1 files changed, 185 insertions, 0 deletions
diff --git a/src/features.rs b/src/features.rs
new file mode 100644
index 00000000..3f59c6b9
--- /dev/null
+++ b/src/features.rs
@@ -0,0 +1,185 @@
+//! Contains code for selecting features
+
+#![deny(missing_docs)]
+#![deny(warnings)]
+#![deny(unused_extern_crates)]
+
+use std::io;
+use std::str::FromStr;
+
+/// Define RustTarget struct definition, Default impl, and conversions
+/// between RustTarget and String.
+macro_rules! rust_target_def {
+    ( $( $( #[$attr:meta] )* => $release:ident => $value:expr; )* ) => {
+        /// Represents the version of the Rust language to target.
+        ///
+        /// To support a beta release, use the corresponding stable release.
+        ///
+        /// This enum will have more variants added as necessary.
+        #[derive(Debug, Copy, Clone, Eq, PartialEq, PartialOrd, Hash)]
+        #[allow(non_camel_case_types)]
+        pub enum RustTarget {
+            $(
+                $(
+                    #[$attr]
+                )*
+                $release,
+            )*
+        }
+
+        impl Default for RustTarget {
+            /// Gives the latest stable Rust version
+            fn default() -> RustTarget {
+                LATEST_STABLE_RUST
+            }
+        }
+
+        impl FromStr for RustTarget {
+            type Err = io::Error;
+
+            /// Create a `RustTarget` from a string.
+            ///
+            /// * The stable/beta versions of Rust are of the form "1.0",
+            /// "1.19", etc.
+            /// * The nightly version should be specified with "nightly".
+            fn from_str(s: &str) -> Result<Self, Self::Err> {
+                match s.as_ref() {
+                    $(
+                        stringify!($value) => Ok(RustTarget::$release),
+                    )*
+                    _ => Err(
+                        io::Error::new(
+                            io::ErrorKind::InvalidInput,
+                            concat!(
+                                "Got an invalid rust target. Accepted values ",
+                                "are of the form ",
+                                "\"1.0\" or \"nightly\"."))),
+                }
+            }
+        }
+
+        impl From<RustTarget> for String {
+            fn from(target: RustTarget) -> Self {
+                match target {
+                    $(
+                        RustTarget::$release => stringify!($value),
+                    )*
+                }.into()
+            }
+        }
+    }
+}
+
+/// Defines an array slice with all RustTarget values
+macro_rules! rust_target_values_def {
+    ( $( $( #[$attr:meta] )* => $release:ident => $value:expr; )* ) => {
+        /// Strings of allowed `RustTarget` values
+        pub static RUST_TARGET_STRINGS: &'static [&str] = &[
+            $(
+                stringify!($value),
+            )*
+        ];
+    }
+}
+
+/// Defines macro which takes a macro
+macro_rules! rust_target_base {
+    ( $x_macro:ident ) => {
+        $x_macro!(
+            /// Rust stable 1.0
+            => Stable_1_0 => 1.0;
+            /// Rust stable 1.19
+            => Stable_1_19 => 1.19;
+            /// Nightly rust
+            => Nightly => nightly;
+        );
+    }
+}
+
+rust_target_base!(rust_target_def);
+rust_target_base!(rust_target_values_def);
+
+/// Latest stable release of Rust
+pub const LATEST_STABLE_RUST: RustTarget = RustTarget::Stable_1_19;
+
+/// Create RustFeatures struct definition, new(), and a getter for each field
+macro_rules! rust_feature_def {
+    ( $( $( #[$attr:meta] )* => $feature:ident; )* ) => {
+        /// Features supported by a rust target
+        #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
+        pub struct RustFeatures {
+            $(
+                $feature: bool,
+            )*
+        }
+
+        impl RustFeatures {
+            /// Gives a RustFeatures struct with all features disabled
+            fn new() -> Self {
+                RustFeatures {
+                    $(
+                        $feature: false,
+                    )*
+                }
+            }
+
+            $(
+                $(
+                    #[$attr]
+                )*
+                pub fn $feature(&self) -> bool {
+                    self.$feature
+                }
+            )*
+        }
+    }
+}
+
+rust_feature_def!(
+    /// Untagged unions ([RFC 1444](https://github.com/rust-lang/rfcs/blob/master/text/1444-union.md))
+    => untagged_union;
+    /// Constant function ([RFC 911](https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md))
+    => const_fn;
+);
+
+impl From<RustTarget> for RustFeatures {
+    fn from(rust_target: RustTarget) -> Self {
+        let mut features = RustFeatures::new();
+
+        if rust_target >= RustTarget::Stable_1_19 {
+            features.untagged_union = true;
+        }
+
+        if rust_target >= RustTarget::Nightly {
+            features.const_fn = true;
+        }
+
+        features
+    }
+}
+
+impl Default for RustFeatures {
+    fn default() -> Self {
+        let default_rust_target: RustTarget = Default::default();
+        Self::from(default_rust_target)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    #![allow(unused_imports)]
+    use super::*;
+
+    fn test_target(target_str: &str, target: RustTarget) {
+        let target_string: String = target.into();
+        assert_eq!(target_str, target_string);
+        assert_eq!(target, RustTarget::from_str(target_str).unwrap());
+    }
+
+    #[test]
+    fn str_to_target() {
+        test_target("1.0", RustTarget::Stable_1_0);
+        test_target("1.19", RustTarget::Stable_1_19);
+        test_target("nightly", RustTarget::Nightly);
+    }
+}