Merge #110

110: No std support r=Ogeon a=mb64

Adds support for using palette without the standard library, as discussed in issue #108.

Fixes #108.

One tiny question: because this would be in a future release (not 0.4.0), what version should be in the README example Cargo.toml for `#![no_std]`?  I put 0.4, but was unsure about if it should be something different.

Co-authored-by: Mark Barbone <mark.l.barbone@gmail.com>

Author: bors[bot]

README.md

@@ -20,16 +20,27 @@ Add the following lines to your `Cargo.toml` file:
 palette = "0.4"
 ```
 
-### Optional Features
+### Features
 
 These features are enabled by default:
 
 * `"named"` - Enables color constants, located in the `named` module.
-* `"named_from_str"` - Enables the `named::from_str`, which maps name string to colors.
+* `"named_from_str"` - Enables the `named::from_str`, which maps name string to colors. This requires the standard library.
+* `"std"` - Enables use of the standard library.
 
 These features are disabled by default:
 
-* `"serde"` - Enables color serializing and deserializing.
+* `"serializing"` - Enables color serializing and deserializing using `serde`.
+
+### Without the standard library
+
+Here is an example `Cargo.toml` entry for using palette on `#![no_std]`:
+
+```toml
+[dependencies.palette]
+version = "0.4"
+default-features = false
+```
 
 ## It's Never "Just RGB"
 
@@ -113,6 +124,18 @@ This library is only meant for color manipulation and conversion. It's not a ful
 
 [pixel_module]: https://ogeon.github.io/docs/palette/master/palette/pixel/index.html
 
+## Using palette in an embedded environment
+
+Palette supports `#![no_std]` environments by disabling the `"std"` feature. However, there are some things that are unavailable without the standard library:
+
+* Gradients are unavailable, because they depend heavily on Vectors
+* The `"named_from_str"` feature requires the standard library as well
+* Serialization using `serde` is unavailable
+
+It uses [`libm`] to provide the floating-point operations that are typically in `std`.
+
+[`libm`]: https://github.com/japaric/libm
+
 ## Contributing
 
 All sorts of contributions are welcome, no matter how huge or tiny, so take a look at [CONTRIBUTING.md](CONTRIBUTING.md) for guidelines, if you are interested.

palette/Cargo.toml

@@ -13,24 +13,26 @@ license = "MIT OR Apache-2.0"
 build = "build/main.rs"
 
 [features]
-default = ["named_from_str"]
-named_from_str = ["named", "phf", "phf_codegen"]
+default = ["named_from_str", "std"]
+named_from_str = ["named", "phf", "phf_codegen", "std"]
 named = []
+std = ["approx/std", "num-traits/std"]
+serializing = ["serde", "std"]
 
 #internal
 strict = []
 
 [dependencies]
 palette_derive = {version = "0.4.1", path = "../palette_derive"}
-num-traits = "0.2"
-approx = "0.2"
+num-traits = {version = "0.2", default-features = false}
+approx = {version = "0.2", default-features = false}
+libm = "0.1.2"
 
 [dependencies.phf]
 version = "0.7"
 optional = true
 
 [dependencies.serde]
-#feature
 version = "1"
 features = ["serde_derive"]
 optional = true