tmp: start addressing reviews

Author: Nadrieril

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -302,7 +302,7 @@
 //!
 //! # `Missing` and relevant constructors
 //!
-//! Take the following example:
+//! Take the following variation on the previous example:
 //!
 //! ```compile_fail,E0004
 //! enum Direction { North, South, East, West }
@@ -313,7 +313,8 @@
 //! }
 //! ```
 //!
-//! Remember that we represent the "everything else" cases with [`Constructor::Missing`]. When we
+//! As explained in [`crate::constructor`], we represent the "everything else" cases with
+//! [`Constructor::Missing`]. This constructor is only covered by wildcards in the column. When we
 //! specialize with `Missing` in the first column, we have one arm left:
 //!
 //! ```ignore(partial code)
@@ -332,10 +333,10 @@
 //!
 //! Because `Missing` only matches wildcard rows, specializing with `Missing` is guaranteed to
 //! result in a subset of the rows obtained from specializing with anything else. This means that
-//! any row with a wildcard found useful when specializing with anything else would also be found
-//! useful in the `Missing` case. In our example, after specializing with `North` here we will not
-//! gain new information regarding the usefulness of arm 2 or of the fake wildcard row used for
-//! exhaustiveness. This allows us to skip cases.
+//! any row with a wildcard found useful when specializing with any non-`Missing` constructor would
+//! also be found useful in the `Missing` case. In our example, after specializing with `North` here
+//! we will not gain new information regarding the usefulness of arm 2 or of the fake wildcard row
+//! used for exhaustiveness. This allows us to skip cases.
 //!
 //! When specializing, if there is a `Missing` case we call the other constructors "irrelevant".
 //! When there is no `Missing` case there are no irrelevant constructors.