Nightly: unsafe code is allowed outside unsafe blocks when the code is unreachable #45087
Description
I'm not sure if this is a feature or a bug, but the compiler behaves differently for the following program on stable, beta, and nightly:
fn main() {
return;
let _ = ();
*(&true as *const bool);
}On stable:
error[E0133]: dereference of raw pointer requires unsafe function or block
--> src/main.rs:4:5
|
4 | *(&true as *const bool);
| ^^^^^^^^^^^^^^^^^^^^^^^ dereference of raw pointer
error: aborting due to previous error
On beta:
warning: unreachable statement
--> src/main.rs:3:5
|
3 | let _ = ();
| ^^^^^^^^^^^
|
= note: #[warn(unreachable_code)] on by default
error[E0133]: dereference of raw pointer requires unsafe function or block
--> src/main.rs:4:5
|
4 | *(&true as *const bool);
| ^^^^^^^^^^^^^^^^^^^^^^^ dereference of raw pointer
error: aborting due to previous error
On nightly:
warning: unreachable statement
--> src/main.rs:3:5
|
3 | let _ = ();
| ^^^^^^^^^^^
|
= note: #[warn(unreachable_code)] on by default
Note that wrapping the final statement in an unsafe block will allow the program to compile on all three versions, but on nightly the compiler will complain:
warning: unnecessary `unsafe` block
--> src/main.rs:4:5
|
4 | unsafe { *(&true as *const bool); }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unnecessary `unsafe` block
|
= note: #[warn(unused_unsafe)] on by default
I suppose that the compiler is technically correct, because dead code tells no tales cannot cause memory unsafety, so no unsafe block is needed... but the current situation does seem like an accident, as the compiler does perform other checks on dead code (typechecking, borrow-checking, ...).