Add some more info to param_env (#610)

* add some more info to param_env

* Remove some trailing spaces

Co-authored-by: Yuki Okushi <huyuumi.dev@gmail.com>

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: Copy>(t: T) {
+fn foo<T: Copy>(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
+You can get the parameter environment for a `def_id` using the
-`tcx.param_env(def_id)`, this `ParamEnv` can be too generic for your use case.
+[`param_env`][query] query. However, this `ParamEnv` can be too generic for
-Using the `ParamEnv` from the surrounding context can allow you to evaluate more
+your use case. Using the `ParamEnv` from the surrounding context can allow you
-things.
+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: T)
+where
+    T: Foo,
+    <T as Foo>::Assoc: Bar
+{
+   bar::<T::Assoc>()
+}
+
+fn bar<T: Baz>() {
+    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 `<T as Foo>::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)`.
+depending on generic parameters (e.g. a `Ty`) by calling the [`and`][and]
-This will produce a `ParamEnvAnd<Ty>`, making clear that you should probably not
+method. This will produce a [`ParamEnvAnd<Ty>`][pea], making clear that you
-be using the inner value without taking care to also use the `ParamEnv`.
+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