Function rustc_expand::base::expr_to_spanned_string

pub fn expr_to_spanned_string<'a>(
    cx: &'a mut ExtCtxt<'_>,
    expr: P<Expr>,
    err_msg: &str
) -> Result<(Symbol, StrStyle, Span), Option<(DiagnosticBuilder<'a, ErrorGuaranteed>, bool)>>

Extracts a string literal from the macro expanded version of expr, returning a diagnostic error of err_msg if expr is not a string literal. The returned bool indicates whether an applicable suggestion has already been added to the diagnostic to avoid emitting multiple suggestions. Err(None) indicates that an ast error was encountered.