warning: `repr` attribute must have a hint
  --> $DIR/repr.rs:13:1
   |
LL | #[repr]
   | ^^^^^^^ needs a hint
   |
   = note: #[warn(bad_repr)] on by default
   = help: valid hints include `#[repr(C)]`, `#[repr(packed)]`, `#[repr(rust)]` and `#[repr(transparent)]`
   = note: for more information, visit <https://doc.rust-lang.org/reference/type-layout.html>

warning: `repr` attribute isn't configurable with a literal
  --> $DIR/repr.rs:17:1
   |
LL | #[repr = "B"]
   | ^^^^^^^^^^^^^ needs a hint
   |
   = help: valid hints include `#[repr(C)]`, `#[repr(packed)]`, `#[repr(rust)]` and `#[repr(transparent)]`
   = note: for more information, visit <https://doc.rust-lang.org/reference/type-layout.html>

warning: `repr` attribute isn't configurable with a literal
  --> $DIR/repr.rs:21:1
   |
LL | #[repr = "C"]
   | ^^^^^^^^^^^^^ help: give `repr` a hint: `#[repr(C)]`