rustc_ast_lowering: make "using _ for array lengths is unstable" translatable

Author: GrigorenkoPV

compiler/rustc_ast_lowering/messages.ftl

@@ -167,6 +167,9 @@ ast_lowering_template_modifier = template modifier
 
 ast_lowering_this_not_async = this is not `async`
 
+ast_lowering_underscore_array_length_unstable =
+    using `_` for array lengths is unstable
+
 ast_lowering_underscore_expr_lhs_assign =
     in expressions, `_` can only be used on the left-hand side of an assignment
     .label = `_` not allowed here

compiler/rustc_ast_lowering/src/lib.rs

@@ -2326,7 +2326,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.expr_block(block)
     }
 
-    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
     fn lower_array_length(&mut self, c: &AnonConst) -> hir::ArrayLen<'hir> {
         match c.value.kind {
             ExprKind::Underscore => {
@@ -2340,7 +2339,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         &self.tcx.sess,
                         sym::generic_arg_infer,
                         c.value.span,
-                        "using `_` for array lengths is unstable",
+                        fluent_generated::ast_lowering_underscore_array_length_unstable,
                     )
                     .stash(c.value.span, StashKey::UnderscoreForArrayLengths);
                     hir::ArrayLen::Body(self.lower_anon_const_to_const_arg(c))