trace-core: Require span IDs to be > 0 (#973)

This branch changes `tokio_trace_core::span::Id::from_u64` to assert
that the integer from which the span ID is constructed is greater than
zero. This is to enable future use of non-zero optimization.

Unfortunately, we can't actually use a `NonZeroU64` _now_, as that type
was only stabilized in Rust 1.28.0, and `tokio`'s current minimum
supported Rust version is 1.26.0.

Adding and documenting the assertion now allows us to change the
internal representation to `NonZeroU64` later (when 1.28.0 is the
minimum supported Rust version), without causing a breaking change.

Signed-off-by: Eliza Weisman <[email protected]>

tokio-trace/tests/support/subscriber.rs

@@ -128,7 +128,7 @@ where
             spans: Mutex::new(HashMap::new()),
             expected,
             current: Mutex::new(Vec::new()),
-            ids: AtomicUsize::new(0),
+            ids: AtomicUsize::new(1),
             filter: self.filter,
         };
         (subscriber, handle)

tokio-trace/tokio-trace-core/src/span.rs

@@ -6,12 +6,13 @@ use {field, Metadata};
 ///
 /// They are generated by [`Subscriber`]s for each span as it is created, by
 /// the [`new_span`] trait method. See the documentation for that method for
-/// more information on span
-/// ID generation.
+/// more information on span ID generation.
 ///
 /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
 /// [`new_span`]: ../subscriber/trait.Subscriber.html#method.new_span
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
+// TODO(eliza): when Tokio's minimum Rust version is >= 1.28, change the
+// internal representation to a `NonZeroU64`.
 pub struct Id(u64);
 
 /// Attributes provided to a `Subscriber` describing a new span when it is
@@ -43,7 +44,13 @@ enum Parent {
 
 impl Id {
     /// Constructs a new span ID from the given `u64`.
+    ///
+    /// **Note**: Span IDs must be greater than zero.
+    ///
+    /// # Panics
+    /// - If the provided `u64` is 0
     pub fn from_u64(u: u64) -> Self {
+        assert!(u > 0);
         Id(u)
     }