Allow cargo test to complete under any set of features.

Previously, tests, examples, and benches would fail to compile under
`cargo test --no-default-features`. Now they are all skipped if they
would not compile.

This does not affect behavior *with* default features — test coverage
is not reduced.

Cargo.toml

@@ -17,6 +17,19 @@ rust-version = "1.48.0"
 [[bench]]
 name = "benchmarks"
 harness = false
+required-features = ["std"]
+
+[[example]]
+name = "base64"
+required-features = ["std"]
+
+[[test]]
+name = "tests"
+required-features = ["alloc"]
+
+[[test]]
+name = "encode"
+required-features = ["alloc"]
 
 [package.metadata.docs.rs]
 rustdoc-args = ["--generate-link-to-definition"]

src/engine/mod.rs

@@ -178,7 +178,8 @@ pub trait Engine: Send + Sync {
     ///
     /// # Example
     ///
-    /// ```rust
+    #[cfg_attr(feature = "alloc", doc = "```")]
+    #[cfg_attr(not(feature = "alloc"), doc = "```ignore")]
     /// use base64::{Engine as _, engine::general_purpose};
     /// let s = b"hello internet!";
     /// let mut buf = Vec::new();

src/lib.rs

@@ -81,7 +81,8 @@
 //!
 //! ## Using predefined engines
 //!
-//! ```
+#![cfg_attr(feature = "alloc", doc = "```")]
+#![cfg_attr(not(feature = "alloc"), doc = "```ignore")]
 //! use base64::{Engine as _, engine::general_purpose};
 //!
 //! let orig = b"data";
@@ -95,7 +96,8 @@
 //!
 //! ## Custom alphabet, config, and engine
 //!
-//! ```
+#![cfg_attr(feature = "alloc", doc = "```")]
+#![cfg_attr(not(feature = "alloc"), doc = "```ignore")]
 //! use base64::{engine, alphabet, Engine as _};
 //!
 //! // bizarro-world base64: +/ as the first symbols instead of the last

src/prelude.rs

@@ -5,7 +5,8 @@
 //!
 //! # Examples
 //!
-//! ```
+#![cfg_attr(feature = "alloc", doc = "```")]
+#![cfg_attr(not(feature = "alloc"), doc = "```ignore")]
 //! use base64::prelude::{Engine as _, BASE64_STANDARD_NO_PAD};
 //!
 //! assert_eq!("c29tZSBieXRlcw", &BASE64_STANDARD_NO_PAD.encode(b"some bytes"));