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Add "# Safety" and "# Examples" section in std::mem::uninitialized #151033

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Add "# Safety" and "# Examples" section in std::mem::uninitialized #151033

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45d065c
library: Add Safety and Examples section in std::mem::uninitialized
CieriA Jan 12, 2026
2c7cad5
library: Add another example in `uninitialized` safety conditions
CieriA Jan 12, 2026
de12e31
library: use allow(deprecated) for uninitialized doctests
CieriA Jan 12, 2026
c7b6315
Reword safety of uninitialized for #151033 (comment)
CieriA Jan 21, 2026
47a2785
fix failed job
CieriA Jan 22, 2026
0a9127d
Merge branch 'main' into patch-6
CieriA Jan 22, 2026
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15 changes: 15 additions & 0 deletions library/core/src/mem/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -705,9 +705,24 @@ pub const unsafe fn zeroed<T>() -> T {
/// 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.
///
/// # Safety
///
/// This function is highly unsafe, as calling this function on nearly all types causes
/// undefined behavior. You should **always** prefer using [`MaybeUninit<T>`] instead.
///
/// If you absolutely must use this function, the following conditions must be upheld:
///
/// - `T` must be *valid* with any sequence of bytes of the appropriate length,
/// initialized or uninitialized.
Comment on lines +715 to +716
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@tgross35 tgross35 Jan 22, 2026

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I think we should hint that non-ZST T effectively needs to be all MaybeUninit to account for the uninitialized requirement. Not sure about best wording but maybe something like:

"... That is, if T is not a ZST, it must be MaybeUninit or an ADT containing only MaybeUninit and padding."

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@CieriA CieriA Jan 22, 2026 •
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I agree, but wouldn't that suggest a correct usage of this function? I removed that part because it would suggest that using uninitialized() with ZSTs is acceptable (and you said that we should discourage usage of uninitialized() by not providing any example, that's why I kept the safety conditions a bit more "abstract"). What do you think?

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@tgross35 tgross35 Jan 22, 2026

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I don't think mentioning the MaybeUninit requirement suggests anything more than what is already there, just makes it more obvious that this isn't sound even for POD types (since that's been a confusion point in the past). Not mentioning ZSTs seems reasonable though.

/// - `T` must be *[inhabited]*, i.e. possible to construct. This means that types
/// like zero-variant enums and [`!`] are unsound to construct with this function.
/// - You must use the value only in ways which do not violate any *safety*
/// invariants of the type.
///
/// [uninit]: MaybeUninit::uninit
/// [assume_init]: MaybeUninit::assume_init
/// [inv]: MaybeUninit#initialization-invariant
/// [inhabited]: https://doc.rust-lang.org/reference/glossary.html#inhabited
#[inline(always)]
#[must_use]
#[deprecated(since = "1.39.0", note = "use `mem::MaybeUninit` instead")]
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36 changes: 18 additions & 18 deletions tests/ui/thir-print/offset_of.stdout
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Expand Up @@ -67,8 +67,8 @@ body:
}
)
else_block: None
hir_id: HirId(DefId(offset_of::concrete).10)
span: $DIR/offset_of.rs:37:5: 1440:57 (#0)
lint_level: Explicit(HirId(DefId(offset_of::concrete).10))
span: $DIR/offset_of.rs:37:5: 1455:57 (#0)
}
}
Stmt {
Expand Down Expand Up @@ -116,8 +116,8 @@ body:
}
)
else_block: None
hir_id: HirId(DefId(offset_of::concrete).20)
span: $DIR/offset_of.rs:38:5: 1440:57 (#0)
lint_level: Explicit(HirId(DefId(offset_of::concrete).20))
span: $DIR/offset_of.rs:38:5: 1455:57 (#0)
}
}
Stmt {
Expand Down Expand Up @@ -165,8 +165,8 @@ body:
}
)
else_block: None
hir_id: HirId(DefId(offset_of::concrete).30)
span: $DIR/offset_of.rs:39:5: 1440:57 (#0)
lint_level: Explicit(HirId(DefId(offset_of::concrete).30))
span: $DIR/offset_of.rs:39:5: 1455:57 (#0)
}
}
Stmt {
Expand Down Expand Up @@ -214,8 +214,8 @@ body:
}
)
else_block: None
hir_id: HirId(DefId(offset_of::concrete).40)
span: $DIR/offset_of.rs:40:5: 1440:57 (#0)
lint_level: Explicit(HirId(DefId(offset_of::concrete).40))
span: $DIR/offset_of.rs:40:5: 1455:57 (#0)
}
}
Stmt {
Expand Down Expand Up @@ -263,8 +263,8 @@ body:
}
)
else_block: None
hir_id: HirId(DefId(offset_of::concrete).50)
span: $DIR/offset_of.rs:41:5: 1440:57 (#0)
lint_level: Explicit(HirId(DefId(offset_of::concrete).50))
span: $DIR/offset_of.rs:41:5: 1455:57 (#0)
}
}
]
Expand Down Expand Up @@ -863,8 +863,8 @@ body:
}
)
else_block: None
hir_id: HirId(DefId(offset_of::generic).12)
span: $DIR/offset_of.rs:45:5: 1440:57 (#0)
lint_level: Explicit(HirId(DefId(offset_of::generic).12))
span: $DIR/offset_of.rs:45:5: 1455:57 (#0)
}
}
Stmt {
Expand Down Expand Up @@ -912,8 +912,8 @@ body:
}
)
else_block: None
hir_id: HirId(DefId(offset_of::generic).24)
span: $DIR/offset_of.rs:46:5: 1440:57 (#0)
lint_level: Explicit(HirId(DefId(offset_of::generic).24))
span: $DIR/offset_of.rs:46:5: 1455:57 (#0)
}
}
Stmt {
Expand Down Expand Up @@ -961,8 +961,8 @@ body:
}
)
else_block: None
hir_id: HirId(DefId(offset_of::generic).36)
span: $DIR/offset_of.rs:47:5: 1440:57 (#0)
lint_level: Explicit(HirId(DefId(offset_of::generic).36))
span: $DIR/offset_of.rs:47:5: 1455:57 (#0)
}
}
Stmt {
Expand Down Expand Up @@ -1010,8 +1010,8 @@ body:
}
)
else_block: None
hir_id: HirId(DefId(offset_of::generic).48)
span: $DIR/offset_of.rs:48:5: 1440:57 (#0)
lint_level: Explicit(HirId(DefId(offset_of::generic).48))
span: $DIR/offset_of.rs:48:5: 1455:57 (#0)
}
}
]
Expand Down
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