warning: trait objects without an explicit `dyn` are deprecated
  --> $DIR/not-on-bare-trait.rs:7:12
   |
LL | fn foo(_x: Foo + Send) {
   |            ^^^^^^^^^^
   |
   = note: `#[warn(bare_trait_objects)]` on by default
   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/warnings-promoted-to-error.html>
help: use `dyn`
   |
LL - fn foo(_x: Foo + Send) {
LL + fn foo(_x: dyn Foo + Send) {
   |

error[E0277]: the size for values of type `(dyn Foo + Send + 'static)` cannot be known at compilation time
  --> $DIR/not-on-bare-trait.rs:7:8
   |
LL | fn foo(_x: Foo + Send) {
   |        ^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `(dyn Foo + Send + 'static)`
   = help: unsized fn params are gated as an unstable feature
help: function arguments must have a statically known size, borrowed types always have a known size
   |
LL | fn foo(_x: &Foo + Send) {
   |            +

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0277`.