feat: add Span::into_scope()

A way to conveniently auto-drop a span after executing a closure.

gix-trace/src/disabled.rs

@@ -1,12 +1,13 @@
 /// A workaround for a clippy warning
 #[doc(hidden)]
-pub struct Unit;
+#[derive(Clone)]
+pub struct Span;
 
 /// A macro to create a span.
 #[macro_export]
 macro_rules! span {
     (target: $target:expr, $lvl:expr, $name:expr, $($fields:tt)*) => {
-        $crate::Unit
+        $crate::Span
     };
     (target: $target:expr, $lvl:expr, $name:expr) => {
         $crate::span!(target: $target, $lvl, $name,)

gix-trace/src/enabled.rs

@@ -4,6 +4,7 @@ use tracing_core::{dispatcher::get_default as with_dispatcher, span::Id, Dispatc
 pub use tracing_core::{field, metadata, Metadata};
 
 /// An entered span which will exit on drop.
+#[derive(Clone)]
 pub struct Span {
     id: Option<(Id, Dispatch)>,
 }

gix-trace/src/lib.rs

@@ -37,14 +37,21 @@ mod enabled;
 #[cfg(feature = "tracing")]
 pub use enabled::Span;
 
+impl Span {
+    /// Execute `f` in with this span active, consuming it.
+    pub fn into_scope<T>(self, f: impl FnOnce() -> T) -> T {
+        f()
+    }
+}
+
 #[cfg(feature = "tracing")]
 #[doc(hidden)]
 pub use enabled::{field, metadata, MetaOnlyCallsite, Metadata};
 
 #[cfg(not(feature = "tracing"))]
 mod disabled;
 #[cfg(not(feature = "tracing"))]
-pub use disabled::Unit;
+pub use disabled::Span;
 
 /// Create a new [coarse][Level::Coarse] span.
 #[macro_export]

gix-trace/tests/trace.rs

@@ -2,7 +2,8 @@ use gix_trace::{coarse, detail, span};
 #[test]
 fn span() {
     let _x = span!(gix_trace::Level::Coarse, "hello");
-    span!(gix_trace::Level::Coarse, "hello", x = "value", y = 42);
+    let fourty_two = span!(gix_trace::Level::Coarse, "hello", x = "value", y = 42).into_scope(|| 42);
+    assert_eq!(fourty_two, 42);
     span!(target: "other", gix_trace::Level::Coarse, "hello", x = "value", y = 42);
 }