Implement GLB algorithm. (Issue #2263)

r=brson

Author: nikomatsakis

src/librustc/metadata/tyencode.rs

@@ -188,6 +188,9 @@ fn enc_bound_region(w: io::Writer, cx: @ctxt, br: ty::bound_region) {
         w.write_char('|');
         enc_bound_region(w, cx, *br);
       }
+      ty::br_fresh(id) => {
+        w.write_uint(id);
+      }
     }
 }
 

src/librustc/middle/astencode.rs

@@ -418,7 +418,8 @@ impl ty::Region: tr {
 impl ty::bound_region: tr {
     fn tr(xcx: extended_decode_ctxt) -> ty::bound_region {
         match self {
-            ty::br_anon(_) | ty::br_named(_) | ty::br_self => self,
+            ty::br_anon(_) | ty::br_named(_) | ty::br_self |
+            ty::br_fresh(_) => self,
             ty::br_cap_avoid(id, br) => ty::br_cap_avoid(xcx.tr_id(id),
                                                          @br.tr(xcx))
         }

src/librustc/middle/ty.rs

@@ -72,7 +72,7 @@ export expr_ty_params_and_ty;
 export expr_is_lval, expr_kind;
 export ExprKind, LvalueExpr, RvalueDatumExpr, RvalueDpsExpr, RvalueStmtExpr;
 export field_ty;
-export fold_ty, fold_sty_to_ty, fold_region, fold_regions;
+export fold_ty, fold_sty_to_ty, fold_region, fold_regions, fold_sig;
 export apply_op_on_t_to_ty_fn;
 export fold_regions_and_ty, walk_regions_and_ty;
 export field;
@@ -145,7 +145,7 @@ export ty_struct;
 export Region, bound_region, encl_region;
 export re_bound, re_free, re_scope, re_static, re_infer;
 export ReVar, ReSkolemized;
-export br_self, br_anon, br_named, br_cap_avoid;
+export br_self, br_anon, br_named, br_cap_avoid, br_fresh;
 export get, type_has_params, type_needs_infer, type_has_regions;
 export type_is_region_ptr;
 export type_id;
@@ -606,6 +606,9 @@ enum bound_region {
     /// Named region parameters for functions (a in &a/T)
     br_named(ast::ident),
 
+    /// Fresh bound identifiers created during GLB computations.
+    br_fresh(uint),
+
     /**
      * Handles capture-avoiding substitution in a rather subtle case.  If you
      * have a closure whose argument types are being inferred based on the
@@ -1311,6 +1314,17 @@ fn fold_sty_to_ty(tcx: ty::ctxt, sty: &sty, foldop: fn(t) -> t) -> t {
     mk_t(tcx, fold_sty(sty, foldop))
 }
 
+fn fold_sig(sig: &FnSig, fldop: fn(t) -> t) -> FnSig {
+    let args = do sig.inputs.map |arg| {
+        { mode: arg.mode, ty: fldop(arg.ty) }
+    };
+
+    FnSig {
+        inputs: move args,
+        output: fldop(sig.output)
+    }
+}
+
 fn fold_sty(sty: &sty, fldop: fn(t) -> t) -> sty {
     fn fold_substs(substs: &substs, fldop: fn(t) -> t) -> substs {
         {self_r: substs.self_r,
@@ -1489,8 +1503,8 @@ fn apply_op_on_t_to_ty_fn(
 fn fold_regions(
     cx: ctxt,
     ty: t,
-    fldr: fn(r: Region, in_fn: bool) -> Region) -> t {
-
+    fldr: fn(r: Region, in_fn: bool) -> Region) -> t
+{
     fn do_fold(cx: ctxt, ty: t, in_fn: bool,
                fldr: fn(Region, bool) -> Region) -> t {
         if !type_has_regions(ty) { return ty; }
@@ -2744,7 +2758,10 @@ impl bound_region : to_bytes::IterBytes {
           to_bytes::iter_bytes_2(&2u8, ident, lsb0, f),
 
           ty::br_cap_avoid(ref id, ref br) =>
-          to_bytes::iter_bytes_3(&3u8, id, br, lsb0, f)
+          to_bytes::iter_bytes_3(&3u8, id, br, lsb0, f),
+
+          ty::br_fresh(ref x) =>
+          to_bytes::iter_bytes_2(&4u8, x, lsb0, f)
         }
     }
 }
@@ -4493,6 +4510,12 @@ impl bound_region : cmp::Eq {
                     _ => false
                 }
             }
+            br_fresh(e0a) => {
+                match (*other) {
+                    br_fresh(e0b) => e0a == e0b,
+                    _ => false
+                }
+            }
         }
     }
     pure fn ne(&self, other: &bound_region) -> bool { !(*self).eq(other) }

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

@@ -80,7 +80,7 @@ enum Assign = combine_fields;
 
 impl Assign {
     fn tys(a: ty::t, b: ty::t) -> ares {
-        debug!("Assign.tys(%s -> %s)",
+        debug!("Assign.tys(%s => %s)",
                a.to_str(self.infcx),
                b.to_str(self.infcx));
         let _r = indenter();
@@ -139,7 +139,7 @@ priv impl Assign {
         a: ty::t, b: ty::t,
         +a_bnd: Option<ty::t>, +b_bnd: Option<ty::t>) -> ares {
 
-        debug!("Assign.assign_tys_or_sub(%s -> %s, %s -> %s)",
+        debug!("Assign.assign_tys_or_sub(%s => %s, %s => %s)",
                a.to_str(self.infcx), b.to_str(self.infcx),
                a_bnd.to_str(self.infcx), b_bnd.to_str(self.infcx));
         let _r = indenter();

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

@@ -69,6 +69,7 @@ trait combine {
     fn infcx() -> infer_ctxt;
     fn tag() -> ~str;
     fn a_is_expected() -> bool;
+    fn span() -> span;
 
     fn sub() -> Sub;
     fn lub() -> Lub;

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

@@ -15,6 +15,8 @@ use middle::typeck::infer::lattice::*;
 use middle::typeck::infer::sub::Sub;
 use middle::typeck::infer::to_str::ToStr;
 
+use std::list;
+
 use syntax::ast::{Many, Once};
 
 enum Glb = combine_fields;  // "greatest lower bound" (common subtype)
@@ -23,6 +25,7 @@ impl Glb: combine {
     fn infcx() -> infer_ctxt { self.infcx }
     fn tag() -> ~str { ~"glb" }
     fn a_is_expected() -> bool { self.a_is_expected }
+    fn span() -> span { self.span }
 
     fn sub() -> Sub { Sub(*self) }
     fn lub() -> Lub { Lub(*self) }
@@ -144,7 +147,122 @@ impl Glb: combine {
     }
 
     fn fns(a: &ty::FnTy, b: &ty::FnTy) -> cres<ty::FnTy> {
-        super_fns(&self, a, b)
+        // Note: this is a subtle algorithm.  For a full explanation,
+        // please see the large comment in `region_inference.rs`.
+
+        debug!("%s.fns(%?, %?)",
+               self.tag(), a.to_str(self.infcx), b.to_str(self.infcx));
+        let _indenter = indenter();
+
+        // Take a snapshot.  We'll never roll this back, but in later
+        // phases we do want to be able to examine "all bindings that
+        // were created as part of this type comparison", and making a
+        // snapshot is a convenient way to do that.
+        let snapshot = self.infcx.region_vars.start_snapshot();
+
+        // Instantiate each bound region with a fresh region variable.
+        let (a_with_fresh, a_isr) =
+            self.infcx.replace_bound_regions_with_fresh_regions(
+                self.span, a);
+        let a_vars = var_ids(&self, a_isr);
+        let (b_with_fresh, b_isr) =
+            self.infcx.replace_bound_regions_with_fresh_regions(
+                self.span, b);
+        let b_vars = var_ids(&self, b_isr);
+
+        // Collect constraints.
+        let fn_ty0 = if_ok!(super_fns(&self, &a_with_fresh, &b_with_fresh));
+        debug!("fn_ty0 = %s", fn_ty0.to_str(self.infcx));
+
+        // Generalize the regions appearing in fn_ty0 if possible
+        let new_vars =
+            self.infcx.region_vars.vars_created_since_snapshot(snapshot);
+        let fn_ty1 =
+            self.infcx.fold_regions_in_sig(
+                &fn_ty0,
+                |r, _in_fn| generalize_region(&self, snapshot,
+                                              new_vars, a_isr, a_vars, b_vars,
+                                              r));
+        debug!("fn_ty1 = %s", fn_ty1.to_str(self.infcx));
+        return Ok(move fn_ty1);
+
+        fn generalize_region(self: &Glb,
+                             snapshot: uint,
+                             new_vars: &[RegionVid],
+                             a_isr: isr_alist,
+                             a_vars: &[RegionVid],
+                             b_vars: &[RegionVid],
+                             r0: ty::Region) -> ty::Region {
+            if !is_var_in_set(new_vars, r0) {
+                return r0;
+            }
+
+            let tainted = self.infcx.region_vars.tainted(snapshot, r0);
+
+            let mut a_r = None, b_r = None, only_new_vars = true;
+            for tainted.each |r| {
+                if is_var_in_set(a_vars, *r) {
+                    if a_r.is_some() {
+                        return fresh_bound_variable(self);
+                    } else {
+                        a_r = Some(*r);
+                    }
+                } else if is_var_in_set(b_vars, *r) {
+                    if b_r.is_some() {
+                        return fresh_bound_variable(self);
+                    } else {
+                        b_r = Some(*r);
+                    }
+                } else if !is_var_in_set(new_vars, *r) {
+                    only_new_vars = false;
+                }
+            }
+
+                // NB---I do not believe this algorithm computes
+                // (necessarily) the GLB.  As written it can
+                // spuriously fail.  In particular, if there is a case
+                // like: fn(fn(&a)) and fn(fn(&b)), where a and b are
+                // free, it will return fn(&c) where c = GLB(a,b).  If
+                // however this GLB is not defined, then the result is
+                // an error, even though something like
+                // "fn<X>(fn(&X))" where X is bound would be a
+                // subtype of both of those.
+                //
+                // The problem is that if we were to return a bound
+                // variable, we'd be computing a lower-bound, but not
+                // necessarily the *greatest* lower-bound.
+
+            if a_r.is_some() && b_r.is_some() && only_new_vars {
+                // Related to exactly one bound variable from each fn:
+                return rev_lookup(self, a_isr, a_r.get());
+            } else if a_r.is_none() && b_r.is_none() {
+                // Not related to bound variables from either fn:
+                return r0;
+            } else {
+                // Other:
+                return fresh_bound_variable(self);
+            }
+        }
+
+        fn rev_lookup(self: &Glb,
+                      a_isr: isr_alist,
+                      r: ty::Region) -> ty::Region
+        {
+            for list::each(a_isr) |pair| {
+                let (a_br, a_r) = *pair;
+                if a_r == r {
+                    return ty::re_bound(a_br);
+                }
+            }
+
+            self.infcx.tcx.sess.span_bug(
+                self.span,
+                fmt!("could not find original bound region for %?", r));
+        }
+
+        fn fresh_bound_variable(self: &Glb) -> ty::Region {
+            self.infcx.region_vars.new_bound()
+        }
     }
 
     fn fn_metas(a: &ty::FnMeta, b: &ty::FnMeta) -> cres<ty::FnMeta> {

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

@@ -14,6 +14,8 @@ use middle::typeck::infer::combine::*;
 use middle::typeck::infer::unify::*;
 use middle::typeck::infer::to_str::ToStr;
 
+use std::list;
+
 // ______________________________________________________________________
 // Lattice operations on variables
 //
@@ -158,3 +160,29 @@ fn lattice_var_and_t<L:lattice_ops combine>(
       }
     }
 }
+
+// ___________________________________________________________________________
+// Random utility functions used by LUB/GLB when computing LUB/GLB of
+// fn types
+
+fn var_ids<T: combine>(self: &T, isr: isr_alist) -> ~[RegionVid] {
+    let mut result = ~[];
+    for list::each(isr) |pair| {
+        match pair.second() {
+            ty::re_infer(ty::ReVar(r)) => { result.push(r); }
+            r => {
+                self.infcx().tcx.sess.span_bug(
+                    self.span(),
+                    fmt!("Found non-region-vid: %?", r));
+            }
+        }
+    }
+    return result;
+}
+
+fn is_var_in_set(new_vars: &[RegionVid], r: ty::Region) -> bool {
+    match r {
+        ty::re_infer(ty::ReVar(ref v)) => new_vars.contains(v),
+        _ => false
+    }
+}

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

@@ -31,6 +31,7 @@ impl Lub: combine {
     fn infcx() -> infer_ctxt { self.infcx }
     fn tag() -> ~str { ~"lub" }
     fn a_is_expected() -> bool { self.a_is_expected }
+    fn span() -> span { self.span }
 
     fn sub() -> Sub { Sub(*self) }
     fn lub() -> Lub { Lub(*self) }
@@ -139,12 +140,10 @@ impl Lub: combine {
         let new_vars =
             self.infcx.region_vars.vars_created_since_snapshot(snapshot);
         let fn_ty1 =
-            ty::apply_op_on_t_to_ty_fn(
-                self.infcx.tcx, &fn_ty0,
-                |t| ty::fold_regions(
-                    self.infcx.tcx, t,
-                    |r, _in_fn| generalize_region(&self, snapshot,
-                                                  new_vars, a_isr, r)));
+            self.infcx.fold_regions_in_sig(
+                &fn_ty0,
+                |r, _in_fn| generalize_region(&self, snapshot, new_vars,
+                                              a_isr, r));
         return Ok(move fn_ty1);
 
         fn generalize_region(self: &Lub,
@@ -153,7 +152,7 @@ impl Lub: combine {
                              a_isr: isr_alist,
                              r0: ty::Region) -> ty::Region {
             // Regions that pre-dated the LUB computation stay as they are.
-            if !is_new_var(new_vars, r0) {
+            if !is_var_in_set(new_vars, r0) {
                 debug!("generalize_region(r0=%?): not new variable", r0);
                 return r0;
             }
@@ -163,7 +162,7 @@ impl Lub: combine {
             // Variables created during LUB computation which are
             // *related* to regions that pre-date the LUB computation
             // stay as they are.
-            if !tainted.all(|r| is_new_var(new_vars, *r)) {
+            if !tainted.all(|r| is_var_in_set(new_vars, *r)) {
                 debug!("generalize_region(r0=%?): \
                         non-new-variables found in %?",
                        r0, tainted);
@@ -190,13 +189,6 @@ impl Lub: combine {
                 fmt!("Region %? is not associated with \
                       any bound region from A!", r0));
         }
-
-        fn is_new_var(new_vars: &[RegionVid], r: ty::Region) -> bool {
-            match r {
-                ty::re_infer(ty::ReVar(ref v)) => new_vars.contains(v),
-                _ => false
-            }
-        }
     }
 
     fn fn_metas(a: &ty::FnMeta, b: &ty::FnMeta) -> cres<ty::FnMeta> {

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

@@ -799,5 +799,16 @@ impl infer_ctxt {
         (fn_ty, isr)
     }
 
+    fn fold_regions_in_sig(
+        &self,
+        fn_ty: &ty::FnTy,
+        fldr: &fn(r: ty::Region, in_fn: bool) -> ty::Region) -> ty::FnTy
+    {
+        let sig = do ty::fold_sig(&fn_ty.sig) |t| {
+            ty::fold_regions(self.tcx, t, fldr)
+        };
+        ty::FnTyBase {meta: fn_ty.meta, sig: sig}
+    }
+
 }