feat(trace): add allocation-free to_hex() for TraceId and SpanId by BRGOVIND · Pull Request #3557 · open-telemetry/opentelemetry-rust

BRGOVIND · 2026-06-14T23:47:48Z

Closes #3544 — adds allocation-free hex encoding methods for TraceId and SpanId.

Problem

Exporters that need the hex representation of a trace/span ID today call .to_string(), which heap-allocates a String on every export event. On hot paths this is two unnecessary allocations per span record.

Solution

This PR adds:

HexEncode<N> — a stack-allocated [u8; N] newtype that:
- implements Deref<Target = str>
- implements Display and Debug
- carries zero heap state
TraceId::to_hex() -> HexEncode<32> — 32-char lowercase hex, no allocation
SpanId::to_hex() -> HexEncode<16> — 16-char lowercase hex, no allocation

HexEncode is re-exported from the crate root alongside TraceId/SpanId.

Usage

Before:

let s: String = trace_id.to_string();   // heap allocation

After:

let hex = trace_id.to_hex();   // stack only
write_field(&*hex);             // deref to &str, zero-copy

Exporters can switch from format!("{}", trace_id) to trace_id.to_hex() and eliminate both allocations per record.

Exporters that need a hex string for trace/span IDs currently have to call .to_string() which heap-allocates a String on every export. On hot paths this is two unnecessary allocations per record. This commit adds: - HexEncode<N> — a stack-allocated buffer of N ASCII bytes that derefs to &str and implements Display/Debug. No heap allocation, no unsafe visible to callers. - TraceId::to_hex() -> HexEncode<32> — 32-char lowercase hex, no alloc. - SpanId::to_hex() -> HexEncode<16> — 16-char lowercase hex, no alloc. HexEncode is re-exported from the crate root alongside TraceId/SpanId. Exporters that currently write: format!("{}", trace_id) can switch to: let hex = trace_id.to_hex(); write_field(&*hex) and avoid the allocation entirely. Closes open-telemetry#3544 Signed-off-by: B R GOVIND <brgovind2005@gmail.com>

linux-foundation-easycla · 2026-06-14T23:47:55Z

The committers listed above are authorized under a signed CLA.

✅ login: BRGOVIND / name: B R GOVIND (c5eac18)

codecov · 2026-06-14T23:50:45Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.9%. Comparing base (c14b570) to head (a8286c5).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3557   +/-   ##
=====================================
  Coverage   82.9%   82.9%           
=====================================
  Files        130     130           
  Lines      27484   27557   +73     
=====================================
+ Hits       22796   22869   +73     
  Misses      4688    4688

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Cover HexEncode<N>: as_str(), Deref to &str, Display, Debug, Clone, PartialEq, and the all-bytes encoding correctness. Also test the INVALID sentinel values for both types. Signed-off-by: B R GOVIND <brgovind2005@gmail.com>

BRGOVIND · 2026-06-15T12:24:22Z

Added tests in the latest commit to cover the new code paths (fixes the Codecov patch coverage):

TraceId::to_hex() and SpanId::to_hex() output matches Display for all test vectors
HexEncode deref to &str, Clone, PartialEq, Display, Debug
INVALID sentinel values for both types produce the expected all-zero hex strings
test_to_hex_all_bytes: round-trips all 256 byte values through the encoder and compares against format!("{}", id)

BRGOVIND · 2026-06-27T04:16:28Z

Gentle ping — this adds an allocation-free to_hex() for TraceId/SpanId. Would appreciate a maintainer's eyes when there's time; happy to add benchmarks or adjust the API surface if you'd prefer a different shape.

github-actions · 2026-07-12T04:19:14Z

Thank you for your contribution! This PR has been automatically marked as stale because it has not had activity in the last 14 days. This may be due to a delay in review on our side or awaiting a response from you; either is fine, and we appreciate your patience.

It will be closed in 14 days if no further activity occurs. Pushing a new commit or leaving a comment will remove the stale label and keep the PR open.

BRGOVIND · 2026-07-12T04:24:04Z

Thank you for your contribution! This PR has been automatically marked as stale because it has not had activity in the last 14 days. This may be due to a delay in review on our side or awaiting a response from you; either is fine, and we appreciate your patience.

It will be closed in 14 days if no further activity occurs. Pushing a new commit or leaving a comment will remove the stale label and keep the PR open.

Maintainers can u please take a look at the pr ?

cijothomas · 2026-07-13T16:34:12Z

Thank you for your contribution! This PR has been automatically marked as stale because it has not had activity in the last 14 days. This may be due to a delay in review on our side or awaiting a response from you; either is fine, and we appreciate your patience.
It will be closed in 14 days if no further activity occurs. Pushing a new commit or leaving a comment will remove the stale label and keep the PR open.

Maintainers can u please take a look at the pr ?

I'll review today

Copilot

Pull request overview

Adds allocation-free, stack-based lowercase hex encoding for TraceId and SpanId to avoid per-export heap allocations, along with a reusable HexEncode<N> wrapper type.

Changes:

Introduces HexEncode<const N: usize> (stack [u8; N]) that derefs to str and implements Display/Debug.
Adds TraceId::to_hex() -> HexEncode<32> and SpanId::to_hex() -> HexEncode<16> using a shared encode_hex_bytes helper.
Re-exports HexEncode from the crate root and adds tests validating formatting behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
opentelemetry/src/trace_context.rs	Adds `HexEncode`, `to_hex()` APIs, encoding helper, and new unit tests.
opentelemetry/src/lib.rs	Re-exports `HexEncode` alongside existing trace ID types.

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+    /// Return the hex string as a `&str`.
+    pub fn as_str(&self) -> &str {
+        // SAFETY: the buffer is always filled with ASCII hex digits.
+        unsafe { std::str::from_utf8_unchecked(&self.0) }
+    }


+    #[test]
+    fn test_to_hex_all_bytes() {
+        // Verify every possible byte value encodes correctly.
+        let mut bytes = [0u8; 16];
+        for i in 0u8..=255 {
+            bytes[i as usize % 16] = i;
+        }
+        let id = TraceId::from_bytes(bytes);
+        let hex = id.to_hex();
+        assert_eq!(hex.as_str(), format!("{}", id));
+        assert_eq!(hex.len(), 32);
+    }


 mod trace_context;
-pub use trace_context::{SpanId, TraceFlags, TraceId};
+pub use trace_context::{HexEncode, SpanId, TraceFlags, TraceId};


BRGOVIND requested a review from a team as a code owner June 14, 2026 23:47

BRGOVIND changed the title ~~trace: add allocation-free to_hex() for TraceId and SpanId~~ feat(trace): add allocation-free to_hex() for TraceId and SpanId Jun 15, 2026

github-actions Bot added the Stale label Jul 12, 2026

github-actions Bot removed the Stale label Jul 13, 2026

cijothomas requested review from cijothomas and Copilot July 13, 2026 16:33

Copilot started reviewing on behalf of cijothomas July 13, 2026 16:34 View session

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feat(trace): add allocation-free to_hex() for TraceId and SpanId#3557

feat(trace): add allocation-free to_hex() for TraceId and SpanId#3557
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BRGOVIND commented Jun 14, 2026

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