librustc: Add a hack to allow unboxed closures to achieve the 'static

bound if possible.

Closes rust-lang#16560.

src/librustc/middle/freevars.rs

@@ -24,7 +24,7 @@ use syntax::codemap::Span;
 use syntax::visit::Visitor;
 use syntax::visit;
 
-#[deriving(Clone, Decodable, Encodable, Show)]
+#[deriving(Clone, Decodable, Encodable, PartialEq, Eq, Show)]
 pub enum CaptureMode {
     /// Copy/move the value from this llvm ValueRef into the environment.
     CaptureByValue,

src/librustc/middle/ty.rs

@@ -2254,6 +2254,8 @@ pub fn type_contents(cx: &ctxt, ty: t) -> TypeContents {
                 // FIXME(#14449): `borrowed_contents` below assumes `&mut`
                 // unboxed closure.
                 let upvars = unboxed_closure_upvars(cx, did);
+                debug!("region of unboxed closure is {}",
+                       r.repr(cx));
                 TypeContents::union(upvars.as_slice(),
                                     |f| tc_ty(cx, f.ty, cache)) |
                     borrowed_contents(r, MutMutable)

src/librustc/middle/typeck/check/regionck.rs

@@ -666,14 +666,23 @@ fn check_expr_fn_block(rcx: &mut Rcx,
             });
         }
         ty::ty_unboxed_closure(_, region) => {
+            debug!("regionck: constraining region of unboxed closure");
             freevars::with_freevars(tcx, expr.id, |freevars| {
-                // No free variables means that there is no environment and
-                // hence the closure has static lifetime. Otherwise, the
-                // closure must not outlive the variables it closes over
-                // by-reference.
-                if !freevars.is_empty() {
-                    constrain_free_variables(rcx, region, expr, freevars);
-                }
+                if freevars.is_empty() {
+                    // No free variables means that there is no
+                    // environment and hence the closure has static
+                    // lifetime. Otherwise, the closure must not outlive
+                    // the variables it closes over by-reference.
+                    rcx.fcx.mk_subr(false,
+                                    infer::MiscRegion(expr.span),
+                                    ty::ReStatic,
+                                    region);
+                } else {
+                    constrain_free_variables(rcx,
+                                             region,
+                                             expr,
+                                             freevars);
+                } 
             })
         }
         _ => ()
@@ -709,36 +718,79 @@ fn check_expr_fn_block(rcx: &mut Rcx,
         let infcx = rcx.fcx.infcx();
         debug!("constrain_free_variables({}, {})",
                region.repr(tcx), expr.repr(tcx));
+        let capture_mode = freevars::get_capture_mode(tcx, expr.id);
+        let mut all_static = capture_mode == freevars::CaptureByValue;
         for freevar in freevars.iter() {
             debug!("freevar def is {:?}", freevar.def);
 
             // Identify the variable being closed over and its node-id.
             let def = freevar.def;
             let def_id = def.def_id();
             assert!(def_id.krate == ast::LOCAL_CRATE);
-            let upvar_id = ty::UpvarId { var_id: def_id.node,
-                                         closure_expr_id: expr.id };
-
-            // Create a region variable to represent this borrow. This borrow
-            // must outlive the region on the closure.
-            let origin = infer::UpvarRegion(upvar_id, expr.span);
-            let freevar_region = infcx.next_region_var(origin);
-            rcx.fcx.mk_subr(true, infer::FreeVariable(freevar.span, def_id.node),
-                            region, freevar_region);
-
-            // Create a UpvarBorrow entry. Note that we begin with a
-            // const borrow_kind, but change it to either mut or
-            // immutable as dictated by the uses.
-            let upvar_borrow = ty::UpvarBorrow { kind: ty::ImmBorrow,
-                                                 region: freevar_region };
-            rcx.fcx.inh.upvar_borrow_map.borrow_mut().insert(upvar_id,
-                                                             upvar_borrow);
-
-            // Guarantee that the closure does not outlive the variable itself.
-            let en_region = region_of_def(rcx.fcx, def);
-            debug!("en_region = {}", en_region.repr(tcx));
-            rcx.fcx.mk_subr(true, infer::FreeVariable(freevar.span, def_id.node),
-                            region, en_region);
+            let upvar_id = ty::UpvarId {
+                var_id: def_id.node,
+                closure_expr_id: expr.id,
+            };
+
+            if capture_mode == freevars::CaptureByRef {
+                // Create a region variable to represent this borrow. This
+                // borrow must outlive the region on the closure.
+                let origin = infer::UpvarRegion(upvar_id, expr.span);
+                let freevar_region = infcx.next_region_var(origin);
+                rcx.fcx.mk_subr(true,
+                                infer::FreeVariable(freevar.span,
+                                                    def_id.node),
+                                region,
+                                freevar_region);
+
+                // Create a UpvarBorrow entry. Note that we begin with a
+                // const borrow_kind, but change it to either mut or
+                // immutable as dictated by the uses.
+                let upvar_borrow = ty::UpvarBorrow {
+                    kind: ty::ImmBorrow,
+                    region: freevar_region,
+                };
+                rcx.fcx
+                   .inh
+                   .upvar_borrow_map
+                   .borrow_mut()
+                   .insert(upvar_id, upvar_borrow);
+
+                // Guarantee that the closure does not outlive the variable
+                // itself.
+                let en_region = region_of_def(rcx.fcx, def);
+                debug!("en_region = {}", en_region.repr(tcx));
+                rcx.fcx.mk_subr(true,
+                                infer::FreeVariable(freevar.span,
+                                                    def_id.node),
+                                region,
+                                en_region);
+            }
+
+            // FIXME(pcwalton): As a hack, if the type of the variable being
+            // closed over has no regions and this is a by-value capture,
+            // record a `'static` bound.
+            let local_type = rcx.fcx.local_ty(freevar.span, def_id.node);
+            let local_type =
+                rcx.fcx.infcx().resolve_type_vars_if_possible(local_type);
+            if all_static && !ty::type_is_static(rcx.tcx(), local_type) {
+                debug!("regionck: not static: {}",
+                       rcx.fcx
+                          .local_ty(freevar.span, def_id.node)
+                          .repr(rcx.tcx()));
+                all_static = false
+            }
+        }
+
+        // FIXME(pcwalton): As a hack, if the type of the variable being
+        // closed over has no regions and this is a by-value capture,
+        // record a `'static` bound.
+        if all_static {
+            debug!("regionck: all static!");
+            rcx.fcx.mk_subr(false,
+                            infer::MiscRegion(expr.span),
+                            ty::ReStatic,
+                            region);
         }
     }
 
@@ -1022,9 +1074,14 @@ fn constrain_regions_in_type_of_node(
                            rcx.fcx.inh.adjustments.borrow().find(&id),
                            |method_call| rcx.resolve_method_type(method_call));
     debug!("constrain_regions_in_type_of_node(\
-            ty={}, ty0={}, id={}, minimum_lifetime={:?})",
+            ty={},\
+            ty0={},\
+            id={},\
+            minimum_lifetime={:?},\
+            origin={:?})",
            ty_to_string(tcx, ty), ty_to_string(tcx, ty0),
-           id, minimum_lifetime);
+           id, minimum_lifetime,
+           origin);
     constrain_regions_in_type(rcx, minimum_lifetime, origin, ty);
 }
 

src/librustc/middle/typeck/infer/error_reporting.rs

@@ -620,6 +620,19 @@ impl<'a> ErrorReporting for InferCtxt<'a> {
                     sup,
                     "");
             }
+            infer::MiscRegion(span) => {
+                self.tcx.sess.span_err(span, "reference is not valid");
+                note_and_explain_region(
+                    self.tcx,
+                    "the reference is valid for ",
+                    sub,
+                    "");
+                note_and_explain_region(
+                    self.tcx,
+                    "but the referenced data is only valid for ",
+                    sup,
+                    "");
+            }
         }
     }
 
@@ -1425,6 +1438,9 @@ impl<'a> ErrorReportingHelpers for InferCtxt<'a> {
                     "...so that the pointer does not outlive the \
                     data it points at");
             }
+            infer::MiscRegion(span) => {
+                self.tcx.sess.span_note(span, "...so that reference is valid")
+            }
         }
     }
 }

src/librustc/middle/typeck/infer/mod.rs

@@ -194,6 +194,9 @@ pub enum SubregionOrigin {
 
     // An auto-borrow that does not enclose the expr where it occurs
     AutoBorrow(Span),
+
+    // Not yet categorized
+    MiscRegion(Span),
 }
 
 /// Reasons to create a region inference variable
@@ -905,6 +908,7 @@ impl SubregionOrigin {
             CallReturn(a) => a,
             AddrOf(a) => a,
             AutoBorrow(a) => a,
+            MiscRegion(a) => a,
         }
     }
 }
@@ -948,6 +952,7 @@ impl Repr for SubregionOrigin {
             CallReturn(a) => format!("CallReturn({})", a.repr(tcx)),
             AddrOf(a) => format!("AddrOf({})", a.repr(tcx)),
             AutoBorrow(a) => format!("AutoBorrow({})", a.repr(tcx)),
+            MiscRegion(a) => format!("MiscRegion({})", a.repr(tcx)),
         }
     }
 }

src/test/run-pass/unboxed-closures-send.rs

@@ -0,0 +1,40 @@
+// Copyright 2014 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.
+
+#![feature(unboxed_closures)]
+
+use std::mem;
+use std::ops::Fn;
+
+// Unsugared closure
+struct Closure(u8);
+
+impl Fn<(u8,), u8> for Closure {
+    extern "rust-call" fn call(&self, (y,): (u8,)) -> u8 {
+        let &Closure(x) = self;
+
+        x + y
+    }
+}
+
+fn main() {
+    let y = 0u8;
+    let closure = |&: x: u8| x + y;
+    let unsugared_closure = Closure(y);
+
+    // Check that both closures are capturing by value
+    println!("{}", mem::size_of_val(&closure));  // prints 1
+    println!("{}", mem::size_of_val(&unsugared_closure));  // prints 1
+
+    spawn(proc() {
+        let ok = unsugared_closure;
+        let err = closure;
+    })
+}