rust-anthology | Learn Rust | Media library

A match expression does not need runtime type information; as a match only accepts a single expression of a single known type, by definition it can leverage compile time information.

See also:

• Is it possible to pattern match in Rust with multiple types?
• How to match on data type in Rust?

At compile time, every match expression will be verified to be exhaustive: all possible shapes of the value are handled.

At run time, the code will determine which specific match arm is executed. You can think of a match as implemented via a fancy if-else chain.

As we humans tend to be not-extremely-precise when communicating, it's highly likely that some resources blur the line between these two aspects.

Enum variants are not standalone types. That is, given an enum Foo, Foo::Bar is not a type — it's a value of type Foo. This is the same as how false is not a type — it's a value of type bool. The same logic applies for i32 (type) and 42 (value).

In most cases, enums are implemented as a sea of bytes that correspond to the values each enum variant might be, with each variant's data layered on top of each other. This is known as a union.

Then a discriminant is added next to this soup of bytes. This is an integer value that specifies which variant the value is. Adding the discriminant makes it into a tagged union.

Matching on an enum is conceptually similar to this pseudo-Rust: