Ignore unsupported doc tests under --no-default-features

There's no pretty way of gating doctests contingent on cfg
presence/absense so this is kind of ugly.

First, a doc comment `///` is converted into its equivalent
`#[doc(...)]`, which is then converted into a conditional rust attribute
via `#[cfg_attr(...)]`. We need two of these, one to ignore the tests if
the cfg predicate isn't met and another to indicate the start of a code
block and enable testing + syntax highlighting.

src/format/mod.rs

@@ -16,7 +16,8 @@
 //! C's `strftime` format. The available options can be found [here](./strftime/index.html).
 //!
 //! # Example
-//! ```rust
+#![cfg_attr(not(feature = "std"), doc = "```ignore")]
+#![cfg_attr(feature = "std", doc = "```rust")]
 //! # use std::error::Error;
 //! use chrono::prelude::*;
 //!

src/lib.rs

@@ -110,7 +110,8 @@
 //! or in the local time zone
 //! ([`Local::now()`](./offset/struct.Local.html#method.now)).
 //!
-//! ```rust
+#![cfg_attr(not(feature = "clock"), doc = "```ignore")]
+#![cfg_attr(feature = "clock", doc = "```rust")]
 //! use chrono::prelude::*;
 //!
 //! let utc: DateTime<Utc> = Utc::now();       // e.g. `2014-11-28T12:45:59.324310806Z`
@@ -122,7 +123,8 @@
 //! This is a bit verbose due to Rust's lack of function and method overloading,
 //! but in turn we get a rich combination of initialization methods.
 //!
-//! ```rust
+#![cfg_attr(not(feature = "std"), doc = "```ignore")]
+#![cfg_attr(feature = "std", doc = "```rust")]
 //! use chrono::prelude::*;
 //! use chrono::offset::LocalResult;
 //!
@@ -314,7 +316,8 @@
 //! [`DateTime.timestamp_subsec_nanos`](./struct.DateTime.html#method.timestamp_subsec_nanos)
 //! to get the number of additional number of nanoseconds.
 //!
-//! ```rust
+#![cfg_attr(not(feature = "std"), doc = "```ignore")]
+#![cfg_attr(feature = "std", doc = "```rust")]
 //! // We need the trait in scope to use Utc::timestamp().
 //! use chrono::{DateTime, TimeZone, Utc};
 //!
@@ -333,7 +336,8 @@
 //! It also has time zones attached, and have to be constructed via time zones.
 //! Most operations available to `DateTime` are also available to `Date` whenever appropriate.
 //!
-//! ```rust
+#![cfg_attr(not(feature = "std"), doc = "```ignore")]
+#![cfg_attr(feature = "std", doc = "```rust")]
 //! use chrono::prelude::*;
 //! use chrono::offset::LocalResult;
 //!

src/naive/time/mod.rs

@@ -818,7 +818,8 @@ impl Timelike for NaiveTime {
     /// ([Why?](#leap-second-handling))
     /// Use the proper [formatting method](#method.format) to get a human-readable representation.
     ///
-    /// ```
+    #[cfg_attr(not(feature = "std"), doc = "```ignore")]
+    #[cfg_attr(feature = "std", doc = "```")]
     /// # use chrono::{NaiveTime, Timelike};
     /// let leap = NaiveTime::from_hms_milli_opt(23, 59, 59, 1_000).unwrap();
     /// assert_eq!(leap.second(), 59);
@@ -846,7 +847,8 @@ impl Timelike for NaiveTime {
     /// You can reduce the range with `time.nanosecond() % 1_000_000_000`, or
     /// use the proper [formatting method](#method.format) to get a human-readable representation.
     ///
-    /// ```
+    #[cfg_attr(not(feature = "std"), doc = "```ignore")]
+    #[cfg_attr(feature = "std", doc = "```")]
     /// # use chrono::{NaiveTime, Timelike};
     /// let leap = NaiveTime::from_hms_milli_opt(23, 59, 59, 1_000).unwrap();
     /// assert_eq!(leap.nanosecond(), 1_000_000_000);

src/offset/fixed.rs

@@ -46,7 +46,8 @@ impl FixedOffset {
     ///
     /// # Example
     ///
-    /// ```
+    #[cfg_attr(not(feature = "std"), doc = "```ignore")]
+    #[cfg_attr(feature = "std", doc = "```")]
     /// use chrono::{FixedOffset, TimeZone};
     /// let hour = 3600;
     /// let datetime = FixedOffset::east_opt(5 * hour).unwrap().ymd_opt(2016, 11, 08).unwrap()
@@ -79,7 +80,8 @@ impl FixedOffset {
     ///
     /// # Example
     ///
-    /// ```
+    #[cfg_attr(not(feature = "std"), doc = "```ignore")]
+    #[cfg_attr(feature = "std", doc = "```")]
     /// use chrono::{FixedOffset, TimeZone};
     /// let hour = 3600;
     /// let datetime = FixedOffset::west_opt(5 * hour).unwrap().ymd_opt(2016, 11, 08).unwrap()