A type alias is not a new type

Level: 201 · working knowledge

One line: type Kilometers = u32; gives u32 a second name and nothing else — a Kilometers and a Miles add together without a word from the compiler — so an alias shortens a long type, and a newtype is what makes two types different.

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

What it has to cover

• The same program with type Km = u32; type Mi = u32; and with struct Km(u32); struct Mi(u32); — the first adds them, the second refuses with E0308
• What aliases are for: shortening Box<dyn Fn(u32) -> Result<(), MyError> + Send> and pinning a parameter, as io::Result<T> pins E (see the Result alias)
• Generic aliases: type Grid<T> = Vec<Vec<T>>;
• Bounds in an alias are not enforced — the type_alias_bounds lint, and what it says
• An alias cannot carry an impl of its own: impl Km { … } is an inherent impl on u32, and rustc 1.98.0 refuses it with E0390 cannot define inherent impl for primitive types
• Aliases in an API: rustdoc shows the alias, the compiler error shows the expanded type, and the reader has to connect them

The trap it exists for

Reaching for type UserId = u64; to stop IDs being mixed up. It documents the intent and enforces none of it — fn delete(user: UserId) accepts an order number without complaint. That job needs a tuple struct.

Where this sits

A score is not a number: the newtype is the version that creates a distinct type. The Result you are reading is probably an alias is the alias you meet most. Units are types is the newtype applied to measurements.

See also

• A score is not a number: the newtype — the distinct type an alias is not
• The Result you are reading is probably an alias — io::Result<T> and friends
• Units are types — kilometres and miles as real types
• What a struct is — the tuple struct a newtype is made of
• Wrapper types — when a newtype should implement Deref
• TYPES.md — where aliases sit among the ways to name a type
• Rust: The Practical Guide, chapter 2 exercises, run — exercise 2.8: type Book = (String, String, u32); declared inside main, and type_name printing the tuple the alias stands for

If you are coming from another language

• C. typedef unsigned int km; is exactly a Rust alias — same type, new spelling, no checking.
• Go. Go has both, with a one-character difference: type Km = uint32 is an alias, type Km uint32 is a new defined type that will not mix with Mi. Rust spells the second as a tuple struct.
• Python. Km: TypeAlias = int is an alias for a checker; NewType("Km", int) is the newtype, checked only by the type checker and gone at run time.
• C++. using Km = unsigned; is an alias; a distinct type needs a wrapper struct, as in Rust.

Po polsku

Alias typu (type Km = u32;) to tylko druga nazwa tego samego typu — kompilator bez słowa doda kilometry do mil. Do skracania długich typów alias jest świetny; do rozróżniania wartości potrzebny jest newtype, czyli struktura krotkowa struct Km(u32);.

Szukaj po polsku: alias typu · newtype w Ruście · rust type alias vs newtype · rust type alias bounds not enforced