contracts: make derive macros do nothing outside nightly channel

This allows us to build Topola in stable rust, as our vendored contracts
library fork uses try blocks which haven't been stabilized yet.

vendored/contracts/Cargo.toml

@@ -38,4 +38,4 @@ mirai_assertions = []
 syn = { version = "1.0", features = ["extra-traits", "full", "visit", "visit-mut"] }
 quote = "1.0"
 proc-macro2 = "1.0"
-
+rustc_version = "0.4"

vendored/contracts/src/lib.rs

@@ -180,6 +180,8 @@ extern crate proc_macro;
 
 mod implementation;
 
+use rustc_version::{version, version_meta, Channel, Version};
+
 use implementation::ContractMode;
 use proc_macro::TokenStream;
 
@@ -197,6 +199,10 @@ use proc_macro::TokenStream;
 /// ```
 #[proc_macro_attribute]
 pub fn requires(attr: TokenStream, toks: TokenStream) -> TokenStream {
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        return toks;
+    }
+
     let attr = attr.into();
     let toks = toks.into();
     implementation::requires(ContractMode::Always, attr, toks).into()
@@ -207,6 +213,10 @@ pub fn requires(attr: TokenStream, toks: TokenStream) -> TokenStream {
 /// [`requires`]: attr.requires.html
 #[proc_macro_attribute]
 pub fn debug_requires(attr: TokenStream, toks: TokenStream) -> TokenStream {
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        return toks;
+    }
+
     let attr = attr.into();
     let toks = toks.into();
     implementation::requires(ContractMode::Debug, attr, toks).into()
@@ -217,6 +227,10 @@ pub fn debug_requires(attr: TokenStream, toks: TokenStream) -> TokenStream {
 /// [`requires`]: attr.requires.html
 #[proc_macro_attribute]
 pub fn test_requires(attr: TokenStream, toks: TokenStream) -> TokenStream {
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        return toks;
+    }
+
     let attr = attr.into();
     let toks = toks.into();
     implementation::requires(ContractMode::Test, attr, toks).into()
@@ -252,6 +266,10 @@ pub fn test_requires(attr: TokenStream, toks: TokenStream) -> TokenStream {
 /// ```
 #[proc_macro_attribute]
 pub fn ensures(attr: TokenStream, toks: TokenStream) -> TokenStream {
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        return toks;
+    }
+
     let attr = attr.into();
     let toks = toks.into();
     implementation::ensures(ContractMode::Always, attr, toks).into()
@@ -262,6 +280,10 @@ pub fn ensures(attr: TokenStream, toks: TokenStream) -> TokenStream {
 /// [`ensures`]: attr.ensures.html
 #[proc_macro_attribute]
 pub fn debug_ensures(attr: TokenStream, toks: TokenStream) -> TokenStream {
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        return toks;
+    }
+
     let attr = attr.into();
     let toks = toks.into();
     implementation::ensures(ContractMode::Debug, attr, toks).into()
@@ -272,6 +294,10 @@ pub fn debug_ensures(attr: TokenStream, toks: TokenStream) -> TokenStream {
 /// [`ensures`]: attr.ensures.html
 #[proc_macro_attribute]
 pub fn test_ensures(attr: TokenStream, toks: TokenStream) -> TokenStream {
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        return toks;
+    }
+
     let attr = attr.into();
     let toks = toks.into();
     implementation::ensures(ContractMode::Test, attr, toks).into()
@@ -320,6 +346,10 @@ pub fn test_ensures(attr: TokenStream, toks: TokenStream) -> TokenStream {
 /// ```
 #[proc_macro_attribute]
 pub fn invariant(attr: TokenStream, toks: TokenStream) -> TokenStream {
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        return toks;
+    }
+
     // Invariant attributes might apply to `impl` blocks as well, where the same
     // level is simply replicated on all methods.
     // Function expansions will resolve the actual mode themselves, so the
@@ -337,6 +367,10 @@ pub fn invariant(attr: TokenStream, toks: TokenStream) -> TokenStream {
 /// [`invariant`]: attr.invariant.html
 #[proc_macro_attribute]
 pub fn debug_invariant(attr: TokenStream, toks: TokenStream) -> TokenStream {
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        return toks;
+    }
+
     let mode = ContractMode::Debug;
     let attr = attr.into();
     let toks = toks.into();
@@ -348,6 +382,10 @@ pub fn debug_invariant(attr: TokenStream, toks: TokenStream) -> TokenStream {
 /// [`invariant`]: attr.invariant.html
 #[proc_macro_attribute]
 pub fn test_invariant(attr: TokenStream, toks: TokenStream) -> TokenStream {
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        return toks;
+    }
+
     let mode = ContractMode::Test;
     let attr = attr.into();
     let toks = toks.into();
@@ -389,6 +427,10 @@ pub fn test_invariant(attr: TokenStream, toks: TokenStream) -> TokenStream {
 /// ```
 #[proc_macro_attribute]
 pub fn contract_trait(attrs: TokenStream, toks: TokenStream) -> TokenStream {
+    if let Channel::Nightly = version_meta().unwrap().channel {
+        return toks;
+    }
+
     let attrs: proc_macro2::TokenStream = attrs.into();
     let toks: proc_macro2::TokenStream = toks.into();