Simplify features and make documentation call out feature gates (#1479)

* Makes use of the nightly `doc_cfg` feature to automatically mark feature-gated items as requiring that feature.

This is possible thanks to the fact that docs.rs runs on nightly. While this may not be stabilized (and therefore may eventually reverse), I think it's extremely useful to users and only requires small additional configurations that would be easy to remove in the future.

* Adds appropriate arguments to CI/CD and removes serde-1, test, and docs features

* Fixes clippy complaining about `return None` instead of question marks

Author: akern40

Diff for: .github/workflows/ci.yaml

@@ -10,7 +10,7 @@ name: Continuous integration
 env:
   CARGO_TERM_COLOR: always
   HOST: x86_64-unknown-linux-gnu
-  FEATURES: "test docs"
+  FEATURES: "approx,serde,rayon"
   RUSTFLAGS: "-D warnings"
   MSRV: 1.64.0
   BLAS_MSRV: 1.71.1
@@ -30,7 +30,7 @@ jobs:
           toolchain: ${{ matrix.rust }}
           components: clippy
       - uses: Swatinem/rust-cache@v2
-      - run: cargo clippy --features docs
+      - run: cargo clippy --features approx,serde,rayon
 
   format:
     runs-on: ubuntu-latest
@@ -139,7 +139,7 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - name: Install cross
         run: cargo install cross
-      - run: ./scripts/cross-tests.sh "docs" ${{ matrix.rust }} ${{ matrix.target }}
+      - run: ./scripts/cross-tests.sh "approx,serde,rayon" ${{ matrix.rust }} ${{ matrix.target }}
 
   cargo-careful:
     #if: ${{ github.event_name == 'merge_group' }}
@@ -161,10 +161,10 @@ jobs:
     strategy:
       matrix:
         rust:
-          - stable
+          - nightly # This is what docs.rs runs on, and is needed for the feature flags
     name: docs/${{ matrix.rust }}
     env:
-      RUSTDOCFLAGS: "-Dwarnings"
+      RUSTDOCFLAGS: "-Dwarnings --cfg docsrs"
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master

Diff for: Cargo.toml

@@ -59,14 +59,6 @@ default = ["std"]
 blas = ["dep:cblas-sys", "dep:libc"]
 
 serde = ["dep:serde"]
-# Old name for the serde feature
-serde-1 = ["dep:serde"]
-
-# These features are used for testing
-test = []
-
-# This feature is used for docs
-docs = ["approx", "serde", "rayon"]
 
 std = ["num-traits/std", "matrixmultiply/std"]
 rayon = ["dep:rayon", "std"]
@@ -121,5 +113,8 @@ opt-level = 2
 no-dev-version = true
 tag-name = "{{version}}"
 
+# Config specific to docs.rs
 [package.metadata.docs.rs]
-features = ["docs"]
+features = ["approx", "serde", "rayon"]
+# Define the configuration attribute `docsrs`
+rustdoc-args = ["--cfg", "docsrs"]

Diff for: src/array_approx.rs

@@ -1,4 +1,5 @@
 #[cfg(feature = "approx")]
+#[cfg_attr(docsrs, doc(cfg(feature = "approx")))]
 mod approx_methods
 {
     use crate::imp_prelude::*;
@@ -10,8 +11,6 @@ mod approx_methods
     {
         /// A test for equality that uses the elementwise absolute difference to compute the
         /// approximate equality of two arrays.
-        ///
-        /// **Requires crate feature `"approx"`**
         pub fn abs_diff_eq<S2>(&self, other: &ArrayBase<S2, D>, epsilon: A::Epsilon) -> bool
         where
             A: ::approx::AbsDiffEq<S2::Elem>,
@@ -23,8 +22,6 @@ mod approx_methods
 
         /// A test for equality that uses an elementwise relative comparison if the values are far
         /// apart; and the absolute difference otherwise.
-        ///
-        /// **Requires crate feature `"approx"`**
         pub fn relative_eq<S2>(&self, other: &ArrayBase<S2, D>, epsilon: A::Epsilon, max_relative: A::Epsilon) -> bool
         where
             A: ::approx::RelativeEq<S2::Elem>,
@@ -192,4 +189,5 @@ macro_rules! impl_approx_traits {
 }
 
 #[cfg(feature = "approx")]
+#[cfg_attr(docsrs, doc(cfg(feature = "approx")))]
 impl_approx_traits!(approx, "**Requires crate feature `\"approx\"`.**");

Diff for: src/arraytraits.rs

@@ -316,6 +316,7 @@ where
 }
 
 #[cfg(feature = "serde")]
+#[cfg_attr(docsrs, doc(cfg(feature = "serde")))]
 // Use version number so we can add a packed format later.
 pub const ARRAY_FORMAT_VERSION: u8 = 1u8;
 

Diff for: src/error.rs

@@ -81,6 +81,7 @@ impl PartialEq for ShapeError
 }
 
 #[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 impl Error for ShapeError {}
 
 impl fmt::Display for ShapeError

Diff for: src/impl_constructors.rs

@@ -99,6 +99,7 @@ where S: DataOwned<Elem = A>
     /// assert!(array == arr1(&[0.0, 0.25, 0.5, 0.75, 1.0]))
     /// ```
     #[cfg(feature = "std")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
     pub fn linspace(start: A, end: A, n: usize) -> Self
     where A: Float
     {
@@ -117,6 +118,7 @@ where S: DataOwned<Elem = A>
     /// assert!(array == arr1(&[0., 1., 2., 3., 4.]))
     /// ```
     #[cfg(feature = "std")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
     pub fn range(start: A, end: A, step: A) -> Self
     where A: Float
     {
@@ -145,6 +147,7 @@ where S: DataOwned<Elem = A>
     /// # }
     /// ```
     #[cfg(feature = "std")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
     pub fn logspace(base: A, start: A, end: A, n: usize) -> Self
     where A: Float
     {
@@ -179,6 +182,7 @@ where S: DataOwned<Elem = A>
     /// # example().unwrap();
     /// ```
     #[cfg(feature = "std")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
     pub fn geomspace(start: A, end: A, n: usize) -> Option<Self>
     where A: Float
     {

Diff for: src/impl_methods.rs

@@ -2299,10 +2299,7 @@ where
         let dim = dim.into_dimension();
 
         // Note: zero strides are safe precisely because we return an read-only view
-        let broadcast_strides = match upcast(&dim, &self.dim, &self.strides) {
-            Some(st) => st,
-            None => return None,
-        };
+        let broadcast_strides = upcast(&dim, &self.dim, &self.strides)?;
         unsafe { Some(ArrayView::new(self.ptr, dim, broadcast_strides)) }
     }
 

Diff for: src/iterators/mod.rs

@@ -139,10 +139,7 @@ impl<A> DoubleEndedIterator for Baseiter<A, Ix1>
     #[inline]
     fn next_back(&mut self) -> Option<Self::Item>
     {
-        let index = match self.index {
-            None => return None,
-            Some(ix) => ix,
-        };
+        let index = self.index?;
         self.dim[0] -= 1;
         let offset = Ix1::stride_offset(&self.dim, &self.strides);
         if index == self.dim {

Diff for: src/lib.rs

@@ -20,6 +20,8 @@
 #![doc(test(attr(allow(unused_variables))))]
 #![doc(test(attr(allow(deprecated))))]
 #![cfg_attr(not(feature = "std"), no_std)]
+// Enable the doc_cfg nightly feature for including feature gate flags in the documentation
+#![cfg_attr(docsrs, feature(doc_cfg))]
 
 //! The `ndarray` crate provides an *n*-dimensional container for general elements
 //! and for numerics.
@@ -120,7 +122,7 @@ extern crate std;
 #[cfg(feature = "blas")]
 extern crate cblas_sys;
 
-#[cfg(feature = "docs")]
+#[cfg(docsrs)]
 pub mod doc;
 
 #[cfg(target_has_atomic = "ptr")]
@@ -148,6 +150,7 @@ use crate::iterators::{ElementsBase, ElementsBaseMut, Iter, IterMut};
 pub use crate::arraytraits::AsArray;
 pub use crate::linalg_traits::LinalgScalar;
 #[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 pub use crate::linalg_traits::NdFloat;
 
 pub use crate::stacking::{concatenate, stack};
@@ -189,9 +192,11 @@ mod layout;
 mod linalg_traits;
 mod linspace;
 #[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 pub use crate::linspace::{linspace, range, Linspace};
 mod logspace;
 #[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 pub use crate::logspace::{logspace, Logspace};
 mod math_cell;
 mod numeric_util;
@@ -1587,6 +1592,7 @@ where
 
 // parallel methods
 #[cfg(feature = "rayon")]
+#[cfg_attr(docsrs, doc(cfg(feature = "rayon")))]
 pub mod parallel;
 
 mod impl_1d;

Diff for: src/linalg_traits.rs

@@ -39,7 +39,10 @@ impl<T> LinalgScalar for T where T: 'static + Copy + Zero + One + Add<Output = T
 /// operations (`ScalarOperand`).
 ///
 /// This trait can only be implemented by `f32` and `f64`.
+///
+/// **Requires default crate feature `"std"`**
 #[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 pub trait NdFloat:
     Float
     + AddAssign
@@ -59,6 +62,8 @@ pub trait NdFloat:
 }
 
 #[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 impl NdFloat for f32 {}
 #[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 impl NdFloat for f64 {}

Diff for: src/numeric/impl_float_maths.rs

@@ -54,6 +54,7 @@ macro_rules! binary_ops {
 ///
 /// Element-wise math functions for any array type that contains float number.
 #[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
 impl<A, S, D> ArrayBase<S, D>
 where
     A: 'static + Float,

Diff for: src/numeric/impl_numeric.rs

@@ -140,6 +140,7 @@ where
     /// ```
     #[track_caller]
     #[cfg(feature = "std")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
     pub fn var(&self, ddof: A) -> A
     where A: Float + FromPrimitive
     {
@@ -205,6 +206,7 @@ where
     /// ```
     #[track_caller]
     #[cfg(feature = "std")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
     pub fn std(&self, ddof: A) -> A
     where A: Float + FromPrimitive
     {
@@ -361,6 +363,7 @@ where
     /// ```
     #[track_caller]
     #[cfg(feature = "std")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
     pub fn var_axis(&self, axis: Axis, ddof: A) -> Array<A, D::Smaller>
     where
         A: Float + FromPrimitive,
@@ -431,6 +434,7 @@ where
     /// ```
     #[track_caller]
     #[cfg(feature = "std")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "std")))]
     pub fn std_axis(&self, axis: Axis, ddof: A) -> Array<A, D::Smaller>
     where
         A: Float + FromPrimitive,

Diff for: src/parallel/impl_par_methods.rs

@@ -8,8 +8,6 @@ use crate::parallel::prelude::*;
 use crate::partial::Partial;
 
 /// # Parallel methods
-///
-/// These methods require crate feature `rayon`.
 impl<A, S, D> ArrayBase<S, D>
 where
     S: DataMut<Elem = A>,

Diff for: src/partial.rs

@@ -37,6 +37,7 @@ impl<T> Partial<T>
     }
 
     #[cfg(feature = "rayon")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "rayon")))]
     pub(crate) fn stub() -> Self
     {
         Self {
@@ -46,6 +47,7 @@ impl<T> Partial<T>
     }
 
     #[cfg(feature = "rayon")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "rayon")))]
     pub(crate) fn is_stub(&self) -> bool
     {
         self.ptr.is_null()
@@ -60,6 +62,7 @@ impl<T> Partial<T>
     }
 
     #[cfg(feature = "rayon")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "rayon")))]
     /// Merge if they are in order (left to right) and contiguous.
     /// Skips merge if T does not need drop.
     pub(crate) fn try_merge(mut left: Self, right: Self) -> Self