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.

Author: tgross35

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
 

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 {

compiler/rustc_expand/src/mbe/metavar_expr.rs

@@ -32,6 +32,9 @@ const EXPR_NAME_ARG_MAP: &[(&str, ArgSpec)] = &[
     ("len", ArgSpec::Between(0, 1)),
 ];
 
+/// List of the above for diagnostics
+const VALID_METAVAR_EXPR_NAMES: &str = "`count`, `ignore`, `index`, `len`, and `concat`";
+
 /// A meta-variable expression, for expansions based on properties of meta-variables.
 #[derive(Debug, PartialEq, Encodable, Decodable)]
 pub(crate) enum MetaVarExpr {
@@ -99,15 +102,11 @@ 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: VALID_METAVAR_EXPR_NAMES,
+                };
+                return Err(psess.dcx().create_err(err));
             }
         };
         check_trailing_tokens(&mut iter, psess, ident)?;

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() {}

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