propagate the error from parsing enum variant to the parser and emit out

compiler/rustc_parse/src/parser/item.rs

@@ -1414,7 +1414,10 @@ impl<'a> Parser<'a> {
 
                 Ok((Some(vr), TrailingToken::MaybeComma))
             },
-        )
+        ).map_err(|mut err|{
+            err.help("enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`");
+            err
+        })
     }
 
     /// Parses `struct Foo { ... }`.

compiler/rustc_parse/src/parser/mod.rs

@@ -943,6 +943,10 @@ impl<'a> Parser<'a> {
                                 Err(e) => {
                                     // Parsing failed, therefore it must be something more serious
                                     // than just a missing separator.
+                                    for xx in &e.children {
+                                        // propagate the help message from sub error 'e' to main error 'expect_err;
+                                        expect_err.children.push(xx.clone());
+                                    }
                                     expect_err.emit();
 
                                     e.cancel();

src/test/ui/macros/syntax-error-recovery.stderr

@@ -7,6 +7,7 @@ LL |                 $token $($inner)? = $value,
 LL | values!(STRING(1) as (String) => cfg(test),);
    | -------------------------------------------- in this macro invocation
    |
+   = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
    = note: this error originates in the macro `values` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: macro expansion ignores token `(String)` and any following

src/test/ui/parser/issue-101477-enum.stderr

@@ -3,6 +3,8 @@ error: unexpected `==`
    |
 LL |     B == 2
    |       ^^ help: try using `=` instead
+   |
+   = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
 
 error: expected item, found `==`
   --> $DIR/issue-101477-enum.rs:6:7

src/test/ui/parser/issue-103869.rs

@@ -0,0 +1,9 @@
+enum VecOrMap{
+    vec: Vec<usize>,
+    //~^ ERROR expected one of `(`, `,`, `=`, `{`, or `}`, found `:`
+    //~| HELP: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
+    //~| ERROR expected item, found `:`
+    map: HashMap<String,usize>
+}
+
+fn main() {}

src/test/ui/parser/issue-103869.stderr

@@ -0,0 +1,16 @@
+error: expected one of `(`, `,`, `=`, `{`, or `}`, found `:`
+  --> $DIR/issue-103869.rs:2:8
+   |
+LL |     vec: Vec<usize>,
+   |        ^ expected one of `(`, `,`, `=`, `{`, or `}`
+   |
+   = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
+
+error: expected item, found `:`
+  --> $DIR/issue-103869.rs:2:8
+   |
+LL |     vec: Vec<usize>,
+   |        ^ expected item
+
+error: aborting due to 2 previous errors
+

src/test/ui/parser/macro/issue-37113.stderr

@@ -9,6 +9,7 @@ LL |             $( $t, )*
 LL |     test_macro!(String,);
    |     -------------------- in this macro invocation
    |
+   = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
    = note: this error originates in the macro `test_macro` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to previous error

src/test/ui/structs/struct-fn-in-definition.rs

@@ -28,6 +28,7 @@ enum E {
     //~^ ERROR functions are not allowed in enum definitions
     //~| HELP unlike in C++, Java, and C#, functions are declared in `impl` blocks
     //~| HELP see https://doc.rust-lang.org/book/ch05-03-method-syntax.html for more information
+    //~| HELP enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
 }
 
 fn main() {}

src/test/ui/structs/struct-fn-in-definition.stderr

@@ -33,6 +33,7 @@ LL |     fn foo() {}
    |
    = help: unlike in C++, Java, and C#, functions are declared in `impl` blocks
    = help: see https://doc.rust-lang.org/book/ch05-03-method-syntax.html for more information
+   = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
 
 error: aborting due to 3 previous errors