Parameter Environment

When working with associated and/or or generic items (types, constants, functions/methods) it is often relevant to have more information about the Self or generic parameters. Trait bounds and similar information is encoded in the ParamEnv. Often this is not enough information to obtain things like the type's Layout, but you can do all kinds of other checks on it (e.g. whether a type implements Copy) or you can evaluate an associated constant whose value does not depend on anything from the parameter environment.

For example if you have a function


#![allow(unused_variables)]
fn main() {
fn foo<T: Copy>(t: T) {
}
}

the parameter environment for that function is [T: Copy]. This means any evaluation within this function will, when accessing the type T, know about its Copy bound via the parameter environment.

Although you can obtain a valid ParamEnv for any item via tcx.param_env(def_id), this ParamEnv can be too generic for your use case. Using the ParamEnv from the surrounding context can allow you to evaluate more things.

Another great thing about ParamEnv is that you can use it to bundle the thing depending on generic parameters (e.g. a Ty) by calling param_env.and(ty). This will produce a ParamEnvAnd<Ty>, making clear that you should probably not be using the inner value without taking care to also use the ParamEnv.