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Glossary: add new entry documenting zero-sized types #2203

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22 changes: 22 additions & 0 deletions src/glossary.md
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Expand Up @@ -211,9 +211,30 @@ r[glossary.uninhabited]

A type is uninhabited if it has no constructors and therefore can never be instantiated. An uninhabited type is "empty" in the sense that there are no values of the type. The canonical example of an uninhabited type is the [never type] `!`, or an enum with no variants `enum Never { }`. Opposite of [Inhabited](#inhabited).

r[glossary.zst]
### Zero-sized type (ZST)

A type is zero-sized (a ZST) if its size is 0. Such types have only one possible value. Examples include:
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@traviscross traviscross Apr 15, 2026 •
edited by rustbot
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Such types have only one possible value.

We may declare the never type (i.e., the empty type) to be a ZST. So it may be more correct to say that a ZST has at most one possible value.

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@DanielEScherzer DanielEScherzer Apr 16, 2026

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Should that be added even before the stabilization of the never type?


- The [unit type] (see [layout.tuple.unit]).
- [Arrays] of zero-sized types (see [layout.array]).
- [Unions] of zero-sized types (see [items.union.common-storage]).

```rust
# use core::mem::size_of;
union U {
f1: (),
f2: [(); 10],
}
assert_eq!(0, size_of::<()>());
assert_eq!(0, size_of::<[(); 10]>());
assert_eq!(0, size_of::<U>());
```

[`extern` blocks]: items.extern
[`extern fn`]: items.fn.extern
[alignment]: type-layout.md#size-and-alignment
[arrays]: type.array
[associated item]: #associated-item
[attributes]: attributes.md
[*entity*]: names.md
Expand Down Expand Up @@ -252,5 +273,6 @@ A type is uninhabited if it has no constructors and therefore can never be insta
[types]: types.md
[undefined-behavior]: behavior-considered-undefined.md
[unions]: items/unions.md
[unit type]: type.tuple.unit
[variable bindings]: patterns.md
[visibility rules]: visibility-and-privacy.md
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