From 7d32407932c9e9d031a948da9471bc6f70116a73 Mon Sep 17 00:00:00 2001
From: Jan Teske <jteske@posteo.net>
Date: Wed, 14 Jun 2023 15:05:12 +0200
Subject: [PATCH] ore/metrics: implement `Borrow` for drop wrapper types

This commit adds `Borrow` implementations to the wrapper types
`DeleteOnDropHistogram`, `DeleteOnDropCounter`, and `DeleteOnDropGauge`.

This makes it possible to write code that is generic over both the
prometheus type and the wrapped type. The existing `Deref` impl does not
work for this.
---
 src/ore/src/metrics/delete_on_drop.rs | 42 +++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/src/ore/src/metrics/delete_on_drop.rs b/src/ore/src/metrics/delete_on_drop.rs
index 6f7fb99cdbe7d..fd2cf9f92aa09 100644
--- a/src/ore/src/metrics/delete_on_drop.rs
+++ b/src/ore/src/metrics/delete_on_drop.rs
@@ -27,6 +27,7 @@
 //! * When using owned data (an extension over what Prometheus allows, which only lets you use
 //!   references to refer to labels), the created metric is also allowed to live for `'static`.
 
+use std::borrow::Borrow;
 use std::collections::BTreeMap;
 use std::marker::PhantomData;
 use std::ops::Deref;
@@ -149,6 +150,10 @@ impl<'a> PromLabelsExt<'a> for BTreeMap<&'a str, &'a str> {
 ///
 /// It adds a method to create a concrete metric from the vector that gets removed from the vector
 /// when the concrete metric is dropped.
+///
+/// NOTE: This type implements [`Borrow`], which imposes some constraints on implementers. To
+/// ensure these constraints, do *not* implement any of the `Eq`, `Ord`, or `Hash` traits on this.
+/// type.
 #[derive(Debug)]
 pub struct DeleteOnDropHistogram<'a, L>
 where
@@ -185,6 +190,15 @@ where
     }
 }
 
+impl<'a, L> Borrow<Histogram> for DeleteOnDropHistogram<'a, L>
+where
+    L: PromLabelsExt<'a>,
+{
+    fn borrow(&self) -> &Histogram {
+        &self.inner
+    }
+}
+
 impl<'a, L> Drop for DeleteOnDropHistogram<'a, L>
 where
     L: PromLabelsExt<'a>,
@@ -200,6 +214,10 @@ where
 ///
 /// It adds a method to create a concrete metric from the vector that gets removed from the vector
 /// when the concrete metric is dropped.
+///
+/// NOTE: This type implements [`Borrow`], which imposes some constraints on implementers. To
+/// ensure these constraints, do *not* implement any of the `Eq`, `Ord`, or `Hash` traits on this.
+/// type.
 #[derive(Debug)]
 pub struct DeleteOnDropCounter<'a, P, L>
 where
@@ -239,6 +257,16 @@ where
     }
 }
 
+impl<'a, P, L> Borrow<GenericCounter<P>> for DeleteOnDropCounter<'a, P, L>
+where
+    P: Atomic,
+    L: PromLabelsExt<'a>,
+{
+    fn borrow(&self) -> &GenericCounter<P> {
+        &self.inner
+    }
+}
+
 impl<'a, P, L> Drop for DeleteOnDropCounter<'a, P, L>
 where
     P: Atomic,
@@ -301,6 +329,10 @@ impl HistogramVecExt for HistogramVec {
 }
 
 /// A [`GenericGauge`] wrapper that deletes its labels from the vec when it is dropped
+///
+/// NOTE: This type implements [`Borrow`], which imposes some constraints on implementers. To
+/// ensure these constraints, do *not* implement any of the `Eq`, `Ord`, or `Hash` traits on this.
+/// type.
 #[derive(Debug)]
 pub struct DeleteOnDropGauge<'a, P, L>
 where
@@ -340,6 +372,16 @@ where
     }
 }
 
+impl<'a, P, L> Borrow<GenericGauge<P>> for DeleteOnDropGauge<'a, P, L>
+where
+    P: Atomic,
+    L: PromLabelsExt<'a>,
+{
+    fn borrow(&self) -> &GenericGauge<P> {
+        &self.inner
+    }
+}
+
 impl<'a, P, L> Drop for DeleteOnDropGauge<'a, P, L>
 where
     P: Atomic,