Require yield types to be sized

src/librustc/traits/error_reporting.rs

@@ -1261,6 +1261,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 err.note("the return type of a function must have a \
                           statically known size");
             }
+            ObligationCauseCode::SizedYieldType => {
+                err.note("the yield type of a generator must have a \
+                          statically known size");
+            }
             ObligationCauseCode::AssignmentLhsSized => {
                 err.note("the left-hand-side of an assignment must have a statically known size");
             }

src/librustc/traits/mod.rs

@@ -138,6 +138,8 @@ pub enum ObligationCauseCode<'tcx> {
     VariableType(ast::NodeId),
     /// Return type must be Sized
     SizedReturnType,
+    /// Yield type must be Sized
+    SizedYieldType,
     /// [T,..n] --> T must be Copy
     RepeatVec,
 

src/librustc/traits/structural_impls.rs

@@ -209,6 +209,7 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
             super::VariableType(id) => Some(super::VariableType(id)),
             super::ReturnType(id) => Some(super::ReturnType(id)),
             super::SizedReturnType => Some(super::SizedReturnType),
+            super::SizedYieldType => Some(super::SizedYieldType),
             super::RepeatVec => Some(super::RepeatVec),
             super::FieldSized(item) => Some(super::FieldSized(item)),
             super::ConstSized => Some(super::ConstSized),
@@ -526,6 +527,7 @@ impl<'tcx> TypeFoldable<'tcx> for traits::ObligationCauseCode<'tcx> {
             super::VariableType(_) |
             super::ReturnType(_) |
             super::SizedReturnType |
+            super::SizedYieldType |
             super::ReturnNoExpression |
             super::RepeatVec |
             super::FieldSized(_) |
@@ -574,6 +576,7 @@ impl<'tcx> TypeFoldable<'tcx> for traits::ObligationCauseCode<'tcx> {
             super::VariableType(_) |
             super::ReturnType(_) |
             super::SizedReturnType |
+            super::SizedYieldType |
             super::ReturnNoExpression |
             super::RepeatVec |
             super::FieldSized(_) |

src/librustc_typeck/check/mod.rs

@@ -1001,7 +1001,9 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
     let span = body.value.span;
 
     if body.is_generator && can_be_generator.is_some() {
-        fcx.yield_ty = Some(fcx.next_ty_var(TypeVariableOrigin::TypeInference(span)));
+        let yield_ty = fcx.next_ty_var(TypeVariableOrigin::TypeInference(span));
+        fcx.require_type_is_sized(yield_ty, span, traits::SizedYieldType);
+        fcx.yield_ty = Some(yield_ty);
     }
 
     GatherLocalsVisitor { fcx: &fcx, }.visit_body(body);

src/test/ui/generator/sized-yield.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 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(generators, generator_trait)]
+
+use std::ops::Generator;
+
+fn main() {
+   let s = String::from("foo");
+   let mut gen = move || { //~ ERROR the trait bound `str: std::marker::Sized` is not satisfied
+       yield s[..];
+   };
+   gen.resume(); //~ ERROR the trait bound `str: std::marker::Sized` is not satisfied
+}

src/test/ui/generator/sized-yield.stderr

@@ -0,0 +1,22 @@
+error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
+  --> $DIR/sized-yield.rs:17:26
+   |
+17 |      let mut gen = move || { //~ ERROR the trait bound `str: std::marker::Sized` is not satisfied
+   |  __________________________^
+18 | |        yield s[..];
+19 | |    };
+   | |____^ `str` does not have a constant size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `str`
+   = note: the yield type of a generator must have a statically known size
+
+error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
+  --> $DIR/sized-yield.rs:20:8
+   |
+20 |    gen.resume(); //~ ERROR the trait bound `str: std::marker::Sized` is not satisfied
+   |        ^^^^^^ `str` does not have a constant size known at compile-time
+   |
+   = help: the trait `std::marker::Sized` is not implemented for `str`
+
+error: aborting due to 2 previous errors
+