[naga wgsl-in]: Rename error variant field for clarity.

Rename `naga::front::wgsl::error::Error::MissingTemplateArg::arg` to
`description`, to make it a little more specific.

Author: jimblandy

naga/src/front/wgsl/error.rs

@@ -431,7 +431,7 @@ pub(crate) enum Error<'a> {
     UnexpectedTemplate(Span),
     MissingTemplateArg {
         span: Span,
-        arg: &'static str,
+        description: &'static str,
     },
     UnexpectedExprForTypeExpression(Span),
 }
@@ -1460,7 +1460,7 @@ impl<'a> Error<'a> {
             },
             Error::MissingTemplateArg {
                 span,
-                arg,
+                description: arg,
             } => ParseError {
                 message: format!(
                     "`{}` needs a template argument specified: {arg}",

naga/src/front/wgsl/lower/template_list.rs

@@ -42,14 +42,14 @@ impl<'iter, 'source> TemplateListIter<'iter, 'source> {
 
     fn expect_next(
         &mut self,
-        arg: &'static str,
+        description: &'static str,
     ) -> Result<'source, Handle<ast::Expression<'source>>> {
         if let Some(expr) = self.template_list.next() {
             Ok(*expr)
         } else {
             Err(Box::new(Error::MissingTemplateArg {
                 span: self.ident_span,
-                arg,
+                description,
             }))
         }
     }