Add long error explanation for E0628

src/librustc_error_codes/error_codes.rs

@@ -349,6 +349,7 @@ E0623: include_str!("./error_codes/E0623.md"),
 E0624: include_str!("./error_codes/E0624.md"),
 E0626: include_str!("./error_codes/E0626.md"),
 E0627: include_str!("./error_codes/E0627.md"),
+E0628: include_str!("./error_codes/E0628.md"),
 E0631: include_str!("./error_codes/E0631.md"),
 E0633: include_str!("./error_codes/E0633.md"),
 E0635: include_str!("./error_codes/E0635.md"),
@@ -580,7 +581,6 @@ E0748: include_str!("./error_codes/E0748.md"),
 //  E0612, // merged into E0609
 //  E0613, // Removed (merged with E0609)
     E0625, // thread-local statics cannot be accessed at compile-time
-    E0628, // generators cannot have explicit parameters
     E0629, // missing 'feature' (rustc_const_unstable)
     // rustc_const_unstable attribute must be paired with stable/unstable
     // attribute

src/librustc_error_codes/error_codes/E0628.md

@@ -0,0 +1,30 @@
+More than one parameter was used for a generator.
+
+Erroneous code example:
+
+```compile_fail,E0628
+#![feature(generators, generator_trait)]
+
+fn main() {
+    let generator = |a: i32, b: i32| {
+        // error: too many parameters for a generator
+        // Allowed only 0 or 1 parameter
+        yield a;
+    };
+}
+```
+
+At present, it is not permitted to pass more than one explicit
+parameter for a generator.This can be fixed by using only
+0 or 1 parameter for generator. So, for example, we might resolve
+the previous example by passing only one parameter.
+
+```
+#![feature(generators, generator_trait)]
+
+fn main() {
+    let generator = |a: i32| {
+        yield a;
+    };
+}
+```

src/test/ui/generator/too-many-parameters.stderr

@@ -6,3 +6,4 @@ LL |     |(), ()| {
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0628`.