From 87e981996fe47826a8f3a69e3d3cc73ec68dbf8d Mon Sep 17 00:00:00 2001
From: Trevor Gross <tmgross@umich.edu>
Date: Sat, 21 Jun 2025 02:59:14 -0400
Subject: [PATCH] mbe: Refactor the diagnostic for unrecognized metavariable
 expressions

Change to a structural diagnostic, update the valid list, and move the
valid list to a note.
---
 compiler/rustc_expand/messages.ftl                 |  5 +++++
 compiler/rustc_expand/src/errors.rs                | 10 ++++++++++
 compiler/rustc_expand/src/mbe/metavar_expr.rs      | 14 +++++---------
 .../ui/macros/metavar-expressions/syntax-errors.rs |  2 +-
 .../metavar-expressions/syntax-errors.stderr       |  6 ++++--
 5 files changed, 25 insertions(+), 12 deletions(-)

diff --git a/compiler/rustc_expand/messages.ftl b/compiler/rustc_expand/messages.ftl
index 7b8c1e076a09..89d6e62834d6 100644
--- a/compiler/rustc_expand/messages.ftl
+++ b/compiler/rustc_expand/messages.ftl
@@ -154,6 +154,11 @@ expand_mve_missing_paren =
     .note = metavariable expressions use function-like parentheses syntax
     .suggestion = try adding parentheses
 
+expand_mve_unrecognized_expr =
+    unrecognized metavariable expression
+    .label = not a valid metavariable expression
+    .note = valid metavariable expressions are {$valid_expr_list}
+
 expand_mve_unrecognized_var =
     variable `{$key}` is not recognized in meta-variable expression
 
diff --git a/compiler/rustc_expand/src/errors.rs b/compiler/rustc_expand/src/errors.rs
index b56a2b043e75..3ac5d2130535 100644
--- a/compiler/rustc_expand/src/errors.rs
+++ b/compiler/rustc_expand/src/errors.rs
@@ -530,6 +530,16 @@ mod metavar_exprs {
         pub insert_span: Option<Span>,
     }
 
+    #[derive(Diagnostic)]
+    #[note]
+    #[diag(expand_mve_unrecognized_expr)]
+    pub(crate) struct MveUnrecognizedExpr {
+        #[primary_span]
+        #[label]
+        pub span: Span,
+        pub valid_expr_list: &'static str,
+    }
+
     #[derive(Diagnostic)]
     #[diag(expand_mve_unrecognized_var)]
     pub(crate) struct MveUnrecognizedVar {
diff --git a/compiler/rustc_expand/src/mbe/metavar_expr.rs b/compiler/rustc_expand/src/mbe/metavar_expr.rs
index 8f15b5794c7f..d2b275ad20a9 100644
--- a/compiler/rustc_expand/src/mbe/metavar_expr.rs
+++ b/compiler/rustc_expand/src/mbe/metavar_expr.rs
@@ -79,15 +79,11 @@ impl MetaVarExpr {
             "index" => MetaVarExpr::Index(parse_depth(&mut iter, psess, ident.span)?),
             "len" => MetaVarExpr::Len(parse_depth(&mut iter, psess, ident.span)?),
             _ => {
-                let err_msg = "unrecognized meta-variable expression";
-                let mut err = psess.dcx().struct_span_err(ident.span, err_msg);
-                err.span_suggestion(
-                    ident.span,
-                    "supported expressions are count, ignore, index and len",
-                    "",
-                    Applicability::MachineApplicable,
-                );
-                return Err(err);
+                let err = errors::MveUnrecognizedExpr {
+                    span: ident.span,
+                    valid_expr_list: "`count`, `ignore`, `index`, `len`, and `concat`",
+                };
+                return Err(psess.dcx().create_err(err));
             }
         };
         check_trailing_tokens(&mut iter, psess, ident)?;
diff --git a/tests/ui/macros/metavar-expressions/syntax-errors.rs b/tests/ui/macros/metavar-expressions/syntax-errors.rs
index a6eebdfa6744..585ea4d5979c 100644
--- a/tests/ui/macros/metavar-expressions/syntax-errors.rs
+++ b/tests/ui/macros/metavar-expressions/syntax-errors.rs
@@ -118,7 +118,7 @@ macro_rules! unknown_ignore_ident {
 
 macro_rules! unknown_metavar {
     ( $( $i:ident ),* ) => { ${ aaaaaaaaaaaaaa(i) } };
-    //~^ ERROR unrecognized meta-variable expression
+    //~^ ERROR unrecognized metavariable expression
 }
 
 fn main() {}
diff --git a/tests/ui/macros/metavar-expressions/syntax-errors.stderr b/tests/ui/macros/metavar-expressions/syntax-errors.stderr
index f3c08f05e2a4..bf1c7673a6ce 100644
--- a/tests/ui/macros/metavar-expressions/syntax-errors.stderr
+++ b/tests/ui/macros/metavar-expressions/syntax-errors.stderr
@@ -192,11 +192,13 @@ error: meta-variables within meta-variable expressions must be referenced using
 LL |         ${ignore(bar)}
    |           ^^^^^^
 
-error: unrecognized meta-variable expression
+error: unrecognized metavariable expression
   --> $DIR/syntax-errors.rs:120:33
    |
 LL |     ( $( $i:ident ),* ) => { ${ aaaaaaaaaaaaaa(i) } };
-   |                                 ^^^^^^^^^^^^^^ help: supported expressions are count, ignore, index and len
+   |                                 ^^^^^^^^^^^^^^ not a valid metavariable expression
+   |
+   = note: valid metavariable expressions are `count`, `ignore`, `index`, `len`, and `concat`
 
 error: expected identifier or string literal
   --> $DIR/syntax-errors.rs:38:14