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genericsrustreferencelifetimelifetime-scoping

Why use the word 'generic' in lifetime parameters?

When we write:

fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'a u32 {
    x
}

Why don't we just refer to the 'as and the 'bs as lifetime parameters as opposed to generic lifetime parameters? It's just a syntactical way to communicate to the compiler a constraint on the returned reference's lifetime in terms of the lifetimes of the arguments. I'm struggling to see the rationale for the inclusion of the word "generic" here.

Solution

  • "Generic" makes me think of generic types, but as far as I can tell it has no relevance here.

    You can think of "generic type" as a shorthand for "type with generic parameters", but those generic parameters can be any or all of:

    1. generic type parameters (as in Vec<T>);

    2. generic lifetime parameters (as in type Ref<'a> = &'a i32);

    3. generic const parameters (real soon now).

    Same for "generic function" = "function with generic parameters".

    You could shorten it to "lifetime parameters" exactly because all lifetime parameters (not all lifetimes!) are generic. The same applies to type parameters.

    but I still don't see how that makes 'a generic as opposed to simply a parameter which takes various values

    Lifetime parameters and type parameters have a few things in common which don't have to go together, but do in Rust:

    1. the most obvious: they are written between < > and not ( );

    2. they can be inferred;

    3. code can't branch depending on their value.

    So they are grouped together as "generic parameters" as opposed to "value parameters".