Added first migration for repeated expressions without syntax vars

Author: nidnogg

compiler/rustc_error_messages/locales/en-US/expand.ftl

@@ -3,3 +3,6 @@ expand_explain_doc_comment_outer =
 
 expand_explain_doc_comment_inner =
     inner doc comments expand to `#![doc = "..."]`, which is what this macro attempted to match
+
+expand_expr_repeat_no_syntax_vars = 
+    attempted to repeat an expression containing no syntax variables matched as repeating at this depth

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -9,6 +9,7 @@ use rustc_errors::{pluralize, PResult};
 use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed};
 use rustc_span::hygiene::{LocalExpnId, Transparency};
 use rustc_span::symbol::{sym, Ident, MacroRulesNormalizedIdent};
+use rustc_macros::SessionDiagnostic;
 use rustc_span::Span;
 
 use smallvec::{smallvec, SmallVec};
@@ -53,6 +54,13 @@ impl<'a> Iterator for Frame<'a> {
     }
 }
 
+#[derive(SessionDiagnostic)]
+#[error(expand::expr_repeat_no_syntax_vars)]
+struct NoSyntaxVarsExprRepeat {
+    #[primary_span]
+    span: Span,
+}
+
 /// This can do Macro-By-Example transcription.
 /// - `interp` is a map of meta-variables to the tokens (non-terminals) they matched in the
 ///   invocation. We are assuming we already know there is a match.
@@ -165,11 +173,7 @@ pub(super) fn transcribe<'a>(
             seq @ mbe::TokenTree::Sequence(_, delimited) => {
                 match lockstep_iter_size(&seq, interp, &repeats) {
                     LockstepIterSize::Unconstrained => {
-                        return Err(cx.struct_span_err(
-                            seq.span(), /* blame macro writer */
-                            "attempted to repeat an expression containing no syntax variables \
-                             matched as repeating at this depth",
-                        ));
+                        return Err(cx.create_err(NoSyntaxVarsExprRepeat { span: seq.span() }));
                     }
 
                     LockstepIterSize::Contradiction(msg) => {