subscriber: change FmtSpan to a combinable bitflag (#1277)

This backports #1277 from `master`.

Fixes #1136.

Allows arbitrarily combining different FmtSpan events to listen to.

Motivation
----------

The idea is to allow any combination of `FmtSpan` options, such as the
currently unachievable combination of `FmtSpan::NEW | FmtSpan::CLOSE`.

Solution
--------

Make `FmtSpan` behave like a bitflag that can be combined using the
bitwise or operator ( `|` ) while maintaining backward compatibility by
keeping the same names for all existing presets and keeping the
implementation details hidden.

tracing-subscriber/src/fmt/fmt_layer.rs

@@ -259,18 +259,33 @@ where
     /// - `FmtSpan::NONE`: No events will be synthesized when spans are
     ///    created, entered, exited, or closed. Data from spans will still be
     ///    included as the context for formatted events. This is the default.
-    /// - `FmtSpan::ACTIVE`: Events will be synthesized when spans are entered
-    ///    or exited.
+    /// - `FmtSpan::NEW`: An event will be synthesized when spans are created.
+    /// - `FmtSpan::ENTER`: An event will be synthesized when spans are entered.
+    /// - `FmtSpan::EXIT`: An event will be synthesized when spans are exited.
     /// - `FmtSpan::CLOSE`: An event will be synthesized when a span closes. If
     ///    [timestamps are enabled][time] for this formatter, the generated
     ///    event will contain fields with the span's _busy time_ (the total
     ///    time for which it was entered) and _idle time_ (the total time that
     ///    the span existed but was not entered).
+    /// - `FmtSpan::ACTIVE`: Events will be synthesized when spans are entered
+    ///    or exited.
     /// - `FmtSpan::FULL`: Events will be synthesized whenever a span is
     ///    created, entered, exited, or closed. If timestamps are enabled, the
     ///    close event will contain the span's busy and idle time, as
     ///    described above.
     ///
+    /// The options can be enabled in any combination. For instance, the following
+    /// will synthesize events whenever spans are created and closed:
+    ///
+    /// ```rust
+    /// use tracing_subscriber::fmt;
+    /// use tracing_subscriber::fmt::format::FmtSpan;
+    ///
+    /// let subscriber = fmt()
+    ///     .with_span_events(FmtSpan::NEW | FmtSpan::CLOSE)
+    ///     .finish();
+    /// ```
+    ///
     /// Note that the generated events will only be part of the log output by
     /// this formatter; they will not be recorded by other `Subscriber`s or by
     /// `Layer`s added to this subscriber.
@@ -627,7 +642,7 @@ where
     }
 
     fn on_enter(&self, id: &Id, ctx: Context<'_, S>) {
-        if self.fmt_span.trace_active() || self.fmt_span.trace_close() && self.fmt_span.fmt_timing {
+        if self.fmt_span.trace_enter() || self.fmt_span.trace_close() && self.fmt_span.fmt_timing {
             let span = ctx.span(id).expect("Span not found, this is a bug");
             let mut extensions = span.extensions_mut();
             if let Some(timings) = extensions.get_mut::<Timings>() {
@@ -636,7 +651,7 @@ where
                 timings.last = now;
             }
 
-            if self.fmt_span.trace_active() {
+            if self.fmt_span.trace_enter() {
                 with_event_from_span!(id, span, "message" = "enter", |event| {
                     drop(extensions);
                     drop(span);
@@ -647,7 +662,7 @@ where
     }
 
     fn on_exit(&self, id: &Id, ctx: Context<'_, S>) {
-        if self.fmt_span.trace_active() || self.fmt_span.trace_close() && self.fmt_span.fmt_timing {
+        if self.fmt_span.trace_exit() || self.fmt_span.trace_close() && self.fmt_span.fmt_timing {
             let span = ctx.span(id).expect("Span not found, this is a bug");
             let mut extensions = span.extensions_mut();
             if let Some(timings) = extensions.get_mut::<Timings>() {
@@ -656,7 +671,7 @@ where
                 timings.last = now;
             }
 
-            if self.fmt_span.trace_active() {
+            if self.fmt_span.trace_exit() {
                 with_event_from_span!(id, span, "message" = "exit", |event| {
                     drop(extensions);
                     drop(span);

tracing-subscriber/src/fmt/format/mod.rs

@@ -1088,40 +1088,89 @@ impl<'a, F> fmt::Debug for FieldFnVisitor<'a, F> {
 ///
 /// See also [`with_span_events`](../struct.SubscriberBuilder.html#method.with_span_events).
 #[derive(Clone, Eq, PartialEq, Ord, PartialOrd)]
-pub struct FmtSpan(FmtSpanInner);
+pub struct FmtSpan(u8);
 
 impl FmtSpan {
-    /// spans are ignored (this is the default)
-    pub const NONE: FmtSpan = FmtSpan(FmtSpanInner::None);
-    /// one event per enter/exit of a span
-    pub const ACTIVE: FmtSpan = FmtSpan(FmtSpanInner::Active);
+    /// one event when span is created
+    pub const NEW: FmtSpan = FmtSpan(1 << 0);
+    /// one event per enter of a span
+    pub const ENTER: FmtSpan = FmtSpan(1 << 1);
+    /// one event per exit of a span
+    pub const EXIT: FmtSpan = FmtSpan(1 << 2);
     /// one event when the span is dropped
-    pub const CLOSE: FmtSpan = FmtSpan(FmtSpanInner::Close);
-    /// events at all points (new, enter, exit, drop)
-    pub const FULL: FmtSpan = FmtSpan(FmtSpanInner::Full);
-}
+    pub const CLOSE: FmtSpan = FmtSpan(1 << 3);
 
-impl Debug for FmtSpan {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self.0 {
-            FmtSpanInner::None => f.write_str("FmtSpan::NONE"),
-            FmtSpanInner::Active => f.write_str("FmtSpan::ACTIVE"),
-            FmtSpanInner::Close => f.write_str("FmtSpan::CLOSE"),
-            FmtSpanInner::Full => f.write_str("FmtSpan::FULL"),
-        }
+    /// spans are ignored (this is the default)
+    pub const NONE: FmtSpan = FmtSpan(0);
+    /// one event per enter/exit of a span
+    pub const ACTIVE: FmtSpan = FmtSpan(FmtSpan::ENTER.0 | FmtSpan::EXIT.0);
+    /// events at all points (new, enter, exit, drop)
+    pub const FULL: FmtSpan =
+        FmtSpan(FmtSpan::NEW.0 | FmtSpan::ENTER.0 | FmtSpan::EXIT.0 | FmtSpan::CLOSE.0);
+
+    /// Check whether or not a certain flag is set for this [`FmtSpan`]
+    fn contains(&self, other: FmtSpan) -> bool {
+        self.clone() & other.clone() == other
     }
 }
 
-#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd)]
-enum FmtSpanInner {
-    /// spans are ignored (this is the default)
-    None,
-    /// one event per enter/exit of a span
-    Active,
-    /// one event when the span is dropped
-    Close,
-    /// events at all points (new, enter, exit, drop)
-    Full,
+macro_rules! impl_fmt_span_bit_op {
+    ($trait:ident, $func:ident, $op:tt) => {
+        impl std::ops::$trait for FmtSpan {
+            type Output = FmtSpan;
+
+            fn $func(self, rhs: Self) -> Self::Output {
+                FmtSpan(self.0 $op rhs.0)
+            }
+        }
+    };
+}
+
+macro_rules! impl_fmt_span_bit_assign_op {
+    ($trait:ident, $func:ident, $op:tt) => {
+        impl std::ops::$trait for FmtSpan {
+            fn $func(&mut self, rhs: Self) {
+                *self = FmtSpan(self.0 $op rhs.0)
+            }
+        }
+    };
+}
+
+impl_fmt_span_bit_op!(BitAnd, bitand, &);
+impl_fmt_span_bit_op!(BitOr, bitor, |);
+impl_fmt_span_bit_op!(BitXor, bitxor, ^);
+
+impl_fmt_span_bit_assign_op!(BitAndAssign, bitand_assign, &);
+impl_fmt_span_bit_assign_op!(BitOrAssign, bitor_assign, |);
+impl_fmt_span_bit_assign_op!(BitXorAssign, bitxor_assign, ^);
+
+impl Debug for FmtSpan {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut wrote_flag = false;
+        let mut write_flags = |flag, flag_str| -> fmt::Result {
+            if self.contains(flag) {
+                if wrote_flag {
+                    f.write_str(" | ")?;
+                }
+
+                f.write_str(flag_str)?;
+                wrote_flag = true;
+            }
+
+            Ok(())
+        };
+
+        if FmtSpan::NONE | self.clone() == FmtSpan::NONE {
+            f.write_str("FmtSpan::NONE")?;
+        } else {
+            write_flags(FmtSpan::NEW, "FmtSpan::NEW")?;
+            write_flags(FmtSpan::ENTER, "FmtSpan::ENTER")?;
+            write_flags(FmtSpan::EXIT, "FmtSpan::EXIT")?;
+            write_flags(FmtSpan::CLOSE, "FmtSpan::CLOSE")?;
+        }
+
+        Ok(())
+    }
 }
 
 pub(super) struct FmtSpanConfig {
@@ -1143,13 +1192,16 @@ impl FmtSpanConfig {
         }
     }
     pub(super) fn trace_new(&self) -> bool {
-        matches!(self.kind, FmtSpan::FULL)
+        self.kind.contains(FmtSpan::NEW)
     }
-    pub(super) fn trace_active(&self) -> bool {
-        matches!(self.kind, FmtSpan::ACTIVE | FmtSpan::FULL)
+    pub(super) fn trace_enter(&self) -> bool {
+        self.kind.contains(FmtSpan::ENTER)
+    }
+    pub(super) fn trace_exit(&self) -> bool {
+        self.kind.contains(FmtSpan::EXIT)
     }
     pub(super) fn trace_close(&self) -> bool {
-        matches!(self.kind, FmtSpan::CLOSE | FmtSpan::FULL)
+        self.kind.contains(FmtSpan::CLOSE)
     }
 }
 
@@ -1193,7 +1245,7 @@ pub(super) mod test {
     use lazy_static::lazy_static;
     use tracing::{self, subscriber::with_default};
 
-    use super::TimingDisplay;
+    use super::{FmtSpan, TimingDisplay};
     use std::{fmt, sync::Mutex};
 
     pub(crate) struct MockTime;
@@ -1380,4 +1432,31 @@ pub(super) mod test {
         assert_eq!(fmt(123456789012), "123s");
         assert_eq!(fmt(1234567890123), "1235s");
     }
+
+    #[test]
+    fn fmt_span_combinations() {
+        let f = FmtSpan::NONE;
+        assert_eq!(f.contains(FmtSpan::NEW), false);
+        assert_eq!(f.contains(FmtSpan::ENTER), false);
+        assert_eq!(f.contains(FmtSpan::EXIT), false);
+        assert_eq!(f.contains(FmtSpan::CLOSE), false);
+
+        let f = FmtSpan::ACTIVE;
+        assert_eq!(f.contains(FmtSpan::NEW), false);
+        assert_eq!(f.contains(FmtSpan::ENTER), true);
+        assert_eq!(f.contains(FmtSpan::EXIT), true);
+        assert_eq!(f.contains(FmtSpan::CLOSE), false);
+
+        let f = FmtSpan::FULL;
+        assert_eq!(f.contains(FmtSpan::NEW), true);
+        assert_eq!(f.contains(FmtSpan::ENTER), true);
+        assert_eq!(f.contains(FmtSpan::EXIT), true);
+        assert_eq!(f.contains(FmtSpan::CLOSE), true);
+
+        let f = FmtSpan::NEW | FmtSpan::CLOSE;
+        assert_eq!(f.contains(FmtSpan::NEW), true);
+        assert_eq!(f.contains(FmtSpan::ENTER), false);
+        assert_eq!(f.contains(FmtSpan::EXIT), false);
+        assert_eq!(f.contains(FmtSpan::CLOSE), true);
+    }
 }

tracing-subscriber/src/fmt/mod.rs

@@ -623,18 +623,33 @@ where
     /// - `FmtSpan::NONE`: No events will be synthesized when spans are
     ///    created, entered, exited, or closed. Data from spans will still be
     ///    included as the context for formatted events. This is the default.
-    /// - `FmtSpan::ACTIVE`: Events will be synthesized when spans are entered
-    ///    or exited.
+    /// - `FmtSpan::NEW`: An event will be synthesized when spans are created.
+    /// - `FmtSpan::ENTER`: An event will be synthesized when spans are entered.
+    /// - `FmtSpan::EXIT`: An event will be synthesized when spans are exited.
     /// - `FmtSpan::CLOSE`: An event will be synthesized when a span closes. If
     ///    [timestamps are enabled][time] for this formatter, the generated
     ///    event will contain fields with the span's _busy time_ (the total
     ///    time for which it was entered) and _idle time_ (the total time that
     ///    the span existed but was not entered).
+    /// - `FmtSpan::ACTIVE`: An event will be synthesized when spans are entered
+    ///    or exited.
     /// - `FmtSpan::FULL`: Events will be synthesized whenever a span is
     ///    created, entered, exited, or closed. If timestamps are enabled, the
     ///    close event will contain the span's busy and idle time, as
     ///    described above.
     ///
+    /// The options can be enabled in any combination. For instance, the following
+    /// will synthesize events whenever spans are created and closed:
+    ///
+    /// ```rust
+    /// use tracing_subscriber::fmt::format::FmtSpan;
+    /// use tracing_subscriber::fmt;
+    ///
+    /// let subscriber = fmt()
+    ///     .with_span_events(FmtSpan::NEW | FmtSpan::CLOSE)
+    ///     .finish();
+    /// ```
+    ///
     /// Note that the generated events will only be part of the log output by
     /// this formatter; they will not be recorded by other `Subscriber`s or by
     /// `Layer`s added to this subscriber.