Make dependency tracking a compile time setting

src/reporting.rs

@@ -0,0 +1,64 @@
+//! Cycle reporting primitives
+//!
+//! This module exposes [`Dep`], which resolves to either something that tracks dependencies or to
+//! something that doesn't.  It should only be assumed to implement the [`Reportable`] trait.
+use std::backtrace::Backtrace;
+use std::borrow::Cow;
+use std::fmt::Write;
+use std::sync::Arc;
+
+#[cfg(feature = "backtraces")]
+pub type Dep = MutexDep<Arc<Backtrace>>;
+#[cfg(not(feature = "backtraces"))]
+pub type Dep = MutexDep<()>;
+
+// Base message to be reported when cycle is detected
+const BASE_MESSAGE: &str = "Found cycle in mutex dependency graph:";
+
+pub trait Reportable: Clone {
+    /// Capture the current state
+    fn capture() -> Self;
+
+    /// Format a trace of state for human readable consumption.
+    fn panic_message(trace: &[Self]) -> Cow<'static, str>;
+}
+
+#[derive(Clone)]
+pub struct MutexDep<T>(T);
+
+/// Use a unit as tracing data: no tracing.
+///
+/// This should have no runtime overhead for capturing traces and should therefore be cheap enough
+/// for most purposes.
+impl Reportable for MutexDep<()> {
+    fn capture() -> Self {
+        Self(())
+    }
+
+    fn panic_message(_trace: &[Self]) -> Cow<'static, str> {
+        Cow::Borrowed(BASE_MESSAGE)
+    }
+}
+
+/// Use a full backtrace as tracing data
+///
+/// Capture the entire backtrace which may be expensive. This implementation does not force capture
+/// in the event that backtraces are disabled at runtime, so the exact overhead can still be
+/// controlled a little.
+///
+/// N.B. the [`Backtrace`] needs to be wrapped in an Arc as backtraces are not [`Clone`].
+impl Reportable for MutexDep<Arc<Backtrace>> {
+    fn capture() -> Self {
+        Self(Arc::new(Backtrace::capture()))
+    }
+
+    fn panic_message(trace: &[Self]) -> Cow<'static, str> {
+        let mut message = format!("{BASE_MESSAGE}\n");
+
+        for entry in trace {
+            let _ = writeln!(message, "{}", entry.0);
+        }
+
+        message.into()
+    }
+}