Diagnostics differ between regular execution and ui tests

[RalfJung]

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

[compiler-errors]

probably due to #105500

[RalfJung]

Cc @jieyouxu -- should this be added to the compiletest project board?

[jieyouxu]

Yes, somehow I missed this one, thanks for the ping.

[cyrgani]

Another case of this is #134471:

pub struct NotClone {}
pub fn foo(v: &Vec<NotClone>) {
    let _v = v.clone();
}

emits an incorrect suggestion with a regular rustc, but it is missing from ui test output:

warning: call to `.clone()` on a reference in this situation does nothing
 --> src/lib.rs:4:15
  |
4 |     let _v = v.clone();
  |               ^^^^^^^^
  |
  = note: the type `Vec<NotClone>` does not implement `Clone`, so calling `clone` on `&Vec<NotClone>` copies the reference, which does not do anything and can be removed
  = note: `#[warn(noop_method_call)]` on by default
help: remove this redundant call
  |
4 -     let _v = v.clone();
4 +     let _v = v;
  |
help: if you meant to clone `Vec<NotClone>`, implement `Clone` for it
 --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:397:1
  |
39+ #[derive(Clone)]
39| pub struct Vec<T, #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
  |

[estebank]

@cyrgani fwiw, this can be addressed by adding a check to the if you meant to clone Vec, implement Clone for it suggestion for Vec being a LocalDefId (if you have a DefId, call .is_local()).