Missing diagnostic for non-sized type of ptr::null_mut

[tesuji]

I tried this code:

trait Trait {}

// Make sure casts between thin-pointer <-> fat pointer obey RFC401
fn main() {
    let a: *mut (dyn Trait + 'static) = 0 as *mut _; //~ ERROR casting
    let x: *mut (dyn Trait + 'static) = std::ptr::null_mut();
}

When I ran this on rustc 1.32.0, I got this error message:

error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
 --> src/lib.rs:6:41
  |
6 |     let x: *mut (dyn Trait + 'static) = std::ptr::null_mut();
  |                                         ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `(dyn Trait + 'static)`
  = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: required by `std::ptr::null_mut`

error[E0606]: casting `usize` as `*mut (dyn Trait + 'static)` is invalid
 --> src/lib.rs:5:41
  |
5 |     let a: *mut (dyn Trait + 'static) = 0 as *mut _; //~ ERROR casting
  |                                         ^^^^^^^^^^^

error: aborting due to 2 previous errors

Some errors occurred: E0277, E0606.
For more information about an error, try `rustc --explain E0277`.

But when I ran this on rustc nightly, I got this error message:

error[E0606]: casting `usize` as `*mut (dyn Trait + 'static)` is invalid
 --> src/lib.rs:5:41
  |
5 |     let a: *mut (dyn Trait + 'static) = 0 as *mut _; //~ ERROR casting
  |                                         ^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0606`.

Meta

% rustc +1.32.0 -Vv 
rustc 1.32.0 (9fda7c223 2019-01-16)
binary: rustc
commit-hash: 9fda7c2237db910e41d6a712e9a2139b352e558b
commit-date: 2019-01-16
host: x86_64-unknown-linux-gnu
release: 1.32.0
LLVM version: 8.0

% rustc +nightly -Vv
rustc 1.37.0-nightly (0e4a56b4b 2019-06-13)
binary: rustc
commit-hash: 0e4a56b4b04ea98bb16caada30cb2418dd06e250
commit-date: 2019-06-13
host: x86_64-unknown-linux-gnu
release: 1.37.0-nightly
LLVM version: 8.0

[scottmcm]

Note that if one comments out the let a line, there's an error about x:

error[E0282]: type annotations needed
 --> src/main.rs:5:41
  |
  |     let x: *mut (dyn Trait + 'static) = std::ptr::null_mut();
  |                                         ^^^^^^^^^^^^^^^^^^ cannot infer type for `T`

[tesuji]

Godbolt link for that

[Skepfyr]

Bisected to find regression in nightly-2018-12-21

[steffahn]

@rustbot modify labels to -E-needs-bisection.

[estebank]

Current output:

error[E0271]: type mismatch resolving `<dyn Trait as Pointee>::Metadata == ()`
   --> f71.rs:6:41
    |
6   |     let x: *mut (dyn Trait + 'static) = std::ptr::null_mut();
    |                                         ^^^^^^^^^^^^^^^^^^ expected `()`, found `DynMetadata<dyn Trait>`
    |
    = note: expected unit type `()`
                  found struct `DynMetadata<(dyn Trait + 'static)>`
    = note: required for `(dyn Trait + 'static)` to implement `Thin`
note: required by a bound in `null_mut`
   --> /home/gh-estebank/rust/library/core/src/ptr/mod.rs:543:35
    |
543 | pub const fn null_mut<T: ?Sized + Thin>() -> *mut T {
    |                                   ^^^^ required by this bound in `null_mut`

error[E0606]: cannot cast `usize` to a pointer that is wide
 --> f71.rs:5:46
  |
5 |     let a: *mut (dyn Trait + 'static) = 0 as *mut _; //~ ERROR casting
  |                                         -    ^^^^^^ creating a `*mut (dyn Trait + 'static)` requires both an address and a vtable
  |                                         |
  |                                         consider casting this expression to `*const ()`, then using `core::ptr::from_raw_parts`

Hiding T: Sized errors is somewhat intended, as they sometimes are knock down effect of other errors so we try to hide them if we infer they are "spurious".