remove the "defaulted unit" type bit during writeback

The defaulted unit bit is only relevant for the surrounding inference
context, and can cause trouble, including spurious lints and ICEs,
outside of it.

Fixes #43853.

5 files changed, 39 insertions, 4 deletions

diff --git a/src/librustc/infer/resolve.rs b/src/librustc/infer/resolve.rs
index 2e8b843d07b..6a1f8f1d069 100644
--- a/src/librustc/infer/resolve.rs
+++ b/src/librustc/infer/resolve.rs
@@ -111,8 +111,10 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for FullTypeResolver<'a, 'gcx, 'tcx>
     }
 
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
-        if !t.needs_infer() {
+        if !t.needs_infer() && !ty::keep_local(&t) {
             t // micro-optimize -- if there is nothing in this type that this fold affects...
+                // ^ we need to have the `keep_local` check to un-default
+                // defaulted tuples.
         } else {
             let t = self.infcx.shallow_resolve(t);
             match t.sty {
@@ -131,6 +133,12 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for FullTypeResolver<'a, 'gcx, 'tcx>
                 ty::TyInfer(_) => {
                     bug!("Unexpected type in full type resolver: {:?}", t);
                 }
+                ty::TyTuple(tys, true) => {
+                    // Un-default defaulted tuples - we are going to a
+                    // different infcx, and the default will just cause
+                    // pollution.
+                    self.tcx().intern_tup(tys, false)
+                }
                 _ => {
                     t.super_fold_with(self)
                 }
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 6b9cbabf20e..003af204f1e 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -1195,7 +1195,7 @@ macro_rules! direct_interners {
     }
 }
 
-fn keep_local<'tcx, T: ty::TypeFoldable<'tcx>>(x: &T) -> bool {
+pub fn keep_local<'tcx, T: ty::TypeFoldable<'tcx>>(x: &T) -> bool {
     x.has_type_flags(ty::TypeFlags::KEEP_IN_LOCAL_TCX)
 }
 
diff --git a/src/librustc/ty/flags.rs b/src/librustc/ty/flags.rs
index ce2bb23660c..31ed61a919e 100644
--- a/src/librustc/ty/flags.rs
+++ b/src/librustc/ty/flags.rs
@@ -151,7 +151,10 @@ impl FlagComputation {
                 self.add_ty(m.ty);
             }
 
-            &ty::TyTuple(ref ts, _) => {
+            &ty::TyTuple(ref ts, is_default) => {
+                if is_default {
+                    self.add_flags(TypeFlags::KEEP_IN_LOCAL_TCX);
+                }
                 self.add_tys(&ts[..]);
             }
 
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 28a73f4a4d3..b91eb3ec68f 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -76,7 +76,7 @@ pub use self::sty::TypeVariants::*;
 pub use self::binding::BindingMode;
 pub use self::binding::BindingMode::*;
 
-pub use self::context::{TyCtxt, GlobalArenas, tls};
+pub use self::context::{TyCtxt, GlobalArenas, tls, keep_local};
 pub use self::context::{Lift, TypeckTables};
 
 pub use self::instance::{Instance, InstanceDef};
diff --git a/src/test/run-pass/issue-43853.rs b/src/test/run-pass/issue-43853.rs
new file mode 100644
index 00000000000..f55d584ea24
--- /dev/null
+++ b/src/test/run-pass/issue-43853.rs
@@ -0,0 +1,24 @@
+// Copyright 2017 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.
+
+use std::panic;
+
+fn test() {
+    wait(|| panic!());
+}
+
+fn wait<T, F: FnOnce() -> T>(f: F) -> F::Output {
+    From::from(f())
+}
+
+fn main() {
+    let result = panic::catch_unwind(move || test());
+    assert!(result.is_err());
+}