error[E0428]: the name `m` is defined multiple times
  --> $DIR/effective_visibilities_invariants.rs:7:1
   |
LL | pub mod m {}
   | --------- previous definition of the module `m` here
LL |
LL | pub mod m {
   | ^^^^^^^^^ `m` redefined here
   |
   = note: `m` must be defined only once in the type namespace of this module

error: module has missing stability attribute
  --> $DIR/effective_visibilities_invariants.rs:3:1
   |
LL | / #![feature(staged_api)]
LL | |
LL | | pub mod m {}
LL | |
...  |
LL | |
LL | | fn main() {}
   | |____________^

error: module has missing stability attribute
  --> $DIR/effective_visibilities_invariants.rs:5:1
   |
LL | pub mod m {}
   | ^^^^^^^^^^^^

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0428`.