unsafe{} when used in macros can emit "unnecessary unsafe block" warning without macro invocation location

[codyps]

The warning itself could be resolved by stmt_expr_attributes (once it is stable) by marking the inner unsafe{} as OK to be unneeded. (If we don't stabilize those, though, we may want to consider whether applying the unused_unsafe warning to unsafe{} generated by macros makes sense.

In any case, we should get an indication of where the macro expansion causing the error is occurring (right now only the macro location is printed, not the invocation location).

Tested with rustc 1.9.0-nightly (600dc3552 2016-04-04)

Example:

macro_rules! another_unsafe {
    ($e:expr) => ({
        unsafe { $e }
    })
}

fn main() {
    let x : usize = unsafe { another_unsafe!(std::mem::transmute(3usize)) };
    println!("{:?}", x);
}

Output:

src/main.rs:4:9: 4:22 warning: unnecessary `unsafe` block, #[warn(unused_unsafe)] on by default
src/main.rs:4         unsafe { $e }
                      ^~~~~~~~~~~~~

Expected: some complaint about line 9 (invocation of another_unsafe!)

[Mark-Simulacrum]

I believe this is fixed:

warning: unnecessary `unsafe` block
 --> test.rs:4:9
  |
4 |         unsafe { $e }
  |         ^^^^^^^^^^^^^ unnecessary `unsafe` block
...
9 |     let x : usize = unsafe { another_unsafe!(std::mem::transmute(3usize)) };
  |                              -------------------------------------------- in this macro invocation
  |
  = note: #[warn(unused_unsafe)] on by default
note: because it's nested under this `unsafe` block
 --> test.rs:9:21
  |
9 |     let x : usize = unsafe { another_unsafe!(std::mem::transmute(3usize)) };
  |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^