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src/param_env.md
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@ -4,7 +4,7 @@
The type system relies on information in the environment in order for it to function correctly. This information is stored in the [`ParamEnv`][pe] type and it is important to use the correct `ParamEnv` when interacting with the type system. The type system relies on information in the environment in order for it to function correctly. This information is stored in the [`ParamEnv`][pe] 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`][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 `ParamEnv`s are initially created via the [`param_env` query][query] which returns a `ParamEnv` derived from the provided item's where clauses. The information represented by `ParamEnv` is a list of in-scope where-clauses, and a [`Reveal`][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 `ParamEnv`s are initially created via the [`param_env` query][query] which returns a `ParamEnv` derived from the provided item's where clauses.
If we have a function such as: If we have a function such as:
```rust ```rust
@ -21,7 +21,7 @@ A more concrete example:
```rust ```rust
// `foo` would have a `ParamEnv` of: // `foo` would have a `ParamEnv` of:
// `[T: Sized, T: Clone]` // `[T: Sized, T: Clone]`
fn foo<T: Clone>(a: T) -> (T, T) { fn foo<T: Clone>(a: T) {
// when typechecking `foo` we require all the where clauses on `bar` // 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 // 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 // prove `T: Clone`. As we are type checking `foo` we use `foo`'s
@ -37,14 +37,15 @@ Or alternatively an example that would not compile:
```rust ```rust
// `foo2` would have a `ParamEnv` of: // `foo2` would have a `ParamEnv` of:
// `[T: Sized]` // `[T: Sized]`
fn foo2<T>(a: T) -> (T, T) { fn foo2<T>(a: T) {
// When typechecking `foo2` we attempt to prove `T: Clone`. // When typechecking `foo2` we attempt to prove `T: Clone`.
// As we are type checking `foo2` we use `foo2`'s environment // As we are type checking `foo2` we use `foo2`'s environment
// when trying to prove `T: Clone`. // when trying to prove `T: Clone`.
// //
// Trying to prove `T: Clone` with a `ParamEnv` of `[T: Sized]` will // Trying to prove `T: Clone` with a `ParamEnv` of `[T: Sized]` will
// fail as there is nothing in the environment telling the trait solver // fail as there is nothing in the environment telling the trait solver
// that `T` implements `Clone`. // that `T` implements `Clone` and there exists no user written impl
// that could apply.
requires_clone(a); requires_clone(a);
} }
``` ```