Add long error explanation for E0739

src/librustc_error_codes/error_codes.rs

@@ -411,6 +411,7 @@ E0735: include_str!("./error_codes/E0735.md"),
 E0736: include_str!("./error_codes/E0736.md"),
 E0737: include_str!("./error_codes/E0737.md"),
 E0738: include_str!("./error_codes/E0738.md"),
+E0739: include_str!("./error_codes/E0739.md"),
 E0740: include_str!("./error_codes/E0740.md"),
 E0741: include_str!("./error_codes/E0741.md"),
 E0742: include_str!("./error_codes/E0742.md"),
@@ -610,5 +611,4 @@ E0748: include_str!("./error_codes/E0748.md"),
     E0722, // Malformed `#[optimize]` attribute
     E0724, // `#[ffi_returns_twice]` is only allowed in foreign functions
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
-    E0739, // invalid track_caller application/syntax
 }

src/librustc_error_codes/error_codes/E0739.md

@@ -0,0 +1,13 @@
+`#[track_caller]` can not be applied on struct. 
+
+Erroneous code example:
+
+```compile_fail,E0739
+#![feature(track_caller)]
+#[track_caller]
+struct Bar {
+    a: u8,
+}
+```
+
+[RFC 2091]: https://github.com/rust-lang/rfcs/blob/master/text/2091-inline-semantic.md

src/test/ui/rfc-2091-track-caller/only-for-fns.stderr

@@ -8,3 +8,4 @@ LL | struct S;
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0739`.