Privatise docsrs cfg flag

Fixes a docs.rs build failure by following the same approach used in
rustls, removing `doc_auto_cfg` and privatizing the docsrs cfg flag.

```
[INFO] [stderr]  Documenting instant-acme v0.8.3 (/opt/rustwide/workdir)
[INFO] [stderr] error[E0557]: feature has been removed
[INFO] [stderr]  --> src/lib.rs:5:38
[INFO] [stderr]   |
[INFO] [stderr] 5 | #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
[INFO] [stderr]   |                                      ^^^^^^^^^^^^ feature has been removed
[INFO] [stderr]   |
[INFO] [stderr]   = note: removed in CURRENT_RUSTC_VERSION; see <rust-lang/rust#138907> for more information
[INFO] [stderr]   = note: merged into `doc_cfg`
[INFO] [stderr]
[INFO] [stderr] error: Compilation failed, aborting rustdoc
```

Author: cpu

.github/workflows/rust.yml

@@ -76,7 +76,7 @@ jobs:
       - uses: dtolnay/rust-toolchain@stable
       - run: cargo doc --locked --all-features --no-deps --document-private-items
         env:
-          RUSTDOCFLAGS: -Dwarnings
+          RUSTDOCFLAGS: -Dwarnings --cfg=instant_acme_docsrs
 
   semver:
     name: Check semver compatibility

Cargo.toml

@@ -68,7 +68,7 @@ required-features = ["hyper-rustls"]
 [package.metadata.docs.rs]
 # all non-default features except fips (cannot build on docs.rs environment)
 features = ["aws-lc-rs", "x509-parser", "time"]
-rustdoc-args = ["--cfg", "docsrs"]
+rustdoc-args = ["--cfg", "instant_acme_docsrs"]
 
 [package.metadata.cargo_check_external_types]
 allowed_external_types = [
@@ -80,3 +80,7 @@ allowed_external_types = [
     "serde::*",
     "serde_json::*",
 ]
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ["cfg(instant_acme_docsrs)"] }
+

src/lib.rs

@@ -2,7 +2,7 @@
 
 #![warn(unreachable_pub)]
 #![warn(missing_docs)]
-#![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
+#![cfg_attr(instant_acme_docsrs, feature(doc_cfg))]
 
 use std::convert::Infallible;
 use std::error::Error as StdError;