param_env.md

The ParamEnv type

What is it

The type system relies on information in the environment in order for it to function correctly. This information is stored in the ParamEnv type and it is important to use the correct ParamEnv when interacting with the type system.

The information represented by ParamEnv is a list of in-scope where-clauses, and a Reveal. A ParamEnv typically corresponds to a specific item's environment however it can also be created with arbitrary data that is not derived from a specific item. In most cases ParamEnvs are initially created via the param_env query which returns a ParamEnv derived from the provided item's where clauses.

If we have a function such as:

// `foo` would have a `ParamEnv` of:
// `[T: Sized, T: Trait, <T as Trait>::Assoc: Clone]`
fn foo<T: Trait>()
where
    <T as Trait>::Assoc: Clone,
{}

If we were interacting with the type system from inside of foo we would use this ParamEnv everywhere that we interact with the type system. This would allow things such as normalization (TODO: write a chapter about normalization and link it), evaluating generic constants, and proving where clauses/goals, to rely on T being sized, implementing Trait, etc.

A more concrete example:

// `foo` would have a `ParamEnv` of:
// `[T: Sized, T: Clone]`
fn foo<T: Clone>(a: T) {
    // when typechecking `foo` we require all the where clauses on `bar`
    // to hold in order for it to be legal to call. This means we have to
    // prove `T: Clone`. As we are type checking `foo` we use `foo`'s
    // environment when trying to check that `T: Clone` holds.
    //
    // Trying to prove `T: Clone` with a `ParamEnv` of `[T: Sized, T: Clone]`
    // will trivially succeed as bound we want to prove is in our environment.
    requires_clone(a);
}

Or alternatively an example that would not compile:

// `foo2` would have a `ParamEnv` of:
// `[T: Sized]`
fn foo2<T>(a: T) {
    // When typechecking `foo2` we attempt to prove `T: Clone`.
    // As we are type checking `foo2` we use `foo2`'s environment
    // when trying to prove `T: Clone`.
    //
    // Trying to prove `T: Clone` with a `ParamEnv` of `[T: Sized]` will
    // fail as there is nothing in the environment telling the trait solver
    // that `T` implements `Clone` and there exists no user written impl
    // that could apply.
    requires_clone(a);
}

It's very important to use the correct ParamEnv when interacting with the type system as otherwise it can lead to ICEs or things compiling when they shouldn't (or vice versa). See #82159 and #82067 as examples of PRs that changed rustc to use the correct param env to avoid ICE.

Construction

Creating a ParamEnv is more complicated than simply using the list of where clauses defined on an item as written by the user. We need to both elaborate supertraits into the env and fully normalize all aliases. This logic is handled by traits::normalize_param_env_or_error (even though it does not mention anything about elaboration).

Elaborating supertraits

When we have a function such as fn foo<T: Copy>() we would like to be able to prove T: Clone inside of the function as the Copy trait has a Clone supertrait. Constructing a ParamEnv looks at all of the trait bounds in the env and explicitly adds new where clauses to the ParamEnv for any supertraits found on the traits.

A concrete example would be the following function:

trait Trait: SuperTrait {}
trait SuperTrait: SuperSuperTrait {}

// `bar`'s unelaborated `ParamEnv` would be:
// `[T: Sized, T: Copy, T: Trait]`
fn bar<T: Copy + Trait>(a: T) {
    requires_impl(a);
}

fn requires_impl<T: Clone + SuperSuperTrait>(a: T) {}

If we did not elaborate the env then the requires_impl call would fail to typecheck as we would not be able to prove T: Clone or T: SuperSuperTrait. In practice we elaborate the env which means that bar's ParamEnv is actually: [T: Sized, T: Copy, T: Clone, T: Trait, T: SuperTrait, T: SuperSuperTrait] This allows us to prove T: Clone and T: SuperSuperTrait when type checking bar.

The Clone trait has a Sized supertrait however we do not end up with two T: Sized bounds in the env (one for the supertrait and one for the implicitly added T: Sized bound). This is because the elaboration process (implemented via util::elaborate) deduplicates the where clauses to avoid this.

As a side effect this also means that even if no actual elaboration of supertraits takes place, the existing where clauses in the env are also deduplicated. See the following example:

trait Trait {}
// The unelaborated `ParamEnv` would be:
// `[T: Sized, T: Trait, T: Trait]`
// but after elaboration it would be:
// `[T: Sized, T: Trait]`
fn foo<T: Trait + Trait>() {}

The next-gen trait solver also requires this elaboration to take place.

Normalizing all bounds

In the old trait solver the where clauses stored in ParamEnv are required to be fully normalized or else the trait solver will not function correctly. A concrete example of needing to normalize the ParamEnv is the following:

trait Trait<T> {
    type Assoc;
}

trait Other {
    type Bar;
}

impl<T> Other for T {
    type Bar = u32;
}

// `foo`'s unnormalized `ParamEnv` would be:
// `[T: Sized, U: Sized, U: Trait<T::Bar>]`
fn foo<T, U>(a: U) 
where
    U: Trait<<T as Other>::Bar>,
{
    requires_impl(a);
}

fn requires_impl<U: Trait<u32>>(_: U) {}

As humans we can tell that <T as Other>::Bar is equal to u32 so the trait bound on U is equivalent to U: Trait<u32>. In practice trying to prove U: Trait<u32> in the old solver in this environment would fail as it is unable to determine that <T as Other>::Bar is equal to u32.

To work around this we normalize ParamEnv's after constructing them so that foo's ParamEnv is actually: [T: Sized, U: Sized, U: Trait<u32>] which means the trait solver is now able to use the U: Trait<u32> in the ParamEnv to determine that the trait bound U: Trait<u32> holds.

This workaround does not work in all cases as normalizing associated types requires a ParamEnv which introduces a bootstrapping problem. We need a normalized ParamEnv in order for normalization to give correct results, but we need to normalize to get that ParamEnv. Currently we normalize the ParamEnv once using the unnormalized param env and it tends to give okay results in practice even though there are some examples where this breaks (example).

In the next-gen trait solver the requirement for all where clauses in the ParamEnv to be fully normalized is not present and so we do not normalize when constructing ParamEnvs.

---

When needing a ParamEnv in the compiler there generally three options for obtaining one:

• The correct env is already in scope simply use it (or pass it down the call stack to where you are)
• Call tcx.param_env(def_id)
• Use ParamEnv::new to construct an env with an arbitrary set of where clauses. Then call traits::normalize_param_env_or_error which will handle normalizing and elaborating all the where clauses in the env for you.

In the large majority of cases a ParamEnv when required already exists somewhere in scope or above in the call stack and should be passed down.

Using the param_env query to obtain an env is generally done at the start of some kind of analysis and then passed everywhere that a ParamEnv is required. For example the type checker will create a ParamEnv for the item it is type checking and then pass it around everywhere.

Creating an env from an arbitrary set of where clauses is usually unnecessary and should only be done if the environment you need does not correspond to an actual item in the source code.

Bundling

A useful API on ParamEnv is the and method which allows bundling a value with the ParamEnv. The and method produces a ParamEnvAnd<T> making it clearer that using the inner value is intended to be done in that specific environment.