feat(stream): Give streams direct control over 'trace' output

src/combinator/debug/internals.rs

@@ -146,7 +146,7 @@ pub(crate) fn start<I: Stream>(
 
     // The debug version of `slice` might be wider, either due to rendering one byte as two nibbles or
     // escaping in strings.
-    let mut debug_slice = format!("{:#?}", input.raw());
+    let mut debug_slice = format!("{:?}", from_fn(|f| input.trace(f)));
     let (debug_slice, eof) = if let Some(debug_offset) = debug_slice
         .char_indices()
         .enumerate()
@@ -299,3 +299,29 @@ fn columns_env() -> Option<usize> {
         .ok()
         .and_then(|c| c.parse::<usize>().ok())
 }
+
+fn from_fn<F: Fn(&mut core::fmt::Formatter<'_>) -> core::fmt::Result>(f: F) -> FromFn<F> {
+    FromFn(f)
+}
+
+struct FromFn<F>(F)
+where
+    F: Fn(&mut core::fmt::Formatter<'_>) -> core::fmt::Result;
+
+impl<F> core::fmt::Debug for FromFn<F>
+where
+    F: Fn(&mut core::fmt::Formatter<'_>) -> core::fmt::Result,
+{
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        (self.0)(f)
+    }
+}
+
+impl<F> core::fmt::Display for FromFn<F>
+where
+    F: Fn(&mut core::fmt::Formatter<'_>) -> core::fmt::Result,
+{
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        (self.0)(f)
+    }
+}

src/stream/mod.rs

@@ -693,6 +693,11 @@ pub trait Stream: Offset<<Self as Stream>::Checkpoint> + crate::lib::std::fmt::D
 
     /// Return the inner-most stream
     fn raw(&self) -> &dyn crate::lib::std::fmt::Debug;
+
+    /// Write out a single-line summary of the current parse location
+    fn trace(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        write!(f, "{:#?}", self.raw())
+    }
 }
 
 impl<'i, T> Stream for &'i [T]