new rules for merging expected/supplied types in closure signatures

Also, fix numbering in mir-opt tests. We are now anonymizing more
consistently, I think, and hence some of the `TyAnon` indices shifted.

18 files changed, 646 insertions, 68 deletions

diff --git a/src/librustc_typeck/check/closure.rs b/src/librustc_typeck/check/closure.rs
index fac2119cb44..df0f3a98858 100644
--- a/src/librustc_typeck/check/closure.rs
+++ b/src/librustc_typeck/check/closure.rs
@@ -13,14 +13,22 @@
 use super::{check_fn, Expectation, FnCtxt};
 
 use astconv::AstConv;
+use rustc::hir::def_id::DefId;
+use rustc::infer::LateBoundRegionConversionTime;
 use rustc::infer::type_variable::TypeVariableOrigin;
 use rustc::ty::{self, ToPolyTraitRef, Ty};
 use rustc::ty::subst::Substs;
+use rustc::ty::TypeFoldable;
 use std::cmp;
 use std::iter;
 use syntax::abi::Abi;
 use rustc::hir;
 
+struct ClosureSignatures<'tcx> {
+    bound_sig: ty::PolyFnSig<'tcx>,
+    liberated_sig: ty::FnSig<'tcx>,
+}
+
 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
     pub fn check_expr_closure(&self,
                               expr: &hir::Expr,
@@ -56,15 +64,13 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                expected_sig);
 
         let expr_def_id = self.tcx.hir.local_def_id(expr.id);
-        let sig = self.ty_of_closure(decl, expected_sig);
 
-        debug!("check_closure: ty_of_closure returns {:?}", sig);
+        let ClosureSignatures { bound_sig, liberated_sig } =
+            self.sig_of_closure(expr_def_id, decl, body, expected_sig);
 
-        // `deduce_expectations_from_expected_type` introduces late-bound
-        // lifetimes defined elsewhere, which we need to anonymize away.
-        let sig = self.tcx.anonymize_late_bound_regions(&sig);
+        debug!("check_closure: ty_of_closure returns {:?}", liberated_sig);
 
-        debug!("check_closure: anonymized sig={:?}", sig);
+        let interior = check_fn(self, self.param_env, liberated_sig, decl, expr.id, body, true).1;
 
         // Create type variables (for now) to represent the transformed
         // types of upvars. These will be unified during the upvar
@@ -75,14 +81,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 |_, _| span_bug!(expr.span, "closure has region param"),
                 |_, _| self.infcx.next_ty_var(TypeVariableOrigin::TransformedUpvar(expr.span))
         );
-
-        let fn_sig = self.liberate_late_bound_regions(expr_def_id, &sig);
-        let fn_sig = self.inh.normalize_associated_types_in(body.value.span,
-                                                            body.value.id,
-                                                            self.param_env,
-                                                            &fn_sig);
-
-        let interior = check_fn(self, self.param_env, fn_sig, decl, expr.id, body, true).1;
+        let closure_type = self.tcx.mk_closure(expr_def_id, substs);
 
         if let Some(interior) = interior {
             let closure_substs = ty::ClosureSubsts {
@@ -91,13 +90,11 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             return self.tcx.mk_generator(expr_def_id, closure_substs, interior);
         }
 
-        let closure_type = self.tcx.mk_closure(expr_def_id, substs);
-
         debug!("check_closure: expr.id={:?} closure_type={:?}", expr.id, closure_type);
 
         // Tuple up the arguments and insert the resulting function type into
         // the `closures` table.
-        let sig = sig.map_bound(|sig| self.tcx.mk_fn_sig(
+        let sig = bound_sig.map_bound(|sig| self.tcx.mk_fn_sig(
             iter::once(self.tcx.intern_tup(sig.inputs(), false)),
             sig.output(),
             sig.variadic,
@@ -271,71 +268,214 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
+    fn sig_of_closure(&self,
+                      expr_def_id: DefId,
+                      decl: &hir::FnDecl,
+                      body: &hir::Body,
+                      expected_sig: Option<ty::FnSig<'tcx>>)
+                      -> ClosureSignatures<'tcx>
+    {
+        if let Some(e) = expected_sig {
+            self.sig_of_closure_with_expectation(expr_def_id, decl, body, e)
+        } else {
+            self.sig_of_closure_no_expectation(expr_def_id, decl, body)
+        }
+    }
+
+    /// If there is no expected signature, then we will convert the
+    /// types that the user gave into a signature.
+    fn sig_of_closure_no_expectation(&self,
+                                     expr_def_id: DefId,
+                                     decl: &hir::FnDecl,
+                                     body: &hir::Body)
+                                     -> ClosureSignatures<'tcx>
+    {
+        debug!("sig_of_closure_no_expectation()");
+
+        let bound_sig = self.supplied_sig_of_closure(decl);
+
+        self.closure_sigs(expr_def_id, body, bound_sig)
+    }
+
     /// Invoked to compute the signature of a closure expression. This
     /// combines any user-provided type annotations (e.g., `|x: u32|
     /// -> u32 { .. }`) with the expected signature.
     ///
-    /// The arguments here are a bit odd-ball:
+    /// The approach is as follows:
+    ///
+    /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
+    /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
+    ///   - If we have no expectation `E`, then the signature of the closure is `S`.
+    ///   - Otherwise, the signature of the closure is E. Moreover:
+    ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
+    ///     - Instantiate all the late-bound regions bound in the closure within `S`
+    ///       with fresh (existential) variables, yielding `S'`
+    ///     - Require that `E' = S'`
+    ///       - We could use some kind of subtyping relationship here,
+    ///         I imagine, but equality is easier and works fine for
+    ///         our purposes.
+    ///
+    /// The key intuition here is that the user's types must be valid
+    /// from "the inside" of the closure, but the expectation
+    /// ultimately drives the overall signature.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// fn with_closure<F>(_: F)
+    ///   where F: Fn(&u32) -> &u32 { .. }
+    ///
+    /// with_closure(|x: &u32| { ... })
+    /// ```
+    ///
+    /// Here:
+    /// - E would be `fn(&u32) -> &u32`.
+    /// - S would be `fn(&u32) ->
+    /// - E' is `&'!0 u32 -> &'!0 u32`
+    /// - S' is `&'?0 u32 -> ?T`
+    ///
+    /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
+    ///
+    /// # Arguments
     ///
+    /// - `expr_def_id`: the def-id of the closure expression
     /// - `decl`: the HIR declaration of the closure
+    /// - `body`: the body of the closure
     /// - `expected_sig`: the expected signature (if any). Note that
     ///   this is missing a binder: that is, there may be late-bound
     ///   regions with depth 1, which are bound then by the closure.
-    fn ty_of_closure(&self,
-                     decl: &hir::FnDecl,
-                     expected_sig: Option<ty::FnSig<'tcx>>)
-                     -> ty::PolyFnSig<'tcx>
+    fn sig_of_closure_with_expectation(&self,
+                                       expr_def_id: DefId,
+                                       decl: &hir::FnDecl,
+                                       body: &hir::Body,
+                                       expected_sig: ty::FnSig<'tcx>)
+                                       -> ClosureSignatures<'tcx>
     {
-        let astconv: &AstConv = self;
-
-        debug!("ty_of_closure(expected_sig={:?})",
-               expected_sig);
+        debug!("sig_of_closure_with_expectation(expected_sig={:?})", expected_sig);
+
+        // Watch out for some surprises and just ignore the
+        // expectation if things don't see to match up with what we
+        // expect.
+        if expected_sig.variadic != decl.variadic {
+            return self.sig_of_closure_no_expectation(expr_def_id, decl, body);
+        } else if expected_sig.inputs_and_output.len() != decl.inputs.len() + 1 {
+            // we could probably handle this case more gracefully
+            return self.sig_of_closure_no_expectation(expr_def_id, decl, body);
+        }
 
-        let input_tys = decl.inputs.iter().enumerate().map(|(i, a)| {
-            let expected_arg_ty = expected_sig.as_ref().and_then(|e| {
-                // no guarantee that the correct number of expected args
-                // were supplied
-                if i < e.inputs().len() {
-                    Some(e.inputs()[i])
-                } else {
-                    None
-                }
-            });
+        // Create a `PolyFnSig`. Note the oddity that late bound
+        // regions appearing free in `expected_sig` are now bound up
+        // in this binder we are creating.
+        assert!(!expected_sig.has_regions_escaping_depth(1));
+        let bound_sig =
+            ty::Binder(self.tcx.mk_fn_sig(
+                expected_sig.inputs().iter().cloned(),
+                expected_sig.output(),
+                decl.variadic,
+                hir::Unsafety::Normal,
+                Abi::RustCall));
+
+        // `deduce_expectations_from_expected_type` introduces
+        // late-bound lifetimes defined elsewhere, which we now
+        // anonymize away, so as not to confuse the user.
+        let bound_sig = self.tcx.anonymize_late_bound_regions(&bound_sig);
+
+        let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
+
+        // Up till this point, we have ignored the annotations that the user
+        // gave. This function will check that they unify successfully.
+        // Along the way, it also writes out entries for types that the user
+        // wrote into our tables, which are then later used by the privacy
+        // check.
+        self.check_supplied_sig_against_expectation(decl, &closure_sigs);
+
+        closure_sigs
+    }
 
-            let input_ty = astconv.ty_of_arg(a, expected_arg_ty);
-            debug!("ty_of_closure: i={} input_ty={:?} expected_arg_ty={:?}",
-                   i, input_ty, expected_arg_ty);
+    /// Enforce the user's types against the expectation.  See
+    /// `sig_of_closure_with_expectation` for details on the overall
+    /// strategy.
+    fn check_supplied_sig_against_expectation(&self,
+                                              decl: &hir::FnDecl,
+                                              expected_sigs: &ClosureSignatures<'tcx>)
+    {
+        // Get the signature S that the user gave.
+        //
+        // (See comment on `sig_of_closure_with_expectation` for the
+        // meaning of these letters.)
+        let supplied_sig = self.supplied_sig_of_closure(decl);
+
+        debug!("check_supplied_sig_against_expectation: supplied_sig={:?}",
+               supplied_sig);
+
+        // The liberated version of this signature should be be a subtype
+        // of the liberated form of the expectation.
+        for ((hir_ty, &supplied_ty), expected_ty) in
+            decl.inputs.iter()
+            .zip(*supplied_sig.inputs().skip_binder()) // binder moved to (*) below
+            .zip(expected_sigs.liberated_sig.inputs()) // `liberated_sig` is E'.
+        {
+            // Instantiate (this part of..) S to S', i.e., with fresh variables.
+            let (supplied_ty, _) = self.infcx.replace_late_bound_regions_with_fresh_var(
+                hir_ty.span,
+                LateBoundRegionConversionTime::FnCall,
+                &ty::Binder(supplied_ty)); // recreated from (*) above
+
+            // Check that E' = S'.
+            self.demand_eqtype(hir_ty.span, expected_ty, supplied_ty);
+        }
 
-            input_ty
-        });
+        let (supplied_output_ty, _) = self.infcx.replace_late_bound_regions_with_fresh_var(
+            decl.output.span(),
+            LateBoundRegionConversionTime::FnCall,
+            &supplied_sig.output());
+        self.demand_eqtype(decl.output.span(),
+                           expected_sigs.liberated_sig.output(),
+                           supplied_output_ty);
+    }
 
-        let expected_ret_ty = expected_sig.as_ref().map(|e| e.output());
+    /// If there is no expected signature, then we will convert the
+    /// types that the user gave into a signature.
+    fn supplied_sig_of_closure(&self,
+                               decl: &hir::FnDecl)
+                               -> ty::PolyFnSig<'tcx>
+    {
+        let astconv: &AstConv = self;
 
-        let output_ty = match decl.output {
-            hir::Return(ref output) => {
-                if let (&hir::TyInfer, Some(expected_ret_ty)) = (&output.node, expected_ret_ty) {
-                    astconv.record_ty(output.hir_id, expected_ret_ty, output.span);
-                    expected_ret_ty
-                } else {
-                    astconv.ast_ty_to_ty(&output)
-                }
-            }
-            hir::DefaultReturn(span) => {
-                if let Some(expected_ret_ty) = expected_ret_ty {
-                    expected_ret_ty
-                } else {
-                    astconv.ty_infer(span)
-                }
-            }
+        // First, convert the types that the user supplied (if any).
+        let supplied_arguments =
+            decl.inputs
+                .iter()
+                .map(|a| astconv.ast_ty_to_ty(a));
+        let supplied_return = match decl.output {
+            hir::Return(ref output) => astconv.ast_ty_to_ty(&output),
+            hir::DefaultReturn(_) => astconv.ty_infer(decl.output.span()),
         };
 
-        debug!("ty_of_closure: output_ty={:?}", output_ty);
-
-        ty::Binder(self.tcx.mk_fn_sig(
-            input_tys,
-            output_ty,
+        let result = ty::Binder(self.tcx.mk_fn_sig(
+            supplied_arguments,
+            supplied_return,
             decl.variadic,
             hir::Unsafety::Normal,
-            Abi::RustCall))
+            Abi::RustCall));
+
+        debug!("supplied_sig_of_closure: result={:?}", result);
+
+        result
+    }
+
+    fn closure_sigs(&self,
+                    expr_def_id: DefId,
+                    body: &hir::Body,
+                    bound_sig: ty::PolyFnSig<'tcx>)
+                    -> ClosureSignatures<'tcx>
+    {
+        let liberated_sig = self.liberate_late_bound_regions(expr_def_id, &bound_sig);
+        let liberated_sig = self.inh.normalize_associated_types_in(body.value.span,
+                                                                   body.value.id,
+                                                                   self.param_env,
+                                                                   &liberated_sig);
+        ClosureSignatures { bound_sig, liberated_sig }
     }
 }
+
diff --git a/src/librustc_typeck/lib.rs b/src/librustc_typeck/lib.rs
index 49ba0499f78..5227955d7b9 100644
--- a/src/librustc_typeck/lib.rs
+++ b/src/librustc_typeck/lib.rs
@@ -76,6 +76,7 @@ This API is completely unstable and subject to change.
 #![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(conservative_impl_trait)]
+#![feature(match_default_bindings)]
 #![feature(never_type)]
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]
diff --git a/src/test/compile-fail/closure-expected-type/README.md b/src/test/compile-fail/closure-expected-type/README.md
new file mode 100644
index 00000000000..9995b00a9a7
--- /dev/null
+++ b/src/test/compile-fail/closure-expected-type/README.md
@@ -0,0 +1 @@
+See `src/test/run-pass/closure-expected-type`.
diff --git a/src/test/compile-fail/closure-expected-type/expect-fn-supply-fn-multiple.rs b/src/test/compile-fail/closure-expected-type/expect-fn-supply-fn-multiple.rs
new file mode 100644
index 00000000000..f1b198a0591
--- /dev/null
+++ b/src/test/compile-fail/closure-expected-type/expect-fn-supply-fn-multiple.rs
@@ -0,0 +1,49 @@
+// Copyright 2016 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.
+
+// must-compile-successfully
+
+#![feature(underscore_lifetimes)]
+#![allow(warnings)]
+
+type Different<'a, 'b> = &'a mut (&'a (), &'b ());
+type Same<'a> = Different<'a, 'a>;
+
+fn with_closure_expecting_different<F>(_: F)
+    where F: for<'a, 'b> FnOnce(Different<'a, 'b>)
+{
+}
+
+fn with_closure_expecting_different_anon<F>(_: F)
+    where F: FnOnce(Different<'_, '_>)
+{
+}
+
+fn supplying_nothing_expecting_anon() {
+    with_closure_expecting_different_anon(|x: Different| {
+    })
+}
+
+fn supplying_nothing_expecting_named() {
+    with_closure_expecting_different(|x: Different| {
+    })
+}
+
+fn supplying_underscore_expecting_anon() {
+    with_closure_expecting_different_anon(|x: Different<'_, '_>| {
+    })
+}
+
+fn supplying_underscore_expecting_named() {
+    with_closure_expecting_different(|x: Different<'_, '_>| {
+    })
+}
+
+fn main() { }
diff --git a/src/test/compile-fail/closure-expected-type/expect-fn-supply-fn.rs b/src/test/compile-fail/closure-expected-type/expect-fn-supply-fn.rs
new file mode 100644
index 00000000000..3f129614ea0
--- /dev/null
+++ b/src/test/compile-fail/closure-expected-type/expect-fn-supply-fn.rs
@@ -0,0 +1,70 @@
+// Copyright 2016 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(underscore_lifetimes)]
+
+fn with_closure_expecting_fn_with_free_region<F>(_: F)
+    where F: for<'a> FnOnce(fn(&'a u32), &i32)
+{
+}
+
+fn with_closure_expecting_fn_with_bound_region<F>(_: F)
+    where F: FnOnce(fn(&u32), &i32)
+{
+}
+
+fn expect_free_supply_free_from_fn<'x>(x: &'x u32) {
+    // Here, the type given for `'x` "obscures" a region from the
+    // expected signature that is bound at closure level.
+    with_closure_expecting_fn_with_free_region(|x: fn(&'x u32), y| {});
+    //~^ ERROR mismatched types
+    //~| ERROR mismatched types
+}
+
+fn expect_free_supply_free_from_closure() {
+    // A variant on the previous test. Here, the region `'a` will be
+    // bound at the closure level, just as is expected, so no error
+    // results.
+    type Foo<'a> = fn(&'a u32);
+    with_closure_expecting_fn_with_free_region(|_x: Foo<'_>, y| {});
+}
+
+fn expect_free_supply_bound() {
+    // Here, we are given a function whose region is bound at closure level,
+    // but we expect one bound in the argument. Error results.
+    with_closure_expecting_fn_with_free_region(|x: fn(&u32), y| {});
+    //~^ ERROR mismatched types
+}
+
+fn expect_bound_supply_free_from_fn<'x>(x: &'x u32) {
+    // Here, we are given a `fn(&u32)` but we expect a `fn(&'x
+    // u32)`. In principle, this could be ok, but we demand equality.
+    with_closure_expecting_fn_with_bound_region(|x: fn(&'x u32), y| {});
+    //~^ ERROR mismatched types
+}
+
+fn expect_bound_supply_free_from_closure() {
+    // A variant on the previous test. Here, the region `'a` will be
+    // bound at the closure level, but we expect something bound at
+    // the argument level.
+    type Foo<'a> = fn(&'a u32);
+    with_closure_expecting_fn_with_bound_region(|_x: Foo<'_>, y| {});
+    //~^ ERROR mismatched types
+}
+
+fn expect_bound_supply_bound<'x>(x: &'x u32) {
+    // No error in this case. The supplied type supplies the bound
+    // regions, and hence we are able to figure out the type of `y`
+    // from the expected type
+    with_closure_expecting_fn_with_bound_region(|x: for<'z> fn(&'z u32), y| {
+    });
+}
+
+fn main() { }
diff --git a/src/test/compile-fail/closure-expected-type/expect-infer-var-appearing-twice.rs b/src/test/compile-fail/closure-expected-type/expect-infer-var-appearing-twice.rs
new file mode 100644
index 00000000000..4ccd1c3b4f1
--- /dev/null
+++ b/src/test/compile-fail/closure-expected-type/expect-infer-var-appearing-twice.rs
@@ -0,0 +1,35 @@
+// Copyright 2016 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.
+
+fn with_closure<F, A>(_: F)
+    where F: FnOnce(A, A)
+{
+}
+
+fn a() {
+    with_closure(|x: u32, y| {
+        // We deduce type of `y` from `x`.
+    });
+}
+
+fn b() {
+    // Here we take the supplied types, resulting in an error later on.
+    with_closure(|x: u32, y: i32| {
+        //~^ ERROR mismatched types
+    });
+}
+
+fn c() {
+    with_closure(|x, y: i32| {
+        // We deduce type of `x` from `y`.
+    });
+}
+
+fn main() { }
diff --git a/src/test/compile-fail/closure-expected-type/expect-infer-var-supply-ty-with-bound-region.rs b/src/test/compile-fail/closure-expected-type/expect-infer-var-supply-ty-with-bound-region.rs
new file mode 100644
index 00000000000..f8cb643c8d6
--- /dev/null
+++ b/src/test/compile-fail/closure-expected-type/expect-infer-var-supply-ty-with-bound-region.rs
@@ -0,0 +1,29 @@
+// Copyright 2016 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.
+
+// must-compile-successfully
+
+fn with_closure<F, A>(_: F)
+    where F: FnOnce(A, &u32)
+{
+}
+
+fn foo() {
+    // This version works; we infer `A` to be `u32`, and take the type
+    // of `y` to be `&u32`.
+    with_closure(|x: u32, y| {});
+}
+
+fn bar() {
+    // This version also works.
+    with_closure(|x: &u32, y| {});
+}
+
+fn main() { }
diff --git a/src/test/compile-fail/closure-expected-type/expect-infer-var-supply-ty-with-free-region.rs b/src/test/compile-fail/closure-expected-type/expect-infer-var-supply-ty-with-free-region.rs
new file mode 100644
index 00000000000..d3c111c5daf
--- /dev/null
+++ b/src/test/compile-fail/closure-expected-type/expect-infer-var-supply-ty-with-free-region.rs
@@ -0,0 +1,29 @@
+// Copyright 2016 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.
+
+// must-compile-successfully
+
+fn with_closure<F, A>(_: F)
+    where F: FnOnce(A, &u32)
+{
+}
+
+fn foo() {
+    // This version works; we infer `A` to be `u32`, and take the type
+    // of `y` to be `&u32`.
+    with_closure(|x: u32, y| {});
+}
+
+fn bar<'x>(x: &'x u32) {
+    // Same.
+    with_closure(|x: &'x u32, y| {});
+}
+
+fn main() { }
diff --git a/src/test/compile-fail/closure-expected-type/expect-region-supply-region.rs b/src/test/compile-fail/closure-expected-type/expect-region-supply-region.rs
new file mode 100644
index 00000000000..9da12dc901f
--- /dev/null
+++ b/src/test/compile-fail/closure-expected-type/expect-region-supply-region.rs
@@ -0,0 +1,80 @@
+// Copyright 2016 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.
+
+#![allow(warnings)]
+
+fn closure_expecting_bound<F>(_: F)
+    where F: FnOnce(&u32)
+{
+}
+
+fn closure_expecting_free<'a, F>(_: F)
+    where F: FnOnce(&'a u32)
+{
+}
+
+fn expect_bound_supply_nothing() {
+    // Because `x` is inferred to have a bound region, we cannot allow
+    // it to escape into `f`:
+    let mut f: Option<&u32> = None;
+    closure_expecting_bound(|x| {
+        f = Some(x); //~ ERROR E0495
+    });
+}
+
+fn expect_bound_supply_bound() {
+    // Because `x` is inferred to have a bound region, we cannot allow
+    // it to escape into `f`, even with an explicit type annotation on
+    // closure:
+    let mut f: Option<&u32> = None;
+    closure_expecting_bound(|x: &u32| {
+        f = Some(x); //~ ERROR E0495
+    });
+}
+
+fn expect_bound_supply_named<'x>() {
+    let mut f: Option<&u32> = None;
+
+    // Here we give a type annotation that `x` should be free. We get
+    // an error because of that.
+    closure_expecting_bound(|x: &'x u32| {
+        //~^ ERROR mismatched types
+        //~| ERROR mismatched types
+
+        // And we still cannot let `x` escape into `f`.
+        f = Some(x);
+        //~^ ERROR cannot infer
+    });
+}
+
+fn expect_free_supply_nothing() {
+    let mut f: Option<&u32> = None;
+    closure_expecting_free(|x| f = Some(x)); // OK
+}
+
+fn expect_free_supply_bound() {
+    let mut f: Option<&u32> = None;
+
+    // Here, even though the annotation `&u32` could be seen as being
+    // bound in the closure, we permit it to be defined as a free
+    // region (which is inferred to something in the fn body).
+    closure_expecting_free(|x: &u32| f = Some(x)); // OK
+}
+
+fn expect_free_supply_named<'x>() {
+    let mut f: Option<&u32> = None;
+
+    // Here, even though the annotation `&u32` could be seen as being
+    // bound in the closure, we permit it to be defined as a free
+    // region (which is inferred to something in the fn body).
+    closure_expecting_free(|x: &'x u32| f = Some(x)); // OK
+}
+
+fn main() { }
diff --git a/src/test/compile-fail/closure-expected-type/expect-two-infer-vars-supply-ty-with-bound-region.rs b/src/test/compile-fail/closure-expected-type/expect-two-infer-vars-supply-ty-with-bound-region.rs
new file mode 100644
index 00000000000..377eaadbd6a
--- /dev/null
+++ b/src/test/compile-fail/closure-expected-type/expect-two-infer-vars-supply-ty-with-bound-region.rs
@@ -0,0 +1,29 @@
+// Copyright 2016 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.
+
+fn with_closure<F, A, B>(_: F)
+    where F: FnOnce(A, B)
+{
+}
+
+fn a() {
+    // Type of `y` is unconstrained.
+    with_closure(|x: u32, y| {}); //~ ERROR E0282
+}
+
+fn b() {
+    with_closure(|x: u32, y: u32| {}); // OK
+}
+
+fn c() {
+    with_closure(|x: u32, y: u32| {}); // OK
+}
+
+fn main() { }
diff --git a/src/test/mir-opt/validate_1.rs b/src/test/mir-opt/validate_1.rs
index 53454c0cc9a..a97ce4e8cbe 100644
--- a/src/test/mir-opt/validate_1.rs
+++ b/src/test/mir-opt/validate_1.rs
@@ -62,7 +62,7 @@ fn main() {
 // fn main::{{closure}}(_1: &ReErased [closure@NodeId(50)], _2: &ReErased mut i32) -> i32 {
 //     ...
 //     bb0: {
-//         Validate(Acquire, [_1: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:11) => validate_1[317d]::main[0]::{{closure}}[0] }, BrEnv) [closure@NodeId(50)], _2: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:11) => validate_1[317d]::main[0]::{{closure}}[0] }, BrAnon(1)) mut i32]);
+//         Validate(Acquire, [_1: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:11) => validate_1[317d]::main[0]::{{closure}}[0] }, BrEnv) [closure@NodeId(50)], _2: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:11) => validate_1[317d]::main[0]::{{closure}}[0] }, BrAnon(0)) mut i32]);
 //         StorageLive(_3);
 //         Validate(Suspend(ReScope(Remainder(BlockRemainder { block: ItemLocalId(22), first_statement_index: 0 }))), [(*_2): i32]);
 //         _3 = &ReErased (*_2);
diff --git a/src/test/mir-opt/validate_4.rs b/src/test/mir-opt/validate_4.rs
index 042edca82a6..3585ac0b8be 100644
--- a/src/test/mir-opt/validate_4.rs
+++ b/src/test/mir-opt/validate_4.rs
@@ -78,8 +78,8 @@ fn main() {
 // fn main::{{closure}}(_1: &ReErased [closure@NodeId(60)], _2: &ReErased mut i32) -> bool {
 //     ...
 //     bb0: {
-//         Validate(Acquire, [_1: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:10) => validate_4[317d]::main[0]::{{closure}}[0] }, BrEnv) [closure@NodeId(60)], _2: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:10) => validate_4[317d]::main[0]::{{closure}}[0] }, BrAnon(1)) mut i32]);
-//         Validate(Release, [_1: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:10) => validate_4[317d]::main[0]::{{closure}}[0] }, BrEnv) [closure@NodeId(60)], _2: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:10) => validate_4[317d]::main[0]::{{closure}}[0] }, BrAnon(1)) mut i32]);
+//         Validate(Acquire, [_1: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:10) => validate_4[317d]::main[0]::{{closure}}[0] }, BrEnv) [closure@NodeId(60)], _2: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:10) => validate_4[317d]::main[0]::{{closure}}[0] }, BrAnon(0)) mut i32]);
+//         Validate(Release, [_1: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:10) => validate_4[317d]::main[0]::{{closure}}[0] }, BrEnv) [closure@NodeId(60)], _2: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:10) => validate_4[317d]::main[0]::{{closure}}[0] }, BrAnon(0)) mut i32]);
 //         StorageLive(_3);
 //         ...
 //         _0 = const write_42(_3) -> bb1;
diff --git a/src/test/mir-opt/validate_5.rs b/src/test/mir-opt/validate_5.rs
index fc849c5aee3..ae09d72942e 100644
--- a/src/test/mir-opt/validate_5.rs
+++ b/src/test/mir-opt/validate_5.rs
@@ -49,7 +49,7 @@ fn main() {
 // fn main::{{closure}}(_1: &ReErased [closure@NodeId(46)], _2: &ReErased mut i32) -> bool {
 //     ...
 //     bb0: {
-//         Validate(Acquire, [_1: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:9) => validate_5[317d]::main[0]::{{closure}}[0] }, BrEnv) [closure@NodeId(46)], _2: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:9) => validate_5[317d]::main[0]::{{closure}}[0] }, BrAnon(1)) mut i32]);
+//         Validate(Acquire, [_1: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:9) => validate_5[317d]::main[0]::{{closure}}[0] }, BrEnv) [closure@NodeId(46)], _2: &ReFree(DefId { krate: CrateNum(0), index: DefIndex(1:9) => validate_5[317d]::main[0]::{{closure}}[0] }, BrAnon(0)) mut i32]);
 //         StorageLive(_3);
 //         StorageLive(_4);
 //         Validate(Suspend(ReScope(Node(ItemLocalId(9)))), [(*_2): i32]);
diff --git a/src/test/run-pass/closure-expected-type/README.md b/src/test/run-pass/closure-expected-type/README.md
new file mode 100644
index 00000000000..fd493e1ff37
--- /dev/null
+++ b/src/test/run-pass/closure-expected-type/README.md
@@ -0,0 +1,8 @@
+Some tests targeted at how we deduce the types of closure arguments.
+This process is a result of some heuristics aimed at combining the
+*expected type* we have with the *actual types* that we get from
+inputs. This investigation was kicked off by #38714, which revealed
+some pretty deep flaws in the ad-hoc way that we were doing things
+before.
+
+See also `src/test/compile-fail/closure-expected-type`.
diff --git a/src/test/run-pass/closure-expected-type/expect-infer-supply-two-infers.rs b/src/test/run-pass/closure-expected-type/expect-infer-supply-two-infers.rs
new file mode 100644
index 00000000000..8a90a491f7e
--- /dev/null
+++ b/src/test/run-pass/closure-expected-type/expect-infer-supply-two-infers.rs
@@ -0,0 +1,26 @@
+// Copyright 2016 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.
+
+fn with_closure<A, F>(_: F)
+    where F: FnOnce(Vec<A>, A)
+{
+}
+
+fn expect_free_supply_free<'x>(x: &'x u32) {
+    with_closure(|mut x: Vec<_>, y| {
+        // Shows that the call to `x.push()` is influencing type of `y`...
+        x.push(22_u32);
+
+        // ...since we now know the type of `y` and can resolve the method call.
+        y.wrapping_add(1);
+    });
+}
+
+fn main() { }
diff --git a/src/test/run-pass/closure-expected-type/issue-38714.rs b/src/test/run-pass/closure-expected-type/issue-38714.rs
new file mode 100644
index 00000000000..a1d512105c9
--- /dev/null
+++ b/src/test/run-pass/closure-expected-type/issue-38714.rs
@@ -0,0 +1,26 @@
+// Copyright 2016 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.
+
+struct UsizeRef<'a> {
+    a: &'a usize
+}
+
+type RefTo = Box<for<'r> Fn(&'r Vec<usize>) -> UsizeRef<'r>>;
+
+fn ref_to<'a>(vec: &'a Vec<usize>) -> UsizeRef<'a> {
+    UsizeRef{ a: &vec[0]}
+}
+
+fn main() {
+    // Regression test: this was causing ICEs; it should compile.
+    let a: RefTo = Box::new(|vec: &Vec<usize>| {
+        UsizeRef{ a: &vec[0] }
+    });
+}
diff --git a/src/test/run-pass/closure-expected-type/supply-just-return-type.rs b/src/test/run-pass/closure-expected-type/supply-just-return-type.rs
new file mode 100644
index 00000000000..0b930b338fd
--- /dev/null
+++ b/src/test/run-pass/closure-expected-type/supply-just-return-type.rs
@@ -0,0 +1,35 @@
+// Copyright 2016 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.
+
+fn with_closure<F, R>(f: F) -> Result<char, R>
+    where F: FnOnce(&char) -> Result<char, R>,
+{
+    f(&'a')
+}
+
+fn main() {
+    // Test that supplying the `-> Result<char, ()>` manually here
+    // (which is needed to constrain `R`) still allows us to figure
+    // out that the type of `x` is `&'a char` where `'a` is bound in
+    // the closure (if we didn't, we'd get a type-error because
+    // `with_closure` requires a bound region).
+    //
+    // This pattern was found in the wild.
+    let z = with_closure(|x| -> Result<char, ()> { Ok(*x) });
+    assert_eq!(z.unwrap(), 'a');
+
+    // It also works with `_`:
+    let z = with_closure(|x: _| -> Result<char, ()> { Ok(*x) });
+    assert_eq!(z.unwrap(), 'a');
+
+    // It also works with `&_`:
+    let z = with_closure(|x: &_| -> Result<char, ()> { Ok(*x) });
+    assert_eq!(z.unwrap(), 'a');
+}
diff --git a/src/test/run-pass/closure-expected-type/supply-nothing.rs b/src/test/run-pass/closure-expected-type/supply-nothing.rs
new file mode 100644
index 00000000000..15d8b393c15
--- /dev/null
+++ b/src/test/run-pass/closure-expected-type/supply-nothing.rs
@@ -0,0 +1,20 @@
+// Copyright 2016 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.
+
+fn with_closure<F>(f: F) -> u32
+    where F: FnOnce(&u32, &u32) -> u32
+{
+    f(&22, &44)
+}
+
+fn main() {
+    let z = with_closure(|x, y| x + y).wrapping_add(1);
+    assert_eq!(z, 22 + 44 + 1);
+}