When libz-sys fails to find zlib, it falls back to building it from source.
However, when curl-rust vendors libcurl (when it falls back), it transitively depends on either zlib being present in system include paths orlibz-sys vendoring it into the same paths:
libz-sys recently attempted to use pkg-config on Windows, which broke curl-sys using an MSVC toolchain (from inside an MSYS2 environment, presumably withoutvcpkg), because it would no longer need to build zlib from source: https://github.com/rust-lang/libz-sys/issues/143
However, because curl-sys does not support pkg-config (#486), on Windows:
on an MSVC host and MSVC target, it will attempt to find libcurl with vcpkg (#509 would slightly improve this)
otherwise, it just builds libcurl from source
When building from source, curl-sys generates a pkg-config file, but doesn't add any linkage or include path information for zlib:
...but otherwise this means zlib.h needs to be in $OUT_DIR/include (which it gets from libz-sys' vendoring) or in the system include paths (which it gets on most non-Windows platforms).
When vendoring libcurl, curl-sys should find zlib properly (ideally, the same way as libz-sys does).
It would be better if you needed to explicitly enable vendoring in this library, because while "silent fallback" can be helpful, it leads to surprising behaviour like this, and makes it more difficult to audit your dependencies.
When
libz-sysfails to findzlib, it falls back to building it from source.However, when
curl-rustvendorslibcurl(when it falls back), it transitively depends on eitherzlibbeing present in system include paths orlibz-sysvendoring it into the same paths:https://github.com/alexcrichton/curl-rust/blob/ff6ad21cc1034826b2ab3f8be0653d8c446e1bdc/curl-sys/build.rs#L57-L59
libz-sysrecently attempted to usepkg-configon Windows, which brokecurl-sysusing an MSVC toolchain (from inside an MSYS2 environment, presumably withoutvcpkg), because it would no longer need to buildzlibfrom source: https://github.com/rust-lang/libz-sys/issues/143However, because
curl-sysdoes not supportpkg-config(#486), on Windows:libcurlwithvcpkg(#509 would slightly improve this)libcurlfrom sourceWhen building from source,
curl-sysgenerates apkg-configfile, but doesn't add any linkage or include path information forzlib:https://github.com/alexcrichton/curl-rust/blob/ff6ad21cc1034826b2ab3f8be0653d8c446e1bdc/curl-sys/build.rs#L98-L100
...and enables
zlibsupport:https://github.com/alexcrichton/curl-rust/blob/ff6ad21cc1034826b2ab3f8be0653d8c446e1bdc/curl-sys/build.rs#L127
...it might get something from
vcpkgif built on an MSVC host (becausebuild.rscfg directives are based on the host, not the target):https://github.com/alexcrichton/curl-rust/blob/ff6ad21cc1034826b2ab3f8be0653d8c446e1bdc/curl-sys/build.rs#L296-L299
...but otherwise this means
zlib.hneeds to be in$OUT_DIR/include(which it gets fromlibz-sys' vendoring) or in the system include paths (which it gets on most non-Windows platforms).When vendoring
libcurl,curl-sysshould findzlibproperly (ideally, the same way aslibz-sysdoes).It would be better if you needed to explicitly enable vendoring in this library, because while "silent fallback" can be helpful, it leads to surprising behaviour like this, and makes it more difficult to audit your dependencies.