Break compilation for invalid feature combinations.

Sadly our feature matrix is not orthogonal.  Some flag combinations don't
make sense together, and may even fail the compilation in confusing ways.

This commit changes the code to fail to compile when incompatible
feature sets are used.

Closes #129.

README.md

@@ -143,7 +143,7 @@ confusing compilation end result.
 
 | Feature | Default? | Description |
 | ------- | -------- | ----------- |
-| `bindgen` | Yes | If set, cargo will run `bindgen` to generate bindings based on the installed ICU library.  The program `icu-config` must be in $PATH for this to work. In the future there may be other approaches for auto-detecting libraries, such as via `pkg-config`. |
+| `use-bindgen` | Yes | If set, cargo will run `bindgen` to generate bindings based on the installed ICU library.  The program `icu-config` must be in $PATH for this to work. In the future there may be other approaches for auto-detecting libraries, such as via `pkg-config`. |
 | `renaming` | Yes | If set, ICU bindings are generated with version numbers appended.  This is called "renaming" in ICU, and is normally needed only when linking against specific ICU version is required, for example to work around having to link different ICU versions.  See [the ICU documentation](http://userguide.icu-project.org/design) for a discussion of renaming. **This feature MUST be used when `bindgen` is NOT used.** |
 | `icu_config` | Yes | If set, the binary icu-config will be used to configure the library.  Turn this feature off if you do not want `build.rs` to try to autodetect the build environment.  You will want to skip this feature if your build environment configures ICU in a different way. **This feature is only meaningful when `bindgen` feature is used; otherwise it has no effect.** |
 | `icu_version_in_env` | No | If set, ICU bindings are made for the ICU version specified in the environment variable `RUST_ICU_MAJOR_VERSION_NUMBER`, which is made available to cargo at build time. See section below for details on how to use this feature. **This feature is only meaningful when `bindgen` feature is NOT used; otherwise it has no effect.** |

rust_icu_sys/src/lib.rs

@@ -20,6 +20,19 @@
     unused_imports
 )]
 
+#[cfg(all(feature="icu_version_in_env", feature="icu_config"))]
+compile_error!(
+    "Features `icu_version_in_env` and `icu_config` are not compatible." +
+    " Choose at most one of them.");
+
+#[cfg(all(feature="icu_config", not(feature="use-bindgen")))]
+compile_error!("Feature `icu_config` is useless without the feature `use-bindgen`");
+
+// This feature combination is not inherrently a problem; we had no use case that
+// required it just yet.
+#[cfg(all(not(feature="renaming"), not(feature="use-bindgen")))]
+compile_error!("You must use `renaming` when not using `use-bindgen`");
+
 #[cfg(feature="use-bindgen")]
 include!(concat!(env!("OUT_DIR"), "/macros.rs"));
 #[cfg(all(feature="use-bindgen",feature="icu_config",not(feature="icu_version_in_env")))]