trace-core: API polish (#962)

This branch makes a handful of `tokio-trace-core` API improvements, mostly
around naming. In particular:

 * Rename `dispatcher::with` to `dispatcher::get_default`
 * Rename `Event::observe` to `Event::dispatch`
 * Make `field::ValidLen` trait private

Closes #948
Closes #960

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

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

@@ -52,11 +52,10 @@ pub fn with_default<T>(dispatcher: Dispatch, f: impl FnOnce() -> T) -> T {
     let _guard = ResetGuard(prior.ok());
     f()
 }
-
 /// Executes a closure with a reference to this thread's current [dispatcher].
 ///
 /// [dispatcher]: ../dispatcher/struct.Dispatch.html
-pub fn with<T, F>(mut f: F) -> T
+pub fn get_default<T, F>(mut f: F) -> T
 where
     F: FnMut(&Dispatch) -> T,
 {

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

@@ -27,9 +27,9 @@ impl<'a> Event<'a> {
     /// Constructs a new `Event` with the specified metadata and set of values,
     /// and observes it with the current subscriber.
     #[inline]
-    pub fn observe(metadata: &'a Metadata<'a>, fields: &'a field::ValueSet) {
+    pub fn dispatch(metadata: &'a Metadata<'a>, fields: &'a field::ValueSet) {
         let event = Event { metadata, fields };
-        ::dispatcher::with(|current| {
+        ::dispatcher::get_default(|current| {
             current.event(&event);
         });
     }

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

@@ -45,6 +45,8 @@ use std::{
     ops::Range,
 };
 
+use self::private::ValidLen;
+
 /// An opaque key allowing _O_(1) access to a field in a `Span`'s key-value
 /// data.
 ///
@@ -227,15 +229,6 @@ pub struct DisplayValue<T: fmt::Display>(T);
 #[derive(Debug, Clone)]
 pub struct DebugValue<T: fmt::Debug>(T);
 
-/// Marker trait implemented by arrays which are of valid length to
-/// construct a `ValueSet`.
-///
-/// `ValueSet`s may only be constructed from arrays containing 32 or fewer
-/// elements, to ensure the array is small enough to always be allocated on the
-/// stack. This trait is only implemented by arrays of an appropriate length,
-/// ensuring that the correct size arrays are used at compile-time.
-pub trait ValidLen<'a>: ::sealed::Sealed + Borrow<[(&'a Field, Option<&'a (Value + 'a)>)]> {}
-
 /// Wraps a type implementing `fmt::Display` as a `Value` that can be
 /// recorded using its `Display` implementation.
 pub fn display<T>(t: T) -> DisplayValue<T>
@@ -615,12 +608,23 @@ impl<'a> fmt::Debug for ValueSet<'a> {
 
 // ===== impl ValidLen =====
 
+mod private {
+    use super::*;
+
+    /// Marker trait implemented by arrays which are of valid length to
+    /// construct a `ValueSet`.
+    ///
+    /// `ValueSet`s may only be constructed from arrays containing 32 or fewer
+    /// elements, to ensure the array is small enough to always be allocated on the
+    /// stack. This trait is only implemented by arrays of an appropriate length,
+    /// ensuring that the correct size arrays are used at compile-time.
+    pub trait ValidLen<'a>: Borrow<[(&'a Field, Option<&'a (Value + 'a)>)]> {}
+}
+
 macro_rules! impl_valid_len {
     ( $( $len:tt ),+ ) => {
         $(
-            impl<'a> ::sealed::Sealed for
-                [(&'a Field, Option<&'a (Value + 'a)>); $len] {}
-            impl<'a> ValidLen<'a> for
+            impl<'a> private::ValidLen<'a> for
                 [(&'a Field, Option<&'a (Value + 'a)>); $len] {}
         )+
     }