Allow into/from inner Arc conversions

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## 0.13.4 (unreleased)
+
+- Add: `into_arc_value()`/`from_arc_value()` unsafe methods to `GenericCounter` and `GenericGauge` (#472)
+- Add: `into_arc_core()`/`from_arc_core()` methods to `Histogram` (#472)
+- Improvement: Mark `GenericCounter`, `GenericGauge` and `Histogram` as `#[repr(transparent)]` (#472)
+
 ## 0.13.3
 
 - Bug fix: Prevent ProcessCollector underflow with CPU time counter (#465)

src/counter.rs

@@ -16,6 +16,7 @@ use crate::vec::{MetricVec, MetricVecBuilder};
 
 /// The underlying implementation for [`Counter`] and [`IntCounter`].
 #[derive(Debug)]
+#[repr(transparent)]
 pub struct GenericCounter<P: Atomic> {
     v: Arc<Value<P>>,
 }
@@ -85,6 +86,36 @@ impl<P: Atomic> GenericCounter<P> {
     pub fn local(&self) -> GenericLocalCounter<P> {
         GenericLocalCounter::new(self.clone())
     }
+
+    /// Unwraps this counter into the inner [`Value`] representing it.
+    ///
+    /// # Safety
+    ///
+    /// This function is `unsafe`, because allows to bypass counter's semantics,
+    /// and so, the improper use of the returned [`Value`] may lead to this
+    /// counter behaving incorrectly.
+    ///
+    /// The caller must ensure that the returned [`Value`] will be used in
+    /// accordance with the counter's semantics.
+    #[inline]
+    pub unsafe fn into_arc_value(this: Self) -> Arc<Value<P>> {
+        this.v
+    }
+
+    /// Wraps the provided [`Value`] into a counter.
+    ///
+    /// # Safety
+    ///
+    /// This function is `unsafe`, because allows to bypass counter's semantics,
+    /// and so, specifying the improper [`Value`] may lead to the counter
+    /// behaving incorrectly.
+    ///
+    /// The caller must ensure that the provided [`Value`] withholds counter's
+    /// semantics.
+    #[inline]
+    pub unsafe fn from_arc_value(v: impl Into<Arc<Value<P>>>) -> Self {
+        Self { v: v.into() }
+    }
 }
 
 impl<P: Atomic> Collector for GenericCounter<P> {

src/gauge.rs

@@ -14,6 +14,7 @@ use crate::vec::{MetricVec, MetricVecBuilder};
 
 /// The underlying implementation for [`Gauge`] and [`IntGauge`].
 #[derive(Debug)]
+#[repr(transparent)]
 pub struct GenericGauge<P: Atomic> {
     v: Arc<Value<P>>,
 }
@@ -88,6 +89,36 @@ impl<P: Atomic> GenericGauge<P> {
     pub fn get(&self) -> P::T {
         self.v.get()
     }
+
+    /// Unwraps this gauge into the inner [`Value`] representing it.
+    ///
+    /// # Safety
+    ///
+    /// This function is `unsafe`, because allows to bypass gauge's semantics,
+    /// and so, the improper use of the returned [`Value`] may lead to this
+    /// gauge behaving incorrectly.
+    ///
+    /// The caller must ensure that the returned [`Value`] will be used in
+    /// accordance with the gauge's semantics.
+    #[inline]
+    pub unsafe fn into_arc_value(this: Self) -> Arc<Value<P>> {
+        this.v
+    }
+
+    /// Wraps the provided [`Value`] into a gauge.
+    ///
+    /// # Safety
+    ///
+    /// This function is `unsafe`, because allows to bypass gauge's semantics,
+    /// and so, specifying the improper [`Value`] may lead to the gauge behaving
+    /// incorrectly.
+    ///
+    /// The caller must ensure that the provided [`Value`] withholds gauge's
+    /// semantics.
+    #[inline]
+    pub unsafe fn from_arc_value(v: impl Into<Arc<Value<P>>>) -> Self {
+        Self { v: v.into() }
+    }
 }
 
 impl<P: Atomic> Collector for GenericGauge<P> {

src/histogram.rs

@@ -667,6 +667,7 @@ impl Drop for HistogramTimer {
 /// [1]: https://prometheus.io/docs/prometheus/latest/querying/functions/#histogram_quantile
 /// [2]: https://prometheus.io/docs/practices/histograms/
 #[derive(Clone, Debug)]
+#[repr(transparent)]
 pub struct Histogram {
     core: Arc<HistogramCore>,
 }
@@ -746,6 +747,18 @@ impl Histogram {
     pub fn get_sample_count(&self) -> u64 {
         self.core.sample_count()
     }
+
+    /// Unwraps this [`Histogram`] into the inner [`HistogramCore`].
+    #[inline]
+    pub fn into_arc_core(this: Self) -> Arc<HistogramCore> {
+        this.core
+    }
+
+    /// Wraps the provided [`HistogramCore`] into a [`Histogram`].
+    #[inline]
+    pub fn from_arc_core(core: impl Into<Arc<HistogramCore>>) -> Self {
+        Self { core: core.into() }
+    }
 }
 
 impl Metric for Histogram {