refactor(encoding): remove as casts (#281)

There was a bug in the `impl EncodeGaugeValue for u64` implementation
due to the `as i64` cast.

The implementation only checked if `self == u64::MAX`, this is wrong
since `u64::MAX` is not the only value that cannot be represented by an
`i64`.

The range of values is actually `self > i64::MAX`. `u64::MAX == 2^64 -
1` whereas `i64::MAX == 2^63 - 1`, this means there are `2^63` `u64`
values that are not representable by an `i64`, not just `i64::MAX`.
Delegating the checks to `i64::try_from(self)` ensures the logic is
right. For this reason I also switched the rest of the implementations
to use `N::from` instead of `as N` since the `N::from` function is only
implemented for types whose conversion is infallible, this guarantees no
such issues will ever happen.

The only `as` cast left is `u64 as f64` in `EncodeExemplarValue`, this
is not possible to remove since there is no `impl TryFrom<u64> for f64`.

Signed-off-by: Jalil David Salamé Messina <[email protected]>

Author: jalil-salame

CHANGELOG.md

@@ -4,6 +4,17 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.24.1]
+
+### Fixed
+
+- `EncodeGaugeValue`, `EncodeCounterValue` and `EncodeExemplarValue` now use
+  fewer `as` casts in their implementation. This caught an issue where
+  `EncodeGaugeValue` would not error when encoding some `u64`s that don't fit
+  in a `i64`. See [PR 281].
+
+[PR 281]: https://github.com/prometheus/client_rust/pull/281
+
 ## [0.24.0]
 
 ### Added

src/encoding.rs

@@ -601,13 +601,9 @@ impl EncodeGaugeValue for i64 {
 impl EncodeGaugeValue for u64 {
     fn encode(&self, encoder: &mut GaugeValueEncoder) -> Result<(), std::fmt::Error> {
         // Between forcing end users to do endless as i64 for things that are
-        // clearly u64 and having one error case for rarely used protobuf when
-        // a gauge is set to u64::MAX, the latter seems like the right choice.
-        if *self == u64::MAX {
-            return Err(std::fmt::Error);
-        }
-
-        encoder.encode_i64(*self as i64)
+        // clearly valid i64 and having one error case for rarely used protobuf when
+        // a gauge is set to >i64::MAX, the latter seems like the right choice.
+        encoder.encode_i64(i64::try_from(*self).map_err(|_err| std::fmt::Error)?)
     }
 }
 
@@ -619,13 +615,13 @@ impl EncodeGaugeValue for f64 {
 
 impl EncodeGaugeValue for i32 {
     fn encode(&self, encoder: &mut GaugeValueEncoder) -> Result<(), std::fmt::Error> {
-        encoder.encode_i64(*self as i64)
+        encoder.encode_i64(i64::from(*self))
     }
 }
 
 impl EncodeGaugeValue for f32 {
     fn encode(&self, encoder: &mut GaugeValueEncoder) -> Result<(), std::fmt::Error> {
-        encoder.encode_f64(*self as f64)
+        encoder.encode_f64(f64::from(*self))
     }
 }
 
@@ -687,13 +683,13 @@ impl EncodeCounterValue for f64 {
 
 impl EncodeCounterValue for u32 {
     fn encode(&self, encoder: &mut CounterValueEncoder) -> Result<(), std::fmt::Error> {
-        encoder.encode_u64(*self as u64)
+        encoder.encode_u64(u64::from(*self))
     }
 }
 
 impl EncodeCounterValue for f32 {
     fn encode(&self, encoder: &mut CounterValueEncoder) -> Result<(), std::fmt::Error> {
-        encoder.encode_f64(*self as f64)
+        encoder.encode_f64(f64::from(*self))
     }
 }
 
@@ -751,13 +747,13 @@ impl EncodeExemplarValue for u64 {
 
 impl EncodeExemplarValue for f32 {
     fn encode(&self, mut encoder: ExemplarValueEncoder) -> Result<(), std::fmt::Error> {
-        encoder.encode(*self as f64)
+        encoder.encode(f64::from(*self))
     }
 }
 
 impl EncodeExemplarValue for u32 {
     fn encode(&self, mut encoder: ExemplarValueEncoder) -> Result<(), std::fmt::Error> {
-        encoder.encode(*self as f64)
+        encoder.encode(f64::from(*self))
     }
 }