Prefer use of const to static where possible (#1270)

- Changed NOOP_SPAN from a lazy static to a const
- Remove branch in build_with_context for the noop tracer.
  The current context's span() method already returns &NOOP_SPAN in the
  case when no active span is present, which already has the right span
  context value to use.
- Changed other constituents of span context to use const default values too.

Author: shaun-cox

opentelemetry/CHANGELOG.md

@@ -6,6 +6,8 @@
 
 - Bump MSRV to 1.64 [#1203](https://github.com/open-telemetry/opentelemetry-rust/pull/1203)
 - `opentelemetry` crate now only carries the API types #1186. Use the `opentelemetry_sdk` crate for the SDK types. 
+- `trace::noop::NoopSpan` no longer implements `Default` and instead exposes
+  a `const DEFAULT` value. [#1270](https://github.com/open-telemetry/opentelemetry-rust/pull/1270)
 
 ## [v0.20.0](https://github.com/open-telemetry/opentelemetry-rust/compare/v0.19.0...v0.20.0)
 This release should been seen as 1.0-rc3 following 1.0-rc2 in v0.19.0. Refer to CHANGELOG.md in individual creates for details on changes made in different creates.

opentelemetry/src/trace/context.rs

@@ -6,7 +6,6 @@ use crate::{
 };
 use futures_core::stream::Stream;
 use futures_sink::Sink;
-use once_cell::sync::Lazy;
 use pin_project_lite::pin_project;
 use std::{
     borrow::Cow,
@@ -16,10 +15,10 @@ use std::{
     task::{Context as TaskContext, Poll},
 };
 
-static NOOP_SPAN: Lazy<SynchronizedSpan> = Lazy::new(|| SynchronizedSpan {
-    span_context: SpanContext::empty_context(),
+const NOOP_SPAN: SynchronizedSpan = SynchronizedSpan {
+    span_context: SpanContext::NONE,
     inner: None,
-});
+};
 
 /// A reference to the currently active span in this context.
 #[derive(Debug)]

opentelemetry/src/trace/noop.rs

@@ -5,7 +5,7 @@
 //! to have minimal resource utilization and runtime impact.
 use crate::{
     propagation::{text_map_propagator::FieldIter, Extractor, Injector, TextMapPropagator},
-    trace::{self, TraceContextExt, TraceFlags, TraceState},
+    trace::{self, TraceContextExt as _},
     Context, InstrumentationLibrary, KeyValue,
 };
 use std::{borrow::Cow, sync::Arc, time::SystemTime};
@@ -38,25 +38,11 @@ pub struct NoopSpan {
     span_context: trace::SpanContext,
 }
 
-impl Default for NoopSpan {
-    fn default() -> Self {
-        NoopSpan::new()
-    }
-}
-
 impl NoopSpan {
-    /// Creates a new `NoopSpan` instance.
-    pub fn new() -> Self {
-        NoopSpan {
-            span_context: trace::SpanContext::new(
-                trace::TraceId::INVALID,
-                trace::SpanId::INVALID,
-                TraceFlags::default(),
-                false,
-                TraceState::default(),
-            ),
-        }
-    }
+    /// The default `NoopSpan`, as a constant
+    pub const DEFAULT: NoopSpan = NoopSpan {
+        span_context: trace::SpanContext::NONE,
+    };
 }
 
 impl trace::Span for NoopSpan {
@@ -135,12 +121,8 @@ impl trace::Tracer for NoopTracer {
     /// If the span builder or the context's current span contains a valid span context, it is
     /// propagated.
     fn build_with_context(&self, _builder: trace::SpanBuilder, parent_cx: &Context) -> Self::Span {
-        if parent_cx.has_active_span() {
-            NoopSpan {
-                span_context: parent_cx.span().span_context().clone(),
-            }
-        } else {
-            NoopSpan::new()
+        NoopSpan {
+            span_context: parent_cx.span().span_context().clone(),
         }
     }
 }
@@ -178,7 +160,7 @@ impl TextMapPropagator for NoopTextMapPropagator {
 mod tests {
     use super::*;
     use crate::testing::trace::TestSpan;
-    use crate::trace::{self, Span, Tracer};
+    use crate::trace::{self, Span, TraceState, Tracer};
 
     fn valid_span_context() -> trace::SpanContext {
         trace::SpanContext::new(

opentelemetry/src/trace/span_context.rs

@@ -20,6 +20,14 @@ use thiserror::Error;
 pub struct TraceFlags(u8);
 
 impl TraceFlags {
+    /// Trace flags with the `sampled` flag set to `0`.
+    ///
+    /// Spans that are not sampled will be ignored by most tracing tools.
+    /// See the `sampled` section of the [W3C TraceContext specification] for details.
+    ///
+    /// [W3C TraceContext specification]: https://www.w3.org/TR/trace-context/#sampled-flag
+    pub const NOT_SAMPLED: TraceFlags = TraceFlags(0x00);
+
     /// Trace flags with the `sampled` flag set to `1`.
     ///
     /// Spans that are not sampled will be ignored by most tracing tools.
@@ -216,6 +224,9 @@ impl fmt::LowerHex for SpanId {
 pub struct TraceState(Option<VecDeque<(String, String)>>);
 
 impl TraceState {
+    /// The default `TraceState`, as a constant
+    pub const NONE: TraceState = TraceState(None);
+
     /// Validates that the given `TraceState` list-member key is valid per the [W3 Spec].
     ///
     /// [W3 Spec]: https://www.w3.org/TR/trace-context/#key
@@ -457,15 +468,18 @@ pub struct SpanContext {
 }
 
 impl SpanContext {
+    /// An invalid span context
+    pub const NONE: SpanContext = SpanContext {
+        trace_id: TraceId::INVALID,
+        span_id: SpanId::INVALID,
+        trace_flags: TraceFlags::NOT_SAMPLED,
+        is_remote: false,
+        trace_state: TraceState::NONE,
+    };
+
     /// Create an invalid empty span context
     pub fn empty_context() -> Self {
-        SpanContext::new(
-            TraceId::INVALID,
-            SpanId::INVALID,
-            TraceFlags::default(),
-            false,
-            TraceState::default(),
-        )
+        SpanContext::NONE
     }
 
     /// Construct a new `SpanContext`