Function expr_block 

pub fn expr_block<'sm>(
    sm: impl HasSourceMap<'sm>,
    expr: &Expr<'_>,
    outer: SyntaxContext,
    default: &str,
    indent_relative_to: Option<Span>,
    app: &mut Applicability,
) -> String

Like snippet_block, but add braces if the expr is not an ExprKind::Block with no label.