Allow deriving for no-std targets

By pulling in `num-traits` as a direct dependency, we corrupt the
feature selection for anybody using `num-derive` too, which makes it
impossible to use on no-std targets. We really only need it for testing
as a dev-dependency though.

Author: cuviper

.travis.yml

@@ -8,6 +8,19 @@ env:
   matrix:
     - # no features
     - FEATURES="full-syntax"
+
+matrix:
+  include:
+    # try a no-std target
+    - rust: stable
+      env: TARGET=thumbv6m-none-eabi
+      before_script:
+        - rustup target add "$TARGET"
+      script:
+        # This test crate is intentionally separate, because we need
+        # independent features for no-std. (rust-lang/cargo#2589)
+        - cd check && cargo check --target "$TARGET"
+
 sudo: false
 script:
   - cargo build --verbose --features="$FEATURES"

Cargo.toml

@@ -13,13 +13,13 @@ readme = "README.md"
 build = "build.rs"
 
 [dependencies]
-num-traits = "0.2"
 proc-macro2 = "0.4.2"
 quote = "0.6"
 syn = "0.15"
 
 [dev-dependencies]
 num = "0.2"
+num-traits = "0.2"
 
 [features]
 full-syntax = ["syn/full"]

check/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "check"
+version = "0.1.0"
+authors = ["Josh Stone <cuviper@gmail.com>"]
+edition = "2018"
+
+[dependencies.num-derive]
+path = ".."
+
+[dependencies.num-traits]
+version = "0.2"
+default-features = false

check/src/lib.rs

@@ -0,0 +1,8 @@
+#![no_std]
+
+#[derive(num_derive::FromPrimitive)]
+pub enum ABC {
+    A,
+    B,
+    C,
+}