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 (e.g. [`compare_method_predicate_entailment`][method_pred_entailment]).


### How are `ParamEnv`s constructed

Creating a [`ParamEnv`][pe] 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`][normalize_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:
```rust
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) as the elaboration process (implemented via [`util::elaborate`][elaborate]) deduplicates where clauses.

A side effect of this is that even if no actual elaboration of supertraits takes place, the existing where clauses in the env are _also_ deduplicated. See the following example:
```rust
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][next-gen-solver] also requires this elaboration to take place.