error[E0659]: `std` is ambiguous
  --> $DIR/ambiguity-macros-nested.rs:8:13
   |
LL |     pub use std::io;
   |             ^^^ ambiguous name
   |
   = note: ambiguous because of a conflict between a macro-expanded name and a less macro-expanded name from outer scope during import or macro resolution
   = note: `std` could refer to a built-in crate
   = help: use `::std` to refer to this crate unambiguously
note: `std` could also refer to the module defined here
  --> $DIR/ambiguity-macros-nested.rs:13:13
   |
LL | /             mod std {
LL | |                 pub struct io;
LL | |             }
   | |_____________^
...
LL |       m!();
   |       ---- in this macro invocation
   = help: use `self::std` to refer to this module unambiguously
   = note: this error originates in the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0659`.