Fix --no-default-features builds

appveyor.yml

@@ -24,4 +24,5 @@ build: false
 
 test_script:
   - cargo test --target %TARGET%
+  - cargo test --no-default-features --target %TARGET%
   - cargo test --manifest-path libc-test/Cargo.toml --target %TARGET%

ci/run.sh

@@ -79,4 +79,5 @@ if [ "$TARGET" = "x86_64-unknown-linux-gnux32" ]; then
   opt="--release"
 fi
 
+cargo test $opt --no-default-features --manifest-path libc-test/Cargo.toml --target $TARGET
 exec cargo test $opt --manifest-path libc-test/Cargo.toml --target $TARGET

libc-test/Cargo.toml

@@ -4,12 +4,17 @@ version = "0.1.0"
 authors = ["Alex Crichton <[email protected]>"]
 build = "build.rs"
 
-[dependencies]
-libc = { path = ".." }
+[dependencies.libc]
+path = ".."
+default-features = false
 
 [build-dependencies]
 ctest = { git = "https://github.com/alexcrichton/ctest" }
 
+[features]
+default = [ "use_std" ]
+use_std = [ "libc/use_std" ]
+
 [[test]]
 name = "main"
 path = "test/main.rs"
@@ -19,3 +24,4 @@ harness = false
 name = "linux-fcntl"
 path = "test/linux_fcntl.rs"
 harness = false
+

src/unix/mod.rs

@@ -277,16 +277,20 @@ cfg_if! {
         // cargo build, don't pull in anything extra as the libstd dep
         // already pulls in all libs.
     } else if #[cfg(target_env = "musl")] {
-        #[link(name = "c", kind = "static", cfg(target_feature = "crt-static"))]
-        #[link(name = "c", cfg(not(target_feature = "crt-static")))]
+        #[cfg_attr(feature = "stdbuild",
+                   link(name = "c", kind = "static",
+                        cfg(target_feature = "crt-static")))]
+        #[cfg_attr(feature = "stdbuild",
+                   link(name = "c", cfg(not(target_feature = "crt-static"))))]
         extern {}
     } else if #[cfg(target_os = "emscripten")] {
         #[link(name = "c")]
         extern {}
-    } else if #[cfg(all(target_os = "netbsd", target_vendor = "rumprun"))] {
+    } else if #[cfg(all(target_os = "netbsd"))] {
         // Since we don't use -nodefaultlibs on Rumprun, libc is always pulled
         // in automatically by the linker. We avoid passing it explicitly, as it
         // causes some versions of binutils to crash with an assertion failure.
+        #[cfg_attr(feature = "stdbuild", target_vendor = "rumprun")]
         #[link(name = "m")]
         extern {}
     } else if #[cfg(any(target_os = "macos",