Auto merge of #98919 - 5225225:stricter-invalid-value, r=RalfJung

Strengthen invalid_value lint to forbid uninit primitives, adjust docs to say that's UB

For context: rust-lang/rust#66151 (comment)

This does not make it a FCW, but it does explicitly state in the docs that uninit integers are UB.

This also doesn't affect any runtime behavior, uninit u32's will still successfully be created through mem::uninitialized.

Author: bors

core/src/mem/maybe_uninit.rs

@@ -54,9 +54,6 @@ use crate::slice;
 /// // The equivalent code with `MaybeUninit<i32>`:
 /// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
 /// ```
-/// (Notice that the rules around uninitialized integers are not finalized yet, but
-/// until they are, it is advisable to avoid them.)
-///
 /// On top of that, remember that most types have additional invariants beyond merely
 /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
 /// is considered initialized (under the current implementation; this does not constitute

core/src/mem/mod.rs

@@ -665,14 +665,14 @@ pub unsafe fn zeroed<T>() -> T {
 /// correctly: it has the same effect as [`MaybeUninit::uninit().assume_init()`][uninit].
 /// As the [`assume_init` documentation][assume_init] explains,
 /// [the Rust compiler assumes][inv] that values are properly initialized.
-/// As a consequence, calling e.g. `mem::uninitialized::<bool>()` causes immediate
-/// undefined behavior for returning a `bool` that is not definitely either `true`
-/// or `false`. Worse, truly uninitialized memory like what gets returned here
+///
+/// Truly uninitialized memory like what gets returned here
 /// is special in that the compiler knows that it does not have a fixed value.
 /// This makes it undefined behavior to have uninitialized data in a variable even
 /// if that variable has an integer type.
-/// (Notice that the rules around uninitialized integers are not finalized yet, but
-/// until they are, it is advisable to avoid them.)
+///
+/// Therefore, it is immediate undefined behavior to call this function on nearly all types,
+/// including integer types and arrays of integer types, and even if the result is unused.
 ///
 /// [uninit]: MaybeUninit::uninit
 /// [assume_init]: MaybeUninit::assume_init