Implement better object defaults (RFC #1156)

src/librustc/diagnostics.rs

@@ -1160,6 +1160,42 @@ static mut FOO: Option<Box<usize>> = None;
 // error: mutable statics are not allowed to have destructors
 static mut BAR: Option<Vec<i32>> = None;
 ```
+"##,
+
+E0398: r##"
+In Rust 1.3, the default object lifetime bounds are expected to
+change, as described in RFC #1156 [1]. You are getting a warning
+because the compiler thinks it is possible that this change will cause
+a compilation error in your code. It is possible, though unlikely,
+that this is a false alarm.
+
+The heart of the change is that where `&'a Box<SomeTrait>` used to
+default to `&'a Box<SomeTrait+'a>`, it now defaults to `&'a
+Box<SomeTrait+'static>` (here, `SomeTrait` is the name of some trait
+type). Note that the only types which are affected are references to
+boxes, like `&Box<SomeTrait>` or `&[Box<SomeTrait>]`.  More common
+types like `&SomeTrait` or `Box<SomeTrait>` are unaffected.
+
+To silence this warning, edit your code to use an explicit bound.
+Most of the time, this means that you will want to change the
+signature of a function that you are calling. For example, if
+the error is reported on a call like `foo(x)`, and `foo` is
+defined as follows:
+
+```
+fn foo(arg: &Box<SomeTrait>) { ... }
+```
+
+you might change it to:
+
+```
+fn foo<'a>(arg: &Box<SomeTrait+'a>) { ... }
+```
+
+This explicitly states that you expect the trait object `SomeTrait` to
+contain references (with a maximum lifetime of `'a`).
+
+[1]: https://github.com/rust-lang/rfcs/pull/1156
 "##
 
 }

src/librustc/metadata/tydecode.rs

@@ -843,15 +843,15 @@ fn parse_type_param_def_<'a, 'tcx, F>(st: &mut PState<'a, 'tcx>, conv: &mut F)
 
 fn parse_object_lifetime_default<'a,'tcx, F>(st: &mut PState<'a,'tcx>,
                                              conv: &mut F)
-                                             -> Option<ty::ObjectLifetimeDefault>
+                                             -> ty::ObjectLifetimeDefault
     where F: FnMut(DefIdSource, ast::DefId) -> ast::DefId,
 {
     match next(st) {
-        'n' => None,
-        'a' => Some(ty::ObjectLifetimeDefault::Ambiguous),
+        'a' => ty::ObjectLifetimeDefault::Ambiguous,
+        'b' => ty::ObjectLifetimeDefault::BaseDefault,
         's' => {
             let region = parse_region_(st, conv);
-            Some(ty::ObjectLifetimeDefault::Specific(region))
+            ty::ObjectLifetimeDefault::Specific(region)
         }
         _ => panic!("parse_object_lifetime_default: bad input")
     }
@@ -887,9 +887,16 @@ fn parse_existential_bounds_<'a,'tcx, F>(st: &mut PState<'a,'tcx>,
         }
     }
 
+    let region_bound_will_change = match next(st) {
+        'y' => true,
+        'n' => false,
+        c => panic!("parse_ty: expected y/n not '{}'", c)
+    };
+
     return ty::ExistentialBounds { region_bound: region_bound,
                                    builtin_bounds: builtin_bounds,
-                                   projection_bounds: projection_bounds };
+                                   projection_bounds: projection_bounds,
+                                   region_bound_will_change: region_bound_will_change };
 }
 
 fn parse_builtin_bounds<F>(st: &mut PState, mut _conv: F) -> ty::BuiltinBounds where

src/librustc/metadata/tyencode.rs

@@ -390,6 +390,8 @@ pub fn enc_existential_bounds<'a,'tcx>(w: &mut Encoder,
     }
 
     mywrite!(w, ".");
+
+    mywrite!(w, "{}", if bs.region_bound_will_change {'y'} else {'n'});
 }
 
 pub fn enc_region_bounds<'a, 'tcx>(w: &mut Encoder,
@@ -414,12 +416,12 @@ pub fn enc_type_param_def<'a, 'tcx>(w: &mut Encoder, cx: &ctxt<'a, 'tcx>,
 
 fn enc_object_lifetime_default<'a, 'tcx>(w: &mut Encoder,
                                          cx: &ctxt<'a, 'tcx>,
-                                         default: Option<ty::ObjectLifetimeDefault>)
+                                         default: ty::ObjectLifetimeDefault)
 {
     match default {
-        None => mywrite!(w, "n"),
-        Some(ty::ObjectLifetimeDefault::Ambiguous) => mywrite!(w, "a"),
-        Some(ty::ObjectLifetimeDefault::Specific(r)) => {
+        ty::ObjectLifetimeDefault::Ambiguous => mywrite!(w, "a"),
+        ty::ObjectLifetimeDefault::BaseDefault => mywrite!(w, "b"),
+        ty::ObjectLifetimeDefault::Specific(r) => {
             mywrite!(w, "s");
             enc_region(w, cx, r);
         }

src/librustc/middle/infer/bivariate.rs

@@ -49,6 +49,11 @@ impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Bivariate<'a, 'tcx> {
 
     fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
 
+    fn will_change(&mut self, _: bool, _: bool) -> bool {
+        // since we are not comparing regions, we don't care
+        false
+    }
+
     fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
                                                variance: ty::Variance,
                                                a: &T,

src/librustc/middle/infer/combine.rs

@@ -56,6 +56,7 @@ pub struct CombineFields<'a, 'tcx: 'a> {
     pub infcx: &'a InferCtxt<'a, 'tcx>,
     pub a_is_expected: bool,
     pub trace: TypeTrace<'tcx>,
+    pub cause: Option<ty_relate::Cause>,
 }
 
 pub fn super_combine_tys<'a,'tcx:'a,R>(infcx: &InferCtxt<'a, 'tcx>,

src/librustc/middle/infer/equate.rs

@@ -34,6 +34,11 @@ impl<'a, 'tcx> TypeRelation<'a,'tcx> for Equate<'a, 'tcx> {
 
     fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
 
+    fn will_change(&mut self, a: bool, b: bool) -> bool {
+        // if either side changed from what it was, that could cause equality to fail
+        a || b
+    }
+
     fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
                                                _: ty::Variance,
                                                a: &T,

src/librustc/middle/infer/error_reporting.rs

@@ -593,7 +593,8 @@ impl<'a, 'tcx> ErrorReporting<'tcx> for InferCtxt<'a, 'tcx> {
                                sub: Region,
                                sup: Region) {
         match origin {
-            infer::Subtype(trace) => {
+            infer::Subtype(trace) |
+            infer::DefaultExistentialBound(trace) => {
                 let terr = ty::terr_regions_does_not_outlive(sup, sub);
                 self.report_and_explain_type_error(trace, &terr);
             }
@@ -1569,7 +1570,8 @@ impl<'a, 'tcx> ErrorReportingHelpers<'tcx> for InferCtxt<'a, 'tcx> {
 
     fn note_region_origin(&self, origin: &SubregionOrigin<'tcx>) {
         match *origin {
-            infer::Subtype(ref trace) => {
+            infer::Subtype(ref trace) |
+            infer::DefaultExistentialBound(ref trace) => {
                 let desc = match trace.origin {
                     infer::Misc(_) => {
                         "types are compatible"

src/librustc/middle/infer/glb.rs

@@ -35,6 +35,16 @@ impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Glb<'a, 'tcx> {
 
     fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
 
+    fn will_change(&mut self, a: bool, b: bool) -> bool {
+        // Hmm, so the result of GLB will still be a LB if one or both
+        // sides change to 'static, but it may no longer be the GLB.
+        // I'm going to go with `a || b` here to be conservative,
+        // since the result of this operation may be affected, though
+        // I think it would mostly be more accepting than before (since the result
+        // would be a bigger region).
+        a || b
+    }
+
     fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
                                                variance: ty::Variance,
                                                a: &T,

src/librustc/middle/infer/lub.rs

@@ -35,6 +35,11 @@ impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Lub<'a, 'tcx> {
 
     fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
 
+    fn will_change(&mut self, a: bool, b: bool) -> bool {
+        // result will be 'static if a || b
+        a || b
+    }
+
     fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
                                                variance: ty::Variance,
                                                a: &T,

src/librustc/middle/infer/mod.rs

@@ -194,6 +194,9 @@ pub enum SubregionOrigin<'tcx> {
     // Arose from a subtyping relation
     Subtype(TypeTrace<'tcx>),
 
+    // Arose from a subtyping relation
+    DefaultExistentialBound(TypeTrace<'tcx>),
+
     // Stack-allocated closures cannot outlive innermost loop
     // or function so as to ensure we only require finite stack
     InfStackClosure(Span),
@@ -658,7 +661,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                       -> CombineFields<'a, 'tcx> {
         CombineFields {infcx: self,
                        a_is_expected: a_is_expected,
-                       trace: trace}
+                       trace: trace,
+                       cause: None}
     }
 
     // public so that it can be used from the rustc_driver unit tests
@@ -1464,6 +1468,7 @@ impl<'tcx> SubregionOrigin<'tcx> {
     pub fn span(&self) -> Span {
         match *self {
             Subtype(ref a) => a.span(),
+            DefaultExistentialBound(ref a) => a.span(),
             InfStackClosure(a) => a,
             InvokeClosure(a) => a,
             DerefPointer(a) => a,

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

@@ -1358,9 +1358,56 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
             }
         }
 
+        // Check for future hostile edges tied to a bad default
+        self.report_future_hostility(&graph);
+
         (0..self.num_vars() as usize).map(|idx| var_data[idx].value).collect()
     }
 
+    fn report_future_hostility(&self, graph: &RegionGraph) {
+        let constraints = self.constraints.borrow();
+        for edge in graph.all_edges() {
+            match constraints[&edge.data] {
+                SubregionOrigin::DefaultExistentialBound(_) => {
+                    // this will become 'static in the future
+                }
+                _ => { continue; }
+            }
+
+            // this constraint will become a 'static constraint in the
+            // future, so walk outward and see if we have any hard
+            // bounds that could not be inferred to 'static
+            for nid in graph.depth_traverse(edge.target()) {
+                for (_, succ) in graph.outgoing_edges(nid) {
+                    match succ.data {
+                        ConstrainVarSubReg(_, r) => {
+                            match r {
+                                ty::ReStatic | ty::ReInfer(_) => {
+                                    /* OK */
+                                }
+                                ty::ReFree(_) | ty::ReScope(_) | ty::ReEmpty => {
+                                    span_warn!(
+                                        self.tcx.sess,
+                                        constraints[&edge.data].span(),
+                                        E0398,
+                                        "this code may fail to compile in Rust 1.3 due to \
+                                         the proposed change in object lifetime bound defaults");
+                                    return; // only issue the warning once per fn
+                                }
+                                ty::ReEarlyBound(..) | ty::ReLateBound(..) => {
+                                    self.tcx.sess.span_bug(
+                                        constraints[&succ.data].span(),
+                                        "relation to bound region");
+                                }
+                            }
+                        }
+                        _ => { }
+                    }
+                }
+            }
+        }
+    }
+
     fn construct_graph(&self) -> RegionGraph {
         let num_vars = self.num_vars();
 

src/librustc/middle/infer/sub.rs

@@ -10,16 +10,17 @@
 
 use super::combine::{self, CombineFields};
 use super::higher_ranked::HigherRankedRelations;
-use super::Subtype;
+use super::SubregionOrigin;
 use super::type_variable::{SubtypeOf, SupertypeOf};
 
 use middle::ty::{self, Ty};
 use middle::ty::TyVar;
-use middle::ty_relate::{Relate, RelateResult, TypeRelation};
+use middle::ty_relate::{Cause, Relate, RelateResult, TypeRelation};
+use std::mem;
 
 /// "Greatest lower bound" (common subtype)
 pub struct Sub<'a, 'tcx: 'a> {
-    fields: CombineFields<'a, 'tcx>
+    fields: CombineFields<'a, 'tcx>,
 }
 
 impl<'a, 'tcx> Sub<'a, 'tcx> {
@@ -33,6 +34,25 @@ impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Sub<'a, 'tcx> {
     fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.fields.infcx.tcx }
     fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
 
+    fn with_cause<F,R>(&mut self, cause: Cause, f: F) -> R
+        where F: FnOnce(&mut Self) -> R
+    {
+        debug!("sub with_cause={:?}", cause);
+        let old_cause = mem::replace(&mut self.fields.cause, Some(cause));
+        let r = f(self);
+        debug!("sub old_cause={:?}", old_cause);
+        self.fields.cause = old_cause;
+        r
+    }
+
+    fn will_change(&mut self, a: bool, b: bool) -> bool {
+        // if we have (Foo+'a) <: (Foo+'b), this requires that 'a:'b.
+        // So if 'a becomes 'static, no additional errors can occur.
+        // OTOH, if 'a stays the same, but 'b becomes 'static, we
+        // could have a problem.
+        !a && b
+    }
+
     fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
                                                variance: ty::Variance,
                                                a: &T,
@@ -84,11 +104,14 @@ impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Sub<'a, 'tcx> {
     }
 
     fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
-        debug!("{}.regions({:?}, {:?})",
-               self.tag(),
-               a,
-               b);
-        let origin = Subtype(self.fields.trace.clone());
+        debug!("{}.regions({:?}, {:?}) self.cause={:?}",
+               self.tag(), a, b, self.fields.cause);
+        let origin = match self.fields.cause {
+            Some(Cause::ExistentialRegionBound(true)) =>
+                SubregionOrigin::DefaultExistentialBound(self.fields.trace.clone()),
+            _ =>
+                SubregionOrigin::Subtype(self.fields.trace.clone()),
+        };
         self.fields.infcx.region_vars.make_subregion(origin, a, b);
         Ok(a)
     }

src/librustc/middle/traits/select.rs

@@ -2445,6 +2445,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     region_bound: data_b.bounds.region_bound,
                     builtin_bounds: data_b.bounds.builtin_bounds,
                     projection_bounds: data_a.bounds.projection_bounds.clone(),
+                    region_bound_will_change: data_b.bounds.region_bound_will_change,
                 };
 
                 let new_trait = tcx.mk_trait(data_a.principal.clone(), bounds);

src/librustc/middle/ty.rs

@@ -2055,6 +2055,11 @@ pub struct ExistentialBounds<'tcx> {
     pub region_bound: ty::Region,
     pub builtin_bounds: BuiltinBounds,
     pub projection_bounds: Vec<PolyProjectionPredicate<'tcx>>,
+
+    // If true, this TyTrait used a "default bound" in the surface
+    // syntax.  This makes no difference to the type system but is
+    // handy for error reporting.
+    pub region_bound_will_change: bool,
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
@@ -2245,6 +2250,9 @@ pub enum ObjectLifetimeDefault {
     /// `T:'a` constraints are found.
     Ambiguous,
 
+    /// Use the base default, typically 'static, but in a fn body it is a fresh variable
+    BaseDefault,
+
     /// Use the given region as the default.
     Specific(Region),
 }
@@ -2256,7 +2264,7 @@ pub struct TypeParameterDef<'tcx> {
     pub space: subst::ParamSpace,
     pub index: u32,
     pub default: Option<Ty<'tcx>>,
-    pub object_lifetime_default: Option<ObjectLifetimeDefault>,
+    pub object_lifetime_default: ObjectLifetimeDefault,
 }
 
 #[derive(RustcEncodable, RustcDecodable, Clone, Debug)]
@@ -7328,6 +7336,7 @@ impl<'tcx> fmt::Debug for ObjectLifetimeDefault {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
             ObjectLifetimeDefault::Ambiguous => write!(f, "Ambiguous"),
+            ObjectLifetimeDefault::BaseDefault => write!(f, "BaseDefault"),
             ObjectLifetimeDefault::Specific(ref r) => write!(f, "{:?}", r),
         }
     }