Assign E0784 to union expression error

compiler/rustc_error_codes/src/error_codes.rs

@@ -478,6 +478,7 @@ E0780: include_str!("./error_codes/E0780.md"),
 E0781: include_str!("./error_codes/E0781.md"),
 E0782: include_str!("./error_codes/E0782.md"),
 E0783: include_str!("./error_codes/E0783.md"),
+E0784: include_str!("./error_codes/E0784.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

compiler/rustc_error_codes/src/error_codes/E0784.md

@@ -0,0 +1,32 @@
+A union expression does not have exactly one field.
+
+Erroneous code example:
+
+```compile_fail,E0784
+union Bird {
+    pigeon: u8,
+    turtledove: u16,
+}
+
+let bird = Bird {}; // error
+let bird = Bird { pigeon: 0, turtledove: 1 }; // error
+```
+
+The key property of unions is that all fields of a union share common storage.
+As a result, writes to one field of a union can overwrite its other fields, and
+size of a union is determined by the size of its largest field.
+
+You can find more information about the union types in the [Rust reference].
+
+Working example:
+
+```
+union Bird {
+    pigeon: u8,
+    turtledove: u16,
+}
+
+let bird = Bird { pigeon: 0 }; // OK
+```
+
+[Rust reference]: https://doc.rust-lang.org/reference/items/unions.html

compiler/rustc_typeck/src/check/expr.rs

@@ -1304,7 +1304,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // Make sure the programmer specified correct number of fields.
         if kind_name == "union" {
             if ast_fields.len() != 1 {
-                tcx.sess.span_err(span, "union expressions should have exactly one field");
+                struct_span_err!(
+                    tcx.sess,
+                    span,
+                    E0784,
+                    "union expressions should have exactly one field",
+                )
+                .emit();
             }
         } else if check_completeness && !error_happened && !remaining_fields.is_empty() {
             let no_accessible_remaining_fields = remaining_fields