reject duplicate `static mut` variables

after #140 landed the entry, exception and interrupt attributes started
accepting code like this:

``` rust
 #[entry]
fn main() -> ! {
    static mut FOO: u32 = 0;
    static mut FOO: i32 = 0;
}
```

because that code expands into:

``` rust
fn main() -> ! {
    let FOO: &'static mut u32 = unsafe {
        static mut FOO: u32 = 0;
        &mut FOO
    };
    // shadows previous variable
    let FOO: &'static mut u32 = unsafe {
        static mut FOO: i32 = 0;
        &mut FOO
    };
}
```

this commit adds a check that rejects `static mut`s with duplicated names to
these three attributes.

0 files changed, 0 insertions, 0 deletions