Add a distinct error code and description for "main function has wrong type"

src/librustc/diagnostics.rs

@@ -1694,6 +1694,30 @@ fn main() {
 https://doc.rust-lang.org/book/closures.html
 "##,
 
+E0579: r##"
+The `main` function was incorrectly declared.
+
+Erroneous code example:
+
+```compile_fail,E0579
+fn main() -> i32 { // error: main function has wrong type
+    0
+}
+```
+
+The `main` function prototype should never take arguments or return type.
+Example:
+
+```
+fn main() {
+    // your code
+}
+```
+
+If you want to get command-line arguments, use `std::env::args`. To exit with a
+specified exit code, use `std::process::exit`.
+"##,
+
 }
 
 

src/librustc/infer/error_reporting.rs

@@ -629,10 +629,13 @@ pub fn report_and_explain_type_error(&self,
         let mut diag = match trace.cause.code {
             ObligationCauseCode::IfExpressionWithNoElse => {
                 struct_span_err!(self.tcx.sess, span, E0317, "{}", failure_str)
-            },
+            }
+            ObligationCauseCode::MainFunctionType => {
+                struct_span_err!(self.tcx.sess, span, E0579, "{}", failure_str)
+            }
             _ => {
                 struct_span_err!(self.tcx.sess, span, E0308, "{}", failure_str)
-            },
+            }
         };
         self.note_type_err(&mut diag, &trace.cause, None, Some(trace.values), terr);
         diag

src/test/compile-fail/E0308-3.rs → src/test/compile-fail/E0579.rs

@@ -8,4 +8,4 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn main() -> i32 { 0 } //~ ERROR E0308
+fn main() -> i32 { 0 } //~ ERROR E0579

src/test/compile-fail/bad-main.rs

@@ -8,4 +8,4 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn main(x: isize) { } //~ ERROR: main function has wrong type
+fn main(x: isize) { } //~ ERROR: main function has wrong type [E0579]

src/test/compile-fail/extern-main-fn.rs

@@ -8,4 +8,4 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-extern fn main() {} //~ ERROR: main function has wrong type
+extern fn main() {} //~ ERROR: main function has wrong type [E0579]

src/test/compile-fail/main-wrong-type-2.rs

@@ -9,6 +9,6 @@
 // except according to those terms.
 
 fn main() -> char {
-//~^ ERROR: main function has wrong type
+//~^ ERROR: main function has wrong type [E0579]
     ' '
 }

src/test/compile-fail/main-wrong-type.rs

@@ -14,5 +14,5 @@ struct S {
 }
 
 fn main(foo: S) {
-//~^ ERROR: main function has wrong type
+//~^ ERROR: main function has wrong type [E0579]
 }