add list of recognized repr attributes to the unrecognized repr error

compiler/rustc_attr/src/builtin.rs

@@ -1040,18 +1040,16 @@ pub fn parse_repr_attr(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
                                 &name,
                             ),
                         });
-                    } else {
+                    } else if matches!(
-                        if matches!(
+                        meta_item.name_or_empty(),
-                            meta_item.name_or_empty(),
+                        sym::C | sym::simd | sym::transparent
-                            sym::C | sym::simd | sym::transparent
+                    ) || int_type_of_word(meta_item.name_or_empty()).is_some()
-                        ) || int_type_of_word(meta_item.name_or_empty()).is_some()
+                    {
-                        {
+                        recognised = true;
-                            recognised = true;
+                        sess.emit_err(session_diagnostics::InvalidReprHintNoValue {
-                            sess.emit_err(session_diagnostics::InvalidReprHintNoValue {
+                            span: meta_item.span,
-                                span: meta_item.span,
+                            name: meta_item.name_or_empty().to_ident_string(),
-                                name: meta_item.name_or_empty().to_ident_string(),
+                        });
-                            });
-                        }
                     }
                 } else if let MetaItemKind::List(_) = meta_item.kind {
                     if meta_item.has_name(sym::align) {

compiler/rustc_passes/src/check_attr.rs

@@ -1658,6 +1658,7 @@ impl CheckAttrVisitor<'_> {
                         E0552,
                         "unrecognized representation hint"
                     )
+                    .help("valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`")
                     .emit();
 
                     continue;

src/test/ui/issues/issue-43988.stderr

@@ -31,12 +31,16 @@ error[E0552]: unrecognized representation hint
    |
 LL |     #[repr(nothing)]
    |            ^^^^^^^
+   |
+   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
 error[E0552]: unrecognized representation hint
   --> $DIR/issue-43988.rs:18:12
    |
 LL |     #[repr(something_not_real)]
    |            ^^^^^^^^^^^^^^^^^^
+   |
+   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
 error[E0518]: attribute should be applied to function or closure
   --> $DIR/issue-43988.rs:30:5

src/test/ui/repr/invalid_repr_list_help.rs

@@ -0,0 +1,17 @@
+#![crate_type = "lib"]
+
+#[repr(uwu)] //~ERROR: unrecognized representation hint
+pub struct OwO;
+
+#[repr(uwu = "a")] //~ERROR: unrecognized representation hint
+pub struct OwO2(i32);
+
+#[repr(uwu(4))] //~ERROR: unrecognized representation hint
+pub struct OwO3 {
+    x: i32,
+}
+
+#[repr(uwu, u8)] //~ERROR: unrecognized representation hint
+pub enum OwO4 {
+    UwU = 1,
+}

src/test/ui/repr/invalid_repr_list_help.stderr

@@ -0,0 +1,35 @@
+error[E0552]: unrecognized representation hint
+  --> $DIR/invalid_repr_list_help.rs:3:8
+   |
+LL | #[repr(uwu)]
+   |        ^^^
+   |
+   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+
+error[E0552]: unrecognized representation hint
+  --> $DIR/invalid_repr_list_help.rs:6:8
+   |
+LL | #[repr(uwu = "a")]
+   |        ^^^^^^^^^
+   |
+   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+
+error[E0552]: unrecognized representation hint
+  --> $DIR/invalid_repr_list_help.rs:9:8
+   |
+LL | #[repr(uwu(4))]
+   |        ^^^^^^
+   |
+   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+
+error[E0552]: unrecognized representation hint
+  --> $DIR/invalid_repr_list_help.rs:14:8
+   |
+LL | #[repr(uwu, u8)]
+   |        ^^^
+   |
+   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0552`.