move E0744 to new error code

src/librustc_error_codes/error_codes.rs

@@ -401,6 +401,7 @@ E0740: include_str!("./error_codes/E0740.md"),
 E0741: include_str!("./error_codes/E0741.md"),
 E0742: include_str!("./error_codes/E0742.md"),
 E0743: include_str!("./error_codes/E0743.md"),
+E0744: include_str!("./error_codes/E0744.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

src/librustc_error_codes/error_codes/E0744.md

@@ -0,0 +1,17 @@
+Control-flow expressions are not allowed inside a const context.
+
+At the moment, `if` and `match`, as well as the looping constructs `for`,
+`while`, and `loop`, are forbidden inside a `const`, `static`, or `const fn`.
+
+```compile_fail,E0744
+const _: i32 = {
+    let mut x = 0;
+    loop {
+        x += 1;
+        if x == 4 {
+            break;
+        }
+    }
+    x
+};
+```

src/librustc_passes/check_const.rs

@@ -17,6 +17,7 @@ use rustc::ty::query::Providers;
 use syntax::ast::Mutability;
 use syntax::span_err;
 use syntax_pos::Span;
+use rustc_error_codes::*;
 
 use std::fmt;