Added long error description & modifed error_codes.rs

Author: jwhite927

src/librustc_error_codes/error_codes.rs

@@ -353,6 +353,7 @@ E0631: include_str!("./error_codes/E0631.md"),
 E0633: include_str!("./error_codes/E0633.md"),
 E0635: include_str!("./error_codes/E0635.md"),
 E0636: include_str!("./error_codes/E0636.md"),
+E0637: include_str!("./error_codes/E0637.md"),
 E0638: include_str!("./error_codes/E0638.md"),
 E0639: include_str!("./error_codes/E0639.md"),
 E0641: include_str!("./error_codes/E0641.md"),
@@ -584,7 +585,6 @@ E0746: include_str!("./error_codes/E0746.md"),
     E0632, // cannot provide explicit generic arguments when `impl Trait` is
            // used in argument position
     E0634, // type has conflicting packed representaton hints
-    E0637, // "'_" is not a valid lifetime bound
     E0640, // infer outlives requirements
 //  E0645, // trait aliases not finished
     E0657, // `impl Trait` can only capture lifetimes bound at the fn level

src/librustc_error_codes/error_codes/E0637.md

@@ -0,0 +1,22 @@
+The underscore `_` character has been used as the identifier for a lifetime. 
+
+Erroneous code example:
+```
+fn some_function<'_>(x: &'_ str, y: &'_ str) -> &'_ str {
+    //Some code
+}
+```
+
+Lifetimes are named with 'ident, where ident is the name of the 
+lifetime or loop. The `_` character, which represents the ignore pattern, 
+cannot be used because it is a reserved lifetime name. 
+To fix this, use a single lowercase letter as the 
+lifetime identifier, such as `'a`. For more information, see [The Rust Book](https://doc.rust-lang.org/stable/book/appendix-02-operators.html#non-operator-symbols).
+
+Corrected code example:
+```
+fn some_function<'a>(x: &'a str, y: &'a str) -> &'a str {
+    //Some code
+}
+```
+