Don't leak flate2 as an implementation detail by default

Cargo.toml

@@ -24,7 +24,7 @@ include = [
 bitflags = "2.0"
 crc32fast = "1.2.0"
 fdeflate = "0.3.3"
-flate2 = "1.0.11"
+flate2 = "1.0.35"
 miniz_oxide = { version = "0.8", features = ["simd"] }
 
 [dev-dependencies]
@@ -39,7 +39,14 @@ rand = "0.8.4"
 term = "1.0.1"
 
 [features]
+# Use nightly-only features for a minor performance boost in PNG decoding
 unstable = ["crc32fast/nightly"]
+# Use zlib-rs for faster PNG encoding at the cost of some `unsafe` code.
+# WARNING: this changes the flate2 backend for your entire dependency tree!
+# While the `png` crate always uses fully memory-safe decoding,
+# this enables zlib-rs and introduces some unsafe code to all other crates
+# that rely on flate2, including the decoding codepaths.
+zlib-rs = ["flate2/zlib-rs"]
 benchmarks = []
 
 [lints.rust]

fuzz/fuzz_targets/roundtrip.rs

@@ -30,7 +30,7 @@ fn encode_png<'a>(width: u8, filter: u8, compression: u8, color_type: u8, raw_bi
     // compression
     let compression = match compression {
         0 => png::DeflateCompression::NoCompression,
-        level @ 1..=9 => png::DeflateCompression::Flate2(level),
+        level @ 1..=9 => png::DeflateCompression::Level(level),
         10 => png::DeflateCompression::FdeflateUltraFast,
         _ => return None,
     };

src/common.rs

@@ -378,11 +378,15 @@ pub enum DeflateCompression {
     /// while still providing a decent compression ratio.
     FdeflateUltraFast,
 
-    /// Uses [flate2](https://crates.io/crates/flate2) crate with the specified [compression level](flate2::Compression::new).
+    /// Compression level between 1 and 9, where higher values mean better compression at the cost of
+    /// speed.
     ///
-    /// Flate2 has several backends that make different trade-offs.
-    /// See the flate2 documentation for the available backends for more information.
-    Flate2(u8),
+    /// This is currently implemented via [flate2](https://crates.io/crates/flate2) crate
+    /// by passing through the [compression level](flate2::Compression::new).
+    ///
+    /// The implementation details and the exact meaning of each level may change in the future,
+    /// including in semver-compatible releases.
+    Level(u8),
     // Other variants can be added in the future
 }
 
@@ -398,16 +402,16 @@ impl DeflateCompression {
             Compression::NoCompression => Self::NoCompression,
             Compression::Fastest => Self::FdeflateUltraFast,
             Compression::Fast => Self::FdeflateUltraFast,
-            Compression::Balanced => Self::Flate2(flate2::Compression::default().level() as u8),
-            Compression::High => Self::Flate2(flate2::Compression::best().level() as u8),
+            Compression::Balanced => Self::Level(flate2::Compression::default().level() as u8),
+            Compression::High => Self::Level(flate2::Compression::best().level() as u8),
         }
     }
 
     pub(crate) fn closest_flate2_level(&self) -> flate2::Compression {
         match self {
             DeflateCompression::NoCompression => flate2::Compression::none(),
             DeflateCompression::FdeflateUltraFast => flate2::Compression::new(1),
-            DeflateCompression::Flate2(level) => flate2::Compression::new(u32::from(*level)),
+            DeflateCompression::Level(level) => flate2::Compression::new(u32::from(*level)),
         }
     }
 }

src/encoder.rs

@@ -823,7 +823,7 @@ impl<W: Write> Writer<W> {
                     compressed
                 }
             }
-            DeflateCompression::Flate2(level) => {
+            DeflateCompression::Level(level) => {
                 let mut current = vec![0; in_len];
 
                 let mut zlib =
@@ -1772,7 +1772,7 @@ mod tests {
                 let mut buf = vec![0; reader.output_buffer_size()];
                 let info = reader.next_frame(&mut buf).unwrap();
                 use DeflateCompression::*;
-                for compression in [NoCompression, FdeflateUltraFast, Flate2(4)] {
+                for compression in [NoCompression, FdeflateUltraFast, Level(4)] {
                     // Encode decoded image
                     let mut out = Vec::new();
                     {
@@ -1830,7 +1830,7 @@ mod tests {
                 let mut buf = vec![0; reader.output_buffer_size()];
                 let info = reader.next_frame(&mut buf).unwrap();
                 use DeflateCompression::*;
-                for compression in [NoCompression, FdeflateUltraFast, Flate2(4)] {
+                for compression in [NoCompression, FdeflateUltraFast, Level(4)] {
                     // Encode decoded image
                     let mut out = Vec::new();
                     {