Improve explanations for E0072, E0073, E0121, E0178.

Author: Nick Hamann

src/librustc_typeck/diagnostics.rs

@@ -133,12 +133,12 @@ size of ListNode = 1 byte for head
                  + size of ListNode
 ```
 
-One way to fix this is by wrapping `ByteList` in a `Box`, like so:
+One way to fix this is by wrapping `ListNode` in a `Box`, like so:
 
 ```
-struct ByteList {
+struct ListNode {
     head: u8,
-    tail: Option<Box<ByteList>>,
+    tail: Option<Box<ListNode>>,
 }
 ```
 
@@ -147,9 +147,8 @@ This works because `Box` is a pointer, so its size is well-known.
 
 E0073: r##"
 You cannot define a struct (or enum) `Foo` that requires an instance of `Foo`
-in order to make a new `Foo` value (think about it: if making an instance of
-`Foo` requires an already-existing instance of `Foo`, how does the first
-instance get made?).
+in order to make a new `Foo` value. This is because there would be no way a
+first instance of `Foo` could be made to initialize another instance!
 
 Here's an example of a struct that has this problem:
 
@@ -163,7 +162,7 @@ One fix is to use `Option`, like so:
 struct Foo { x: Option<Box<Foo>> }
 ```
 
-Now it's possible to create at least one instance of Foo: `Foo { x: None }`.
+Now it's possible to create at least one instance of `Foo`: `Foo { x: None }`.
 "##,
 
 E0081: r##"
@@ -324,17 +323,8 @@ RFC. It is, however, [currently unimplemented][iss15872].
 "##,
 
 E0121: r##"
-When `_` is in a type, it can act as a placeholder for another type. The
-compiler interprets it as a request to infer the type. This enables partial
-type hints, as in the following example:
-
-```
-// x is a Vec<T> for some T. The compiler infers T.
-let x: Vec<_> = (0..4).collect();
-```
-
-However, in order to be consistent with Rust's lack of global type inference,
-type placeholders are disallowed by design in item signatures.
+In order to be consistent with Rust's lack of global type inference, type
+placeholders are disallowed by design in item signatures.
 
 Examples of this error include:
 
@@ -392,8 +382,7 @@ struct Bar<'a> {
 
 More details can be found in [RFC 438].
 
-[RFC 438]: https://github.com/rust-lang/rfcs/blob/master/text/0438-precedence-\
-of-plus.md
+[RFC 438]: https://github.com/rust-lang/rfcs/pull/438
 "##,
 
 E0184: r##"