generalize on_missing_args diagnostics

Author: chenyukang

compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -76,7 +76,7 @@ pub(super) fn failed_to_match_macro(
     let span = token.span.substitute_dummy(sp);
     let CustomDiagnostic {
         message: custom_message, label: custom_label, notes: custom_notes, ..
-    } = if matches!(token.kind, token::Eof) {
+    } = {
         let macro_name = name.to_string();
         on_missing_args
             .map(|directive| {
@@ -90,9 +90,14 @@ pub(super) fn failed_to_match_macro(
                     },
                 )
             })
+            .map(|diag| {
+                if matches!(token.kind, token::Eof) {
+                    diag
+                } else {
+                    CustomDiagnostic { message: None, label: None, ..diag }
+                }
+            })
             .unwrap_or_default()
-    } else {
-        CustomDiagnostic::default()
     };
 
     let mut err = match custom_message {

src/doc/unstable-book/src/language-features/diagnostic-on-missing-args.md

@@ -12,8 +12,9 @@ It lets a macro definition customize the diagnostic that is emitted when an invo
 all required arguments were provided.
 
 This attribute currently applies to declarative macros such as `macro_rules!` and `pub macro`.
-It only affects diagnostics for incomplete invocations; other matcher failures continue to use the
-usual macro diagnostics.
+Custom `message` and `label` are only used for incomplete invocations.
+Custom `note`s are also emitted for other matcher failures where no macro arm matches, such as
+trailing extra arguments.
 
 ```rust,compile_fail
 #![feature(diagnostic_on_missing_args)]

tests/ui/diagnostic_namespace/on_missing_args/notes_on_extra_args.rs

@@ -0,0 +1,19 @@
+#![feature(diagnostic_on_missing_args)]
+
+#[diagnostic::on_missing_args(
+    note = "this macro expects a type and a value, like `pair!(u8, 0)`",
+    note = "make sure to pass both arguments",
+)]
+macro_rules! pair {
+    //~^ NOTE when calling this macro
+    ($ty:ty, $value:expr) => {};
+    //~^ NOTE while trying to match meta-variable `$value:expr`
+}
+
+fn main() {
+    pair!(u8, 0, 42);
+    //~^ ERROR no rules expected `,`
+    //~| NOTE no rules expected this token in macro call
+    //~| NOTE this macro expects a type and a value, like `pair!(u8, 0)`
+    //~| NOTE make sure to pass both arguments
+}

tests/ui/diagnostic_namespace/on_missing_args/notes_on_extra_args.stderr

@@ -0,0 +1,19 @@
+error: no rules expected `,`
+  --> $DIR/notes_on_extra_args.rs:14:16
+   |
+LL | macro_rules! pair {
+   | ----------------- when calling this macro
+...
+LL |     pair!(u8, 0, 42);
+   |                ^ no rules expected this token in macro call
+   |
+note: while trying to match meta-variable `$value:expr`
+  --> $DIR/notes_on_extra_args.rs:9:14
+   |
+LL |     ($ty:ty, $value:expr) => {};
+   |              ^^^^^^^^^^^
+   = note: this macro expects a type and a value, like `pair!(u8, 0)`
+   = note: make sure to pass both arguments
+
+error: aborting due to 1 previous error
+