fix(format/io): implement AVSEEK_SIZE seeking on stable

Cargo.toml

@@ -13,7 +13,6 @@ keywords    = ["audio", "video"]
 
 [features]
 default = ["codec", "device", "filter", "format", "resampling", "software-scaling"]
-unstable = []
 serde = ["serde_", "serde_derive", "ffmpeg-sys/serde"]
 
 static = ["ffmpeg-sys/static"]

src/format/io.rs

@@ -90,18 +90,29 @@ unsafe extern "C" fn read_packet(opaque: *mut c_void, buf: *mut u8, size: c_int)
 	result.try_into().unwrap()
 }
 
+// copy of unstable `Seek::stream_len`
+// https://github.com/rust-lang/rust/issues/59359
+fn stream_len(mut seek: impl Seek) -> io::Result<u64> {
+	let old_pos = seek.stream_position()?;
+	let len = seek.seek(SeekFrom::End(0))?;
+
+	// Avoid seeking a third time when we were already at the end of the
+	// stream. The branch is usually way cheaper than a seek operation.
+	if old_pos != len {
+		seek.seek(SeekFrom::Start(old_pos))?;
+	}
+
+	Ok(len)
+}
+
 unsafe extern "C" fn seek(opaque: *mut c_void, offset: i64, whence: c_int) -> i64 {
 	let mut proxy = Box::<Proxy>::from_raw(opaque.cast());
 
 	let result: i64 = if whence == AVSEEK_SIZE {
-		#[cfg(feature = "unstable")]
-		match proxy.as_seek().stream_len() {
+		match stream_len(proxy.as_seek()) {
 			Ok(size) => size.try_into().unwrap(),
 			Err(err) => as_error(err).into(),
 		}
-
-		#[cfg(not(feature = "unstable"))]
-		0
 	}
 	else {
 		let whence = match whence {

src/lib.rs

@@ -1,5 +1,4 @@
 #![allow(non_camel_case_types)]
-#![cfg_attr(feature = "unstable", feature(seek_stream_len))]
 #![cfg_attr(feature = "cargo-clippy", allow(inline_always))]
 
 #[macro_use]