How are ParamEnv's constructed internally?

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:

fn main() {
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) {

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:

fn main() {
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:

fn main() {
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) 
    U: Trait<<T as Other>::Bar>,

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.