Corrected E0637.md based on test failures

Author: jwhite927

src/librustc_error_codes/error_codes/E0637.md

@@ -1,59 +1,32 @@
-An underscore `_` character has been used as the identifier for a lifetime,
-or a const generic has been borrowed without an explicit lifetime.
+An underscore `_` character has been used as the identifier for a lifetime.
 
-Erroneous example with an underscore:
+Erroneous example:
 ```compile_fail,E0106,E0637
-fn foo<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {}
-    // ^^ `'_` is a reserved lifetime name
-```
-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
-as the identifier because it is a reserved lifetime name. To fix
-this, use a lowercase letter, or a series of lowercase letters as the lifetime
-identifier. Often a single lowercase letter, such as `'a`, is sufficient.  For
-more information, see [the book][bk-no].
-
-Corrected underscore example:
-```
-fn foo<'a>(str1: &'a str, str2: &'a str) -> &'a str {}
-```
-
-Erroneous example with const generic:
-```compile_fail,E0261,E0637,E0658
-struct A<const N: &u8>;
-//~^ ERROR `&` without an explicit lifetime name cannot be used here
-trait B {}
-
-impl<const N: &u8> A<N> {
-//~^ ERROR `&` without an explicit lifetime name cannot be used here
-    fn foo<const M: &u8>(&self) {}
-    //~^ ERROR `&` without an explicit lifetime name cannot be used here
+fn longest<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
+         //^^ `'_` is a reserved lifetime name
+    if str1.len() > str2.len() {
+        str1
+    } else {
+        str2
+    }
 }
-
-impl<const N: &u8> B for A<N> {}
-//~^ ERROR `&` without an explicit lifetime name cannot be used here
-
-fn bar<const N: &u8>() {}
-//~^ ERROR `&` without an explicit lifetime name cannot be used here
 ```
+`'_`, cannot be used as a lifetime identifier because it is a reserved for the
+anonymous lifetime. To fix this, use a lowercase letter such as 'a, or a series
+of lowercase letters such as `'foo`.  For more information, see [the book][bk-no].
+For more information on using the anonymous lifetime in rust nightly, see [the
+nightly book][bk-al].
 
-Const generics cannot be borrowed without specifying a lifetime.The
-compiler handles memory allocation of constants differently than that of
-variables and it cannot infer the lifetime of the borrowed constant.
-To fix this, explicitly specify a lifetime for the const generic.
-
-Corrected const generic example:
+Corrected example:
 ```
-struct A<const N: &'a u8>;
-
-trait B {}
-
-impl<const N: &'a u8> A<N> {
-    fn foo<const M: &'a u8>(&self) {}
+fn longest<'a>(str1: &'a str, str2: &'a str) -> &'a str {
+    if str1.len() > str2.len() {
+        str1
+    } else {
+        str2
+    }
 }
-
-impl<const N: &'a u8> B for A<N> {}
-
-fn bar<const N: &'a u8>() {}
 ```
+
 [bk-no]: https://doc.rust-lang.org/book/appendix-02-operators.html#non-operator-symbols
+[bk-al]: https://doc.rust-lang.org/nightly/edition-guide/rust-2018/ownership-and-lifetimes/the-anonymous-lifetime.html