Make it harder to accidentally use private features

In particular if built with `--all-features` or `features = "all"` we don't
want to accidentally make it look like `YearFlags` are something that people
should use. So: hide it from docs and put it behind a dunder.

Cargo.toml

@@ -29,6 +29,7 @@ alloc = []
 std = []
 clock = ["time", "std"]
 wasmbind = ["wasm-bindgen", "js-sys"]
+__internal_bench = []
 
 [dependencies]
 time = { version = "0.1.39", optional = true }

benches/chrono.rs

@@ -6,7 +6,7 @@ extern crate criterion;
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
 
 use chrono::prelude::*;
-use chrono::{Utc, FixedOffset, DateTime, YearFlags};
+use chrono::{Utc, FixedOffset, DateTime, __BenchYearFlags};
 
 fn bench_datetime_parse_from_rfc2822(c: &mut Criterion) {
     c.bench_function("bench_datetime_parse_from_rfc2822", |b| {
@@ -56,7 +56,7 @@ fn bench_year_flags_from_year(c: &mut Criterion) {
     c.bench_function("bench_year_flags_from_year", |b|
                      b.iter(|| {
         for year in -999i32..1000 {
-            YearFlags::from_year(year);
+            __BenchYearFlags::from_year(year);
         }
     }));
 }

src/lib.rs

@@ -477,10 +477,7 @@ pub mod naive {
     //! (e.g. [`TimeZone`](../offset/trait.TimeZone.html)),
     //! but can be also used for the simpler date and time handling.
 
-    #[cfg(not(feature = "__internal_bench"))]
     mod internals;
-    #[cfg(feature = "__internal_bench")]
-    pub mod internals;
     mod date;
     mod isoweek;
     mod time;
@@ -494,6 +491,10 @@ pub mod naive {
     #[allow(deprecated)]
     pub use self::datetime::rustc_serialize::TsSeconds;
 
+    #[cfg(feature = "__internal_bench")]
+    #[doc(hidden)]
+    pub use self::internals::YearFlags as __BenchYearFlags;
+
 
     /// Serialization/Deserialization of naive types in alternate formats
     ///
@@ -514,7 +515,8 @@ pub mod format;
 mod round;
 
 #[cfg(feature = "__internal_bench")]
-pub use naive::internals::YearFlags;
+#[doc(hidden)]
+pub use naive::__BenchYearFlags;
 
 /// Serialization/Deserialization in alternate formats
 ///