1 files changed, 91 insertions, 0 deletions
diff --git a/src/combinator/debug/mod.rs b/src/combinator/debug/mod.rs
new file mode 100644
index 0000000..ee4c293
--- /dev/null
+++ b/src/combinator/debug/mod.rs
@@ -0,0 +1,91 @@
+#![cfg_attr(feature = "debug", allow(clippy::std_instead_of_core))]
+
+#[cfg(feature = "debug")]
+mod internals;
+
+use crate::error::ErrMode;
+use crate::stream::Stream;
+use crate::Parser;
+
+#[cfg(all(feature = "debug", not(feature = "std")))]
+compile_error!("`debug` requires `std`");
+
+/// Trace the execution of the parser
+///
+/// Note that [`Parser::context`] also provides high level trace information.
+///
+/// See [tutorial][crate::_tutorial::chapter_8] for more details.
+///
+/// # Example
+///
+/// ```rust
+/// # use winnow::{error::ErrMode, error::{InputError, ErrorKind}, error::Needed};
+/// # use winnow::token::take_while;
+/// # use winnow::stream::AsChar;
+/// # use winnow::prelude::*;
+/// use winnow::combinator::trace;
+///
+/// fn short_alpha<'s>(s: &mut &'s [u8]) -> PResult<&'s [u8], InputError<&'s [u8]>> {
+///   trace("short_alpha",
+///     take_while(3..=6, AsChar::is_alpha)
+///   ).parse_next(s)
+/// }
+///
+/// assert_eq!(short_alpha.parse_peek(b"latin123"), Ok((&b"123"[..], &b"latin"[..])));
+/// assert_eq!(short_alpha.parse_peek(b"lengthy"), Ok((&b"y"[..], &b"length"[..])));
+/// assert_eq!(short_alpha.parse_peek(b"latin"), Ok((&b""[..], &b"latin"[..])));
+/// assert_eq!(short_alpha.parse_peek(b"ed"), Err(ErrMode::Backtrack(InputError::new(&b"ed"[..], ErrorKind::Slice))));
+/// assert_eq!(short_alpha.parse_peek(b"12345"), Err(ErrMode::Backtrack(InputError::new(&b"12345"[..], ErrorKind::Slice))));
+/// ```
+#[cfg_attr(not(feature = "debug"), allow(unused_variables))]
+#[cfg_attr(not(feature = "debug"), allow(unused_mut))]
+#[cfg_attr(not(feature = "debug"), inline(always))]
+pub fn trace<I: Stream, O, E>(
+    name: impl crate::lib::std::fmt::Display,
+    parser: impl Parser<I, O, E>,
+) -> impl Parser<I, O, E> {
+    #[cfg(feature = "debug")]
+    {
+        internals::Trace::new(parser, name)
+    }
+    #[cfg(not(feature = "debug"))]
+    {
+        parser
+    }
+}
+
+#[cfg_attr(not(feature = "debug"), allow(unused_variables))]
+pub(crate) fn trace_result<T, E>(
+    name: impl crate::lib::std::fmt::Display,
+    res: &Result<T, ErrMode<E>>,
+) {
+    #[cfg(feature = "debug")]
+    {
+        let depth = internals::Depth::existing();
+        let severity = internals::Severity::with_result(res);
+        internals::result(*depth, &name, severity);
+    }
+}
+
+#[test]
+#[cfg(feature = "std")]
+#[cfg_attr(miri, ignore)]
+#[cfg(unix)]
+#[cfg(feature = "debug")]
+fn example() {
+    use term_transcript::{test::TestConfig, ShellOptions};
+
+    let path = snapbox::cmd::compile_example("string", ["--features=debug"]).unwrap();
+
+    let current_dir = path.parent().unwrap();
+    let cmd = path.file_name().unwrap();
+    // HACK: term_transcript doesn't allow non-UTF8 paths
+    let cmd = format!("./{}", cmd.to_string_lossy());
+
+    TestConfig::new(
+        ShellOptions::default()
+            .with_current_dir(current_dir)
+            .with_env("CLICOLOR_FORCE", "1"),
+    )
+    .test("assets/trace.svg", [cmd.as_str()]);
+}