From baadd90ae6502753bebdc330a160fa6364d43715 Mon Sep 17 00:00:00 2001
From: Tyler Levine <tyler.levine@falconx.io>
Date: Mon, 8 Jan 2024 13:04:43 -0800
Subject: [PATCH] refactor(encoding): replace `EncodeLabelSet` impl for `()`
 with private uninhabited type

Signed-off-by: Tyler Levine <tyler.levine@falconx.io>
---
 src/encoding.rs          |  5 ++++-
 src/encoding/text.rs     | 32 ++++++--------------------------
 src/metrics/counter.rs   |  6 +++---
 src/metrics/histogram.rs |  4 ++--
 4 files changed, 15 insertions(+), 32 deletions(-)

diff --git a/src/encoding.rs b/src/encoding.rs
index a8ad8615..2e122484 100644
--- a/src/encoding.rs
+++ b/src/encoding.rs
@@ -244,6 +244,9 @@ pub trait EncodeLabel {
 #[derive(Debug)]
 pub struct LabelEncoder<'a>(LabelEncoderInner<'a>);
 
+/// Uninhabited type to represent the lack of a label set for a metric
+pub(crate) enum NoLabelSet {}
+
 #[derive(Debug)]
 enum LabelEncoderInner<'a> {
     Text(text::LabelEncoder<'a>),
@@ -349,7 +352,7 @@ impl<T: EncodeLabel> EncodeLabelSet for Vec<T> {
     }
 }
 
-impl EncodeLabelSet for () {
+impl EncodeLabelSet for NoLabelSet {
     fn encode(&self, _encoder: LabelSetEncoder) -> Result<(), std::fmt::Error> {
         Ok(())
     }
diff --git a/src/encoding/text.rs b/src/encoding/text.rs
index 26b3cd5d..5fb8c226 100644
--- a/src/encoding/text.rs
+++ b/src/encoding/text.rs
@@ -24,7 +24,7 @@
 //! assert_eq!(expected, buffer);
 //! ```
 
-use crate::encoding::{EncodeExemplarValue, EncodeLabelSet};
+use crate::encoding::{EncodeExemplarValue, EncodeLabelSet, NoLabelSet};
 use crate::metrics::exemplar::Exemplar;
 use crate::metrics::MetricType;
 use crate::registry::{Prefix, Registry, Unit};
@@ -186,7 +186,7 @@ impl<'a> MetricEncoder<'a> {
 
         self.write_suffix("total")?;
 
-        self.encode_labels::<()>(None)?;
+        self.encode_labels::<NoLabelSet>(None)?;
 
         v.encode(
             &mut CounterValueEncoder {
@@ -210,7 +210,7 @@ impl<'a> MetricEncoder<'a> {
     ) -> Result<(), std::fmt::Error> {
         self.write_prefix_name_unit()?;
 
-        self.encode_labels::<()>(None)?;
+        self.encode_labels::<NoLabelSet>(None)?;
 
         v.encode(
             &mut GaugeValueEncoder {
@@ -266,14 +266,14 @@ impl<'a> MetricEncoder<'a> {
     ) -> Result<(), std::fmt::Error> {
         self.write_prefix_name_unit()?;
         self.write_suffix("sum")?;
-        self.encode_labels::<()>(None)?;
+        self.encode_labels::<NoLabelSet>(None)?;
         self.writer.write_str(" ")?;
         self.writer.write_str(dtoa::Buffer::new().format(sum))?;
         self.newline()?;
 
         self.write_prefix_name_unit()?;
         self.write_suffix("count")?;
-        self.encode_labels::<()>(None)?;
+        self.encode_labels::<NoLabelSet>(None)?;
         self.writer.write_str(" ")?;
         self.writer.write_str(itoa::Buffer::new().format(count))?;
         self.newline()?;
@@ -747,24 +747,6 @@ mod tests {
         parse_with_python_client(encoded);
     }
 
-    #[test]
-    fn encode_histogram_family_with_empty_family_labels() {
-        let mut registry = Registry::default();
-        let family =
-            Family::new_with_constructor(|| Histogram::new(exponential_buckets(1.0, 2.0, 10)));
-        registry.register("my_histogram", "My histogram", family.clone());
-
-        family
-            .get_or_create(&())
-            .observe(1.0);
-
-        let mut encoded = String::new();
-
-        encode(&mut encoded, &registry).unwrap();
-
-        parse_with_python_client(encoded);
-    }
-
     #[test]
     fn encode_histogram_family_with_empty_struct_family_labels() {
         let mut registry = Registry::default();
@@ -781,9 +763,7 @@ mod tests {
             }
         }
 
-        family
-            .get_or_create(&EmptyLabels {})
-            .observe(1.0);
+        family.get_or_create(&EmptyLabels {}).observe(1.0);
 
         let mut encoded = String::new();
 
diff --git a/src/metrics/counter.rs b/src/metrics/counter.rs
index c1c5e511..dff710c7 100644
--- a/src/metrics/counter.rs
+++ b/src/metrics/counter.rs
@@ -2,7 +2,7 @@
 //!
 //! See [`Counter`] for details.
 
-use crate::encoding::{EncodeMetric, MetricEncoder};
+use crate::encoding::{EncodeMetric, MetricEncoder, NoLabelSet};
 
 use super::{MetricType, TypedMetric};
 use std::marker::PhantomData;
@@ -179,7 +179,7 @@ where
     A: Atomic<N>,
 {
     fn encode(&self, mut encoder: MetricEncoder) -> Result<(), std::fmt::Error> {
-        encoder.encode_counter::<(), _, u64>(&self.get(), None)
+        encoder.encode_counter::<NoLabelSet, _, u64>(&self.get(), None)
     }
 
     fn metric_type(&self) -> MetricType {
@@ -211,7 +211,7 @@ where
     N: crate::encoding::EncodeCounterValue,
 {
     fn encode(&self, mut encoder: MetricEncoder) -> Result<(), std::fmt::Error> {
-        encoder.encode_counter::<(), _, u64>(&self.value, None)
+        encoder.encode_counter::<NoLabelSet, _, u64>(&self.value, None)
     }
 
     fn metric_type(&self) -> MetricType {
diff --git a/src/metrics/histogram.rs b/src/metrics/histogram.rs
index 66f4496d..1e418ad9 100644
--- a/src/metrics/histogram.rs
+++ b/src/metrics/histogram.rs
@@ -2,7 +2,7 @@
 //!
 //! See [`Histogram`] for details.
 
-use crate::encoding::{EncodeMetric, MetricEncoder};
+use crate::encoding::{EncodeMetric, MetricEncoder, NoLabelSet};
 
 use super::{MetricType, TypedMetric};
 use parking_lot::{MappedRwLockReadGuard, RwLock, RwLockReadGuard};
@@ -133,7 +133,7 @@ pub fn linear_buckets(start: f64, width: f64, length: u16) -> impl Iterator<Item
 impl EncodeMetric for Histogram {
     fn encode(&self, mut encoder: MetricEncoder) -> Result<(), std::fmt::Error> {
         let (sum, count, buckets) = self.get();
-        encoder.encode_histogram::<()>(sum, count, &buckets, None)
+        encoder.encode_histogram::<NoLabelSet>(sum, count, &buckets, None)
     }
 
     fn metric_type(&self) -> MetricType {