Diagnostics differ between regular execution and ui tests

Consider this example:

fn main() {
    let val = 2;
    let ptr = std::ptr::addr_of!(val);
    unsafe { *ptr = 3; }
}

On the playground and when I run it locally, this produces:

error[E0594]: cannot assign to `*ptr`, which is behind a `*const` pointer
 --> src/main.rs:6:14
  |
6 |     unsafe { *ptr = 3; }
  |              ^^^^^^^^ `ptr` is a `*const` pointer, so the data it refers to cannot be written
  |
help: consider changing this to be a mutable pointer
 --> /playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:2207:6
  |
22|     &mut raw const $place
  |      +++

However, when I make this a ui test, I get different output:

error[E0594]: cannot assign to `*ptr`, which is behind a `*const` pointer
  --> /home/r/src/rust/rustc.2/tests/ui/mytest.rs:5:14
   |
LL |     unsafe { *ptr = 3; }
   |              ^^^^^^^^ `ptr` is a `*const` pointer, so the data it refers to cannot be written
   |
help: consider specifying this binding's type
   |
LL |     let ptr: *mut i32 = std::ptr::addr_of!(val);
   |            ++++++++++

That's... not great? We're supposed to test what users see, and something seems to be going wrong.

Cc @rust-lang/wg-diagnostics

Start with the reproducer in tests/ui/mytest.rs and compare its UI-test output with the diagnostic from regular execution or the playground. Trace how the UI test harness selects and renders the help suggestion. Done means the UI test reports the same relevant suggestion and source location as regular execution.