Change:
<del> // Copyright 2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. fn illegal_cast<U:?Sized,V:?Sized>(u: *const U) -> *const V { u as *const V //~^ ERROR casting //~^^ NOTE vtable kinds } fn illegal_cast_2<U:?Sized>(u: *const U) -> *const str { u as *const str //~^ ERROR casting //~^^ NOTE vtable kinds } trait Foo { fn foo(&self) {} } impl<T> Foo for T {} trait Bar { fn foo(&self) {} } impl<T> Bar for T {} enum E { A, B } fn main() { let f: f32 = 1.2; let v = 0 as *const u8; let fat_v : *const [u8] = unsafe { &*(0 as *const [u8; 1])}; let fat_sv : *const [i8] = unsafe { &*(0 as *const [i8; 1])}; let foo: &Foo = &f; let _ = v as &u8; //~ ERROR non-scalar let _ = v as E; //~ ERROR non-scalar let _ = v as fn(); //~ ERROR non-scalar let _ = v as (u32,); //~ ERROR non-scalar let _ = Some(&v) as *const u8; //~ ERROR non-scalar let _ = v as f32; //~^ ERROR casting let _ = main as f64; //~^ ERROR casting let _ = &v as usize; //~^ ERROR casting //~^^ HELP through a raw pointer first let _ = f as *const u8; //~^ ERROR casting let _ = 3_i32 as bool; //~^ ERROR cannot cast as `bool` [E0054] //~| unsupported cast //~| HELP compare with zero let _ = E::A as bool; //~^ ERROR cannot cast as `bool` [E0054] //~| unsupported cast //~| HELP compare with zero let _ = 0x61u32 as char; //~ ERROR only `u8` can be cast let _ = false as f32; //~^ ERROR casting //~^^ HELP through an integer first let _ = E::A as f32; //~^ ERROR casting //~^^ HELP through an integer first let _ = 'a' as f32; //~^ ERROR casting //~^^ HELP through an integer first let _ = false as *const u8; //~^ ERROR casting let _ = E::A as *const u8; //~^ ERROR casting let _ = 'a' as *const u8; //~^ ERROR casting let _ = 42usize as *const [u8]; //~ ERROR casting let _ = v as *const [u8]; //~ ERROR cannot cast let _ = fat_v as *const Foo; //~^ ERROR the trait bound `[u8]: std::marker::Sized` is not satisfied //~| NOTE the trait `std::marker::Sized` is not implemented for `[u8]` //~| NOTE `[u8]` does not have a constant size known at compile-time //~| NOTE required for the cast to the object type `Foo` let _ = foo as *const str; //~ ERROR casting let _ = foo as *mut str; //~ ERROR casting let _ = main as *mut str; //~ ERROR casting let _ = &f as *mut f32; //~ ERROR casting let _ = &f as *const f64; //~ ERROR casting let _ = fat_sv as usize; //~^ ERROR casting //~^^ HELP through a thin pointer first let a : *const str = "hello"; let _ = a as *const Foo; //~^ ERROR the trait bound `str: std::marker::Sized` is not satisfied //~| NOTE the trait `std::marker::Sized` is not implemented for `str` //~| NOTE `str` does not have a constant size known at compile-time //~| NOTE required for the cast to the object type `Foo` // check no error cascade let _ = main.f as *const u32; //~ no field `f` on type `fn() {main}` let cf: *const Foo = &0; let _ = cf as *const [u16]; //~^ ERROR casting //~^^ NOTE vtable kinds let _ = cf as *const Bar; //~^ ERROR casting //~^^ NOTE vtable kinds vec![0.0].iter().map(|s| s as f32).collect::<Vec<f32>>(); //~^ ERROR casting `&{float}` as `f32` is invalid //~| NOTE cannot cast `&{float}` as `f32` //~| NOTE did you mean `*s`? } </del>

Discussion:
When working on PR , I ended up with the following output: The second span label points at the first character of the span, but because there's already a larger span there, the actual extent of that span is lost. I believe that either the shortest span should be shown within the existing span: there should be a new character introduced for when multiple labels overlap: or at the very least, colorize the smaller span with the secondary label's color:
Will be fixed when PR gets merged.