auto merge of #9863 : csainty/rust/issue-9755-once-fns-feature-directive, r=alexcrichton

Hello,

First time rust contributor here, please let me know if I need to sort out the contribution agreement for this.

I picked issue #9755 to dip my toe in the water, this pull request isn't quite complete though as I have not updated the documentation. The reason for this is that I haven't tracked down why this feature is gated so I don't feel I can write a justification of the same quality as the other features have been documented.
If someone would like to explain or point me at a mail thread I am happy to update with this change.

Hopefully I have understood the process of converting the old flag into a directive correctly.

Also just to call out what I am sure if a known quirk when adding feature directives, you can't build this code unless you have a snapshot of the compiler which knows about the feature directive. Chicken and the egg. I split the change into two commits, the first should be able to build a snapshot that can compile the second.

Author: bors

doc/rust.md

@@ -2067,6 +2067,10 @@ The currently implemented features of the compiler are:
                      For now this style of variant is hidden behind a feature
                      flag.
 
+* `once_fns` - Onceness guarantees a closure is only executed once. Defining a
+               closure as `once` is unlikely to be supported going forward. So
+               they are hidden behind this feature until they are to be removed. 
+
 If a feature is promoted to a language feature, then all existing programs will
 start to receive compilation warnings about #[feature] directives which enabled
 the new feature (because the directive is no longer necessary). However, if

src/librustc/driver/session.rs

@@ -1,4 +1,4 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -74,12 +74,11 @@ pub static statik:                  uint = 1 << 21;
 pub static print_link_args:         uint = 1 << 22;
 pub static no_debug_borrows:        uint = 1 << 23;
 pub static lint_llvm:               uint = 1 << 24;
-pub static once_fns:                uint = 1 << 25;
-pub static print_llvm_passes:       uint = 1 << 26;
-pub static no_vectorize_loops:      uint = 1 << 27;
-pub static no_vectorize_slp:        uint = 1 << 28;
-pub static no_prepopulate_passes:   uint = 1 << 29;
-pub static use_softfp:              uint = 1 << 30;
+pub static print_llvm_passes:       uint = 1 << 25;
+pub static no_vectorize_loops:      uint = 1 << 26;
+pub static no_vectorize_slp:        uint = 1 << 27;
+pub static no_prepopulate_passes:   uint = 1 << 28;
+pub static use_softfp:              uint = 1 << 29;
 
 pub fn debugging_opts_map() -> ~[(&'static str, &'static str, uint)] {
     ~[("verbose", "in general, enable more debug printouts", verbose),
@@ -118,9 +117,6 @@ pub fn debugging_opts_map() -> ~[(&'static str, &'static str, uint)] {
      ("lint-llvm",
       "Run the LLVM lint pass on the pre-optimization IR",
       lint_llvm),
-     ("once-fns",
-      "Allow 'once fn' closures to deinitialize captured variables",
-      once_fns),
      ("print-llvm-passes",
       "Prints the llvm optimization passes being run",
       print_llvm_passes),
@@ -326,7 +322,6 @@ impl Session_ {
     pub fn debug_borrows(&self) -> bool {
         self.opts.optimize == No && !self.debugging_opt(no_debug_borrows)
     }
-    pub fn once_fns(&self) -> bool { self.debugging_opt(once_fns) }
     pub fn print_llvm_passes(&self) -> bool {
         self.debugging_opt(print_llvm_passes)
     }

src/librustc/front/feature_gate.rs

@@ -33,6 +33,7 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("globs", Active),
     ("macro_rules", Active),
     ("struct_variant", Active),
+    ("once_fns", Active),
 
     // These are used to test this portion of the compiler, they don't actually
     // mean anything
@@ -126,6 +127,20 @@ impl Visitor<()> for Context {
 
         visit::walk_item(self, i, ());
     }
+
+    fn visit_ty(&mut self, t: &ast::Ty, _: ()) {
+        match t.node {
+            ast::ty_closure(closure) if closure.onceness == ast::Once => {
+                self.gate_feature("once_fns", t.span,
+                                  "once functions are \
+                                   experimental and likely to be removed");
+
+            },
+            _ => {}
+        }
+
+        visit::walk_ty(self, t, ());
+    }
 }
 
 pub fn check_crate(sess: Session, crate: &ast::Crate) {

src/librustc/middle/borrowck/gather_loans/gather_moves.rs

@@ -1,4 +1,4 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -102,25 +102,13 @@ fn check_is_legal_to_move_from(bccx: &BorrowckCtxt,
     match cmt.cat {
         mc::cat_deref(_, _, mc::region_ptr(*)) |
         mc::cat_deref(_, _, mc::gc_ptr(*)) |
-        mc::cat_deref(_, _, mc::unsafe_ptr(*)) => {
-            bccx.span_err(
-                cmt0.span,
-                format!("cannot move out of {}",
-                     bccx.cmt_to_str(cmt)));
-            false
-        }
-
-        // These are separate from the above cases for a better error message.
+        mc::cat_deref(_, _, mc::unsafe_ptr(*)) |
         mc::cat_stack_upvar(*) |
         mc::cat_copied_upvar(mc::CopiedUpvar { onceness: ast::Many, _ }) => {
-            let once_hint = if bccx.tcx.sess.once_fns() {
-                " (unless the destination closure type is `once fn')"
-            } else {
-                ""
-            };
             bccx.span_err(
                 cmt0.span,
-                format!("cannot move out of {}{}", bccx.cmt_to_str(cmt), once_hint));
+                format!("cannot move out of {}",
+                     bccx.cmt_to_str(cmt)));
             false
         }
 

src/librustc/middle/mem_categorization.rs

@@ -1,4 +1,4 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -508,12 +508,10 @@ impl mem_categorization_ctxt {
                       let var_is_refd = match (closure_ty.sigil, closure_ty.onceness) {
                           // Many-shot stack closures can never move out.
                           (ast::BorrowedSigil, ast::Many) => true,
-                          // 1-shot stack closures can move out with "-Z once-fns".
-                          (ast::BorrowedSigil, ast::Once)
-                              if self.tcx.sess.once_fns() => false,
-                          (ast::BorrowedSigil, ast::Once) => true,
+                          // 1-shot stack closures can move out.
+                          (ast::BorrowedSigil, ast::Once) => false,
                           // Heap closures always capture by copy/move, and can
-                          // move out iff they are once.
+                          // move out if they are once.
                           (ast::OwnedSigil, _) |
                           (ast::ManagedSigil, _) => false,
 

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

@@ -3645,7 +3645,7 @@ pub fn check_intrinsic_type(ccx: @mut CrateCtxt, it: @ast::foreign_item) {
               let fty = ty::mk_closure(ccx.tcx, ty::ClosureTy {
                   purity: ast::impure_fn,
                   sigil: ast::BorrowedSigil,
-                  onceness: ast::Once,
+                  onceness: ast::Many,
                   region: ty::re_bound(ty::br_anon(0)),
                   bounds: ty::EmptyBuiltinBounds(),
                   sig: ty::FnSig {

src/libstd/unstable/intrinsics.rs

@@ -331,7 +331,8 @@ extern "rust-intrinsic" {
 
     pub fn visit_tydesc(td: *TyDesc, tv: &mut TyVisitor);
 
-    pub fn frame_address(f: &once fn(*u8));
+    #[cfg(not(stage0))]
+    pub fn frame_address(f: &fn(*u8));
 
     /// Get the address of the `__morestack` stack growth function.
     pub fn morestack_addr() -> *();

src/test/compile-fail/once-cant-call-twice-on-heap.rs

@@ -11,6 +11,7 @@
 // Testing guarantees provided by once functions.
 // This program would segfault if it were legal.
 
+#[feature(once_fns)];
 extern mod extra;
 use extra::arc;
 use std::util;

src/test/compile-fail/once-cant-call-twice-on-stack.rs

@@ -11,7 +11,7 @@
 // Testing guarantees provided by once functions.
 // This program would segfault if it were legal.
 
-// compile-flags:-Z once-fns
+#[feature(once_fns)];
 extern mod extra;
 use extra::arc;
 use std::util;

src/test/compile-fail/once-fn-subtyping.rs

@@ -1,4 +1,4 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#[feature(once_fns)];
 fn main() {
     let f: &once fn() = ||();
     let g: &fn() = f;  //~ ERROR mismatched types

src/test/run-pass/intrinsic-frame-address.rs

@@ -1,4 +1,4 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -12,7 +12,7 @@
 
 mod rusti {
     extern "rust-intrinsic" {
-        pub fn frame_address(f: &once fn(*u8));
+        pub fn frame_address(f: &fn(*u8));
     }
 }
 

src/test/run-pass/once-move-out-on-heap.rs

@@ -12,6 +12,7 @@
 
 // xfail-fast
 
+#[feature(once_fns)];
 extern mod extra;
 use extra::arc;
 use std::util;

src/test/run-pass/once-move-out-on-stack.rs

@@ -12,7 +12,7 @@
 
 // xfail-fast
 
-// compile-flags:-Z once-fns
+#[feature(once_fns)];
 extern mod extra;
 use extra::arc;
 use std::util;