src/test/ui/unsafe/unsafe-trait-impl.rs

// Check that safe fns are not a subtype of unsafe fns.

trait Foo {
    unsafe fn len(&self) -> u32;
}

impl Foo for u32 {
    fn len(&self) -> u32 { *self }
    //~^ ERROR method `len` has an incompatible type for trait
    //~| expected fn pointer `unsafe fn(&u32) -> _`
    //~| found fn pointer `fn(&u32) -> _`
}

fn main() { }