn_once)
+                                    .find(|it| it.kind == ty::AssociatedKind::Method)
+                                    .unwrap().def_id;
+                                // Now create its substs [Closure, Tuple]
+                                let input = tcx.closure_type(def_id, substs).sig.input(0);
+                                let substs = Substs::for_item(tcx,
+                                    call_once,
+                                    |_, _| {bug!()},
+                                    |def, _| { match def.index {
+                                                0 => operand.ty.clone(),
+                                                1 => input.skip_binder(),
+                                                _ => bug!(),
+                                            } }
+                                );
+
+                                Callee::def(self.ccx, call_once, substs)
+                                    .reify(self.ccx)
+                            }
+                            _ => {
+                                bug!("{} cannot be cast to a fn ptr", operand.ty)
+                            }
+                        }
+                    }
                     mir::CastKind::UnsafeFnPointer => {
                         // this is a no-op at the LLVM level
                         operand.llval

src/librustc_trans/mir/rvalue.rs

@@ -12,6 +12,7 @@ use llvm::{self, ValueRef};
 use rustc::ty::{self, Ty};
 use rustc::ty::cast::{CastTy, IntTy};
 use rustc::ty::layout::Layout;
+use rustc::ty::subst::Substs;
 use rustc::mir::tcx::LvalueTy;
 use rustc::mir;
 use middle::lang_items::ExchangeMallocFnLangItem;
@@ -190,6 +191,36 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
                             }
                         }
                     }
+                    mir::CastKind::ClosureFnPointer => {
+                        match operand.ty.sty {
+                            ty::TyClosure(def_id, substs) => {
+                                // Get the def_id for FnOnce::call_once
+                                let fn_once = bcx.tcx().lang_items.fn_once_trait().unwrap();
+                                let call_once = bcx.tcx()
+                                    .global_tcx().associated_items(fn_once)
+                                    .find(|it| it.kind == ty::AssociatedKind::Method)
+                                    .unwrap().def_id;
+                                // Now create its substs [Closure, Tuple]
+                                let input = bcx.tcx().closure_type(def_id, substs).sig.input(0);
+                                let substs = Substs::for_item(bcx.tcx(),
+                                    call_once,
+                                    |_, _| {bug!()},
+                                    |def, _| { match def.index {
+                                                0 => operand.ty.clone(),
+                                                1 => input.skip_binder(),
+                                                _ => bug!(),
+                                          } }
+                                );
+
+                                OperandValue::Immediate(
+                                    Callee::def(bcx.ccx, call_once, substs)
+                                        .reify(bcx.ccx))
+                            }
+                            _ => {
+                                bug!("{} cannot be cast to a fn ptr", operand.ty)
+                            }
+                        }
+                    }
                     mir::CastKind::UnsafeFnPointer => {
                         // this is a no-op at the LLVM level
                         operand.val

src/librustc_typeck/check/coercion.rs

@@ -63,13 +63,17 @@
 use check::FnCtxt;
 
 use rustc::hir;
+use rustc::hir::def_id::DefId;
 use rustc::infer::{Coercion, InferOk, TypeTrace};
 use rustc::traits::{self, ObligationCause, ObligationCauseCode};
 use rustc::ty::adjustment::{Adjustment, Adjust, AutoBorrow};
-use rustc::ty::{self, LvaluePreference, TypeAndMut, Ty};
+use rustc::ty::{self, LvaluePreference, TypeVariants, TypeAndMut,
+                Ty, ClosureSubsts};
 use rustc::ty::fold::TypeFoldable;
 use rustc::ty::error::TypeError;
 use rustc::ty::relate::RelateResult;
+use syntax::ast::NodeId;
+use syntax::abi;
 use util::common::indent;
 
 use std::cell::RefCell;
@@ -196,6 +200,11 @@ impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
                 // unsafe qualifier.
                 self.coerce_from_fn_pointer(a, a_f, b)
             }
+            ty::TyClosure(def_id_a, substs_a) => {
+                // Non-capturing closures are coercible to
+                // function pointers
+                self.coerce_closure_to_fn(a, def_id_a, substs_a, b)
+            }
             _ => {
                 // Otherwise, just use unification rules.
                 self.unify_and_identity(a, b)
@@ -551,6 +560,52 @@ impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
         }
     }
 
+    fn coerce_closure_to_fn(&self,
+                           a: Ty<'tcx>,
+                           def_id_a: DefId,
+                           substs_a: ClosureSubsts<'tcx>,
+                           b: Ty<'tcx>)
+                           -> CoerceResult<'tcx> {
+        //! Attempts to coerce from the type of a non-capturing closure
+        //! into a function pointer.
+        //!
+
+        let b = self.shallow_resolve(b);
+
+        let node_id_a :NodeId = self.tcx.hir.as_local_node_id(def_id_a).unwrap();
+        match b.sty {
+            ty::TyFnPtr(_) if self.tcx.with_freevars(node_id_a, |v| v.is_empty()) => {
+                // We coerce the closure, which has fn type
+                //     `extern "rust-call" fn((arg0,arg1,...)) -> _`
+                // to
+                //     `fn(arg0,arg1,...) -> _`
+                let sig = self.closure_type(def_id_a, substs_a).sig;
+                let converted_sig = sig.input(0).map_bound(|v| {
+                    let params_iter = match v.sty {
+                        TypeVariants::TyTuple(params, _) => {
+                            params.into_iter().cloned()
+                        }
+                        _ => bug!(),
+                    };
+                    self.tcx.mk_fn_sig(params_iter,
+                                       sig.output().skip_binder(),
+                                       sig.variadic())
+                });
+                let fn_ty = self.tcx.mk_bare_fn(ty::BareFnTy {
+                    unsafety: hir::Unsafety::Normal,
+                    abi: abi::Abi::Rust,
+                    sig: converted_sig,
+                });
+                let pointer_ty = self.tcx.mk_fn_ptr(&fn_ty);
+                debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})",
+                       a, b, pointer_ty);
+                self.unify_and_identity(pointer_ty, b)
+                    .map(|(ty, _)| (ty, Adjust::ClosureFnPointer))
+            }
+            _ => self.unify_and_identity(a, b),
+        }
+    }
+
     fn coerce_unsafe_ptr(&self,
                          a: Ty<'tcx>,
                          b: Ty<'tcx>,

src/librustc_typeck/check/writeback.rs

@@ -412,6 +412,10 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                         adjustment::Adjust::MutToConstPointer
                     }
 
+                    adjustment::Adjust::ClosureFnPointer => {
+                        adjustment::Adjust::ClosureFnPointer
+                    }
+
                     adjustment::Adjust::UnsafeFnPointer => {
                         adjustment::Adjust::UnsafeFnPointer
                     }

src/test/run-pass/closure-to-fn-coercion.rs

@@ -0,0 +1,41 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-stage0: new feature, remove this when SNAP
+
+// #![feature(closure_to_fn_coercion)]
+
+const FOO :fn(u8) -> u8 = |v: u8| { v };
+
+const BAR: [fn(&mut u32); 5] = [
+    |_: &mut u32| {},
+    |v: &mut u32| *v += 1,
+    |v: &mut u32| *v += 2,
+    |v: &mut u32| *v += 3,
+    |v: &mut u32| *v += 4,
+];
+fn func_specific() -> (fn() -> u32) {
+    || return 42
+}
+
+fn main() {
+    // Items
+    assert_eq!(func_specific()(), 42);
+    let foo :fn(u8) -> u8 = |v: u8| { v };
+    assert_eq!(foo(31), 31);
+    // Constants
+    assert_eq!(FOO(31), 31);
+    let mut a :u32 = 0;
+    assert_eq!({BAR[0](&mut a); a }, 0);
+    assert_eq!({BAR[1](&mut a); a }, 1);
+    assert_eq!({BAR[2](&mut a); a }, 3);
+    assert_eq!({BAR[3](&mut a); a }, 6);
+    assert_eq!({BAR[4](&mut a); a }, 10);
+}