add doc comment detail
diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index 97daf89..7fffead 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -148,6 +148,12 @@ fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, G> {
/// converted rather than on `DiagArgValue`, which enables types from other `rustc_*` crates to
/// implement this.
pub trait IntoDiagArg {
+ /// Convert `Self` into a `DiagArgValue` suitable for rendering in a diagnostic.
+ ///
+ /// It takes a `path` where "long values" could be written to, if the `DiagArgValue` is too big
+ /// for displaying on the terminal. This path comes from the `Diag` itself. When rendering
+ /// values that come from `TyCtxt`, like `Ty<'_>`, they can use `TyCtxt::short_string`. If a
+ /// value has no shortening logic that could be used, the argument can be safely ignored.
fn into_diag_arg(self, path: &mut Option<std::path::PathBuf>) -> DiagArgValue;
}
