rustc: Add long diagnostics for E0301

src/librustc/diagnostics.rs

@@ -139,6 +139,20 @@ for item in xs {
 }
 "##,
 
+E0301: r##"
+Mutable borrows are not allowed in pattern guards, because matching cannot have
+side effects. Side effects could alter the matched object or the environment
+on which the match depends in such a way, that the match would not be
+exhaustive. For instance, the following would not match any arm if mutable
+borrows were allowed:
+
+match Some(()) {
+    None => { },
+    option if option.take().is_none() => { /* impossible, option is `Some` */ },
+    Some(_) => { } // When the previous match failed, the option became `None`.
+}
+"##,
+
 E0303: r##"
 In certain cases it is possible for sub-bindings to violate memory safety.
 Updates to the borrow checker in a future version of Rust may remove this
@@ -224,7 +238,6 @@ register_diagnostics! {
     E0298, // mismatched types between arms
     E0299, // mismatched types between arms
     E0300, // unexpanded macro
-    E0301, // cannot mutable borrow in a pattern guard
     E0302, // cannot assign in a pattern guard
     E0304, // expected signed integer constant
     E0305, // expected constant