better error message when compiling with --no-default-features or default-features = false

.github/workflows/main.yml

@@ -38,12 +38,19 @@ jobs:
     - run: cargo test
     - run: cargo test --features zlib
     - run: cargo test --features zlib --no-default-features
+    - run: cargo test --features zlib-default --no-default-features
     - run: cargo test --features zlib-ng-compat --no-default-features
       if: matrix.build != 'mingw'
     - run: cargo test --features zlib-ng --no-default-features
       if: matrix.build != 'mingw'
     - run: cargo test --features cloudflare_zlib --no-default-features
       if: matrix.build != 'mingw'
+    - run: |
+        if ! cargo check --no-default-features 2>&1 | grep "You need to choose"; then 
+          echo "expected message stating a zlib backend must be chosen"
+          exit 1
+        fi
+      if: matrix.build == 'stable' 
 
   rustfmt:
     name: Rustfmt

Cargo.toml

@@ -32,13 +32,14 @@ quickcheck = { version = "1.0", default-features = false }
 
 [features]
 default = ["rust_backend"]
-any_zlib = [] # note: this is not a real user-facing feature
+any_zlib = ["any_impl"] # note: this is not a real user-facing feature
+any_impl = [] # note: this is not a real user-facing feature
 zlib = ["any_zlib", "libz-sys"]
 zlib-default = ["any_zlib", "libz-sys/default"]
 zlib-ng-compat = ["zlib", "libz-sys/zlib-ng"]
 zlib-ng = ["any_zlib", "libz-ng-sys"]
 cloudflare_zlib = ["any_zlib", "cloudflare-zlib-sys"]
-rust_backend = ["miniz_oxide"]
+rust_backend = ["miniz_oxide", "any_impl"]
 miniz-sys = ["rust_backend"] # For backwards compatibility
 
 [package.metadata.docs.rs]

src/ffi/mod.rs

@@ -40,9 +40,9 @@ mod c;
 #[cfg(feature = "any_zlib")]
 pub use self::c::*;
 
-#[cfg(not(feature = "any_zlib"))]
+#[cfg(all(not(feature = "any_zlib"), feature = "miniz_oxide"))]
 mod rust;
-#[cfg(not(feature = "any_zlib"))]
+#[cfg(all(not(feature = "any_zlib"), feature = "miniz_oxide"))]
 pub use self::rust::*;
 
 impl std::fmt::Debug for ErrorMessage {

src/lib.rs

@@ -78,6 +78,9 @@
 #![cfg_attr(test, deny(warnings))]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 
+#[cfg(not(feature = "any_impl",))]
+compile_error!("You need to choose a zlib backend");
+
 pub use crate::crc::{Crc, CrcReader, CrcWriter};
 pub use crate::gz::GzBuilder;
 pub use crate::gz::GzHeader;