error[E0277]: `F` cannot be sent between threads safely --> $DIR/closure-bounds-cant-promote-superkind-in-struct.rs:5:22 | LL | fn foo(blk: F) -> X where F: FnOnce() + 'static { | ^^^^ `F` cannot be sent between threads safely | note: required by a bound in `X` --> $DIR/closure-bounds-cant-promote-superkind-in-struct.rs:1:43 | LL | struct X where F: FnOnce() + 'static + Send { | ^^^^ required by this bound in `X` help: consider further restricting this bound | LL | fn foo(blk: F) -> X where F: FnOnce() + 'static + std::marker::Send { | +++++++++++++++++++ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0277`.