Fix macro_metavar_expr_concat behavior with nested repetitions

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -558,26 +558,29 @@ fn metavar_expr_concat<'tx>(
             MetaVarExprConcatElem::Ident(elem) => elem.name,
             MetaVarExprConcatElem::Literal(elem) => *elem,
             MetaVarExprConcatElem::Var(ident) => {
-                match matched_from_ident(dcx, *ident, tscx.interp)? {
-                    NamedMatch::MatchedSeq(named_matches) => {
-                        let Some((curr_idx, _)) = tscx.repeats.last() else {
-                            return Err(dcx.struct_span_err(dspan.entire(), "invalid syntax"));
-                        };
-                        match &named_matches[*curr_idx] {
-                            // FIXME(c410-f3r) Nested repetitions are unimplemented
-                            MatchedSeq(_) => {
+                let mut matched = matched_from_ident(dcx, *ident, tscx.interp)?;
+                let mut repeats_iter = tscx.repeats.iter();
+
+                loop {
+                    match matched {
+                        NamedMatch::MatchedSingle(pnr) => {
+                            let symbol = extract_symbol_from_pnr(dcx, pnr, ident.span)?;
+                            concatenated.push_str(symbol.as_str());
+                            break;
+                        }
+                        NamedMatch::MatchedSeq(named_matches) => {
+                            if let Some((idx, _)) = repeats_iter.next() {
+                                matched = &named_matches[*idx];
+                            } else {
                                 return Err(dcx.struct_span_err(
                                     ident.span,
-                                    "nested repetitions with `${concat(...)}` metavariable expressions are not yet supported",
+                                    "`${concat(...)}` variable is still repeating at this depth",
                                 ));
                             }
-                            MatchedSingle(pnr) => extract_symbol_from_pnr(dcx, pnr, ident.span)?,
                         }
                     }
-                    NamedMatch::MatchedSingle(pnr) => {
-                        extract_symbol_from_pnr(dcx, pnr, ident.span)?
-                    }
                 }
+                continue;
             }
         };
         concatenated.push_str(symbol.as_str());

tests/ui/macros/concat-nested-repetition.rs

@@ -0,0 +1,35 @@
+//@ check-pass
+#![feature(macro_metavar_expr_concat)]
+
+struct A;
+struct B;
+const AA: A = A;
+const BB: B = B;
+
+macro_rules! define_ioctl_data {
+    (struct $s:ident {
+        $($field:ident: $ty:ident $([$opt:ident])?,)*
+    }) => {
+        pub struct $s {
+            $($field: $ty,)*
+        }
+
+        impl $s {
+            $($(
+                fn ${concat(get_, $field)}(&self) -> $ty {
+                    let _ = $opt;
+                    todo!()
+                }
+            )?)*
+        }
+    };
+}
+
+define_ioctl_data! {
+    struct Foo {
+        a: A [AA],
+        b: B [BB],
+    }
+}
+
+fn main() {}

tests/ui/macros/macro-metavar-expr-concat/in-repetition.rs

@@ -11,7 +11,7 @@ macro_rules! InRepetition {
      ) => {
         $(
             $(
-                ${concat(_, $arg)} //~ ERROR nested repetitions with `${concat(...)}` metavariable expressions are not yet supported
+                ${concat(_, $arg)} //~ ERROR macro expansion ends with an incomplete expression: expected one of `!` or `::`
             )*
         )*
     };

tests/ui/macros/macro-metavar-expr-concat/in-repetition.stderr

@@ -1,8 +1,8 @@
-error: nested repetitions with `${concat(...)}` metavariable expressions are not yet supported
-  --> $DIR/in-repetition.rs:14:30
+error: macro expansion ends with an incomplete expression: expected one of `!` or `::`
+  --> $DIR/in-repetition.rs:14:35
    |
 LL |                 ${concat(_, $arg)}
-   |                              ^^^
+   |                                   ^ expected one of `!` or `::`
 
 error: aborting due to 1 previous error
 

tests/ui/macros/metavar-expressions/concat-repetitions.rs

@@ -11,17 +11,16 @@ macro_rules! one_rep {
 macro_rules! issue_128346 {
     ( $($a:ident)* ) => {
         A(
-            const ${concat($a, Z)}: i32 = 3;
-            //~^ ERROR invalid syntax
+            const ${concat($a, Z)}: i32 = 3; //~ ERROR `${concat(...)}` variable is still repeating at this depth
         )*
     };
 }
 
 macro_rules! issue_131393 {
     ($t:ident $($en:ident)?) => {
         read::<${concat($t, $en)}>()
-        //~^ ERROR invalid syntax
-        //~| ERROR invalid syntax
+        //~^ ERROR `${concat(...)}` variable is still repeating at this depth
+        //~| ERROR `${concat(...)}` variable is still repeating at this depth
     }
 }
 

tests/ui/macros/metavar-expressions/concat-repetitions.stderr

@@ -1,20 +1,20 @@
-error: invalid syntax
-  --> $DIR/concat-repetitions.rs:14:20
+error: `${concat(...)}` variable is still repeating at this depth
+  --> $DIR/concat-repetitions.rs:14:29
    |
 LL |             const ${concat($a, Z)}: i32 = 3;
-   |                    ^^^^^^^^^^^^^^^
+   |                             ^
 
-error: invalid syntax
-  --> $DIR/concat-repetitions.rs:22:17
+error: `${concat(...)}` variable is still repeating at this depth
+  --> $DIR/concat-repetitions.rs:21:30
    |
 LL |         read::<${concat($t, $en)}>()
-   |                 ^^^^^^^^^^^^^^^^^
+   |                              ^^
 
-error: invalid syntax
-  --> $DIR/concat-repetitions.rs:22:17
+error: `${concat(...)}` variable is still repeating at this depth
+  --> $DIR/concat-repetitions.rs:21:30
    |
 LL |         read::<${concat($t, $en)}>()
-   |                 ^^^^^^^^^^^^^^^^^
+   |                              ^^
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`