Auto merge of #87697 - GuillaumeGomez:add-e0784, r=nagisa

Assign E0784 error code for union expression errors

compiler/rustc_error_codes/src/error_codes.rs

@@ -479,6 +479,7 @@ 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
@@ -636,7 +637,7 @@ E0783: include_str!("./error_codes/E0783.md"),
     E0711, // a feature has been declared with conflicting stability attributes
     E0717, // rustc_promotable without stability attribute
 //  E0721, // `await` keyword
-//    E0723, unstable feature in `const` context
+//  E0723, unstable feature in `const` context
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
     E0772, // `'static' obligation coming from `impl dyn Trait {}` or `impl Foo for dyn Bar {}`.

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