diff --git a/src/param_env.md b/src/param_env.md index 2474886e..9ca117b3 100644 --- a/src/param_env.md +++ b/src/param_env.md @@ -3,28 +3,67 @@ 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 +the [`ParamEnv`][pe]. 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. +[pe]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/struct.ParamEnv.html + For example if you have a function ```rust -fn foo(t: T) { -} +fn foo(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. +You can get the parameter environment for a `def_id` using the +[`param_env`][query] query. However, this `ParamEnv` can be too generic for +your use case. Using the `ParamEnv` from the surrounding context can allow you +to evaluate more things. For example, suppose we had something the following: + +[query]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_ty/ty/fn.param_env.html + +```rust +trait Foo { + type Assoc; +} + +trait Bar { } + +trait Baz { + fn stuff() -> bool; +} + +fn foo(t: T) +where + T: Foo, + ::Assoc: Bar +{ + bar::() +} + +fn bar() { + if T::stuff() { mep() } else { mop() } +} +``` + +We may know some things inside `bar` that we wouldn't know if we just fetched +`bar`'s param env because of the `::Assoc: Bar` bound in `foo`. This +is a contrived example that makes no sense in our existing analyses, but we may +run into similar cases when doing analyses with associated constants on generic +traits or traits with assoc types. + +## Bundling 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`, making clear that you should probably not -be using the inner value without taking care to also use the `ParamEnv`. +depending on generic parameters (e.g. a `Ty`) by calling the [`and`][and] +method. This will produce a [`ParamEnvAnd`][pea], making clear that you +should probably not be using the inner value without taking care to also use +the [`ParamEnv`][pe]. + +[and]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/struct.ParamEnv.html#method.and +[pea]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/struct.ParamEnvAnd.html