From a46e8fa0bcc0519da377189928f61904de0c2c8e Mon Sep 17 00:00:00 2001 From: Daniel Scherzer Date: Tue, 10 Mar 2026 12:51:21 -0700 Subject: [PATCH] Glossary: add new entry documenting zero-sized types --- src/glossary.md | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/src/glossary.md b/src/glossary.md index 9d39173e01..694ed95a72 100644 --- a/src/glossary.md +++ b/src/glossary.md @@ -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: + +- 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::()); +``` + [`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 @@ -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