Support unstable-less nightly

This change attempts to put unstable features behind an opt-in feature
flag as requested by issue #120. Unstable lints and compile_test.rs are
both gated by this feature which is added unconditionally to all CI
that touches more than just stable.

As expressed in issue #120, I have some concerns about *needing* to gate
compile_test.rs behind feature = "unstable". I'm not certain that this
unstable-less nightly eyre will work in production due to the reason
that compile_test.rs fails: three of our dependencies, anyhow,
thiserror, and proc-macro, all use a test for a nightly toolchain to
gate unstable features. Maybe I'm wrong though and this change is
sufficient!

.github/workflows/ci.yml

@@ -44,9 +44,9 @@ jobs:
         features:
           - # default
           - --no-default-features
-          - --features track-caller
-          - --features pyo3
-          - --features auto-install
+          - --features unstable track-caller
+          - --features unstable pyo3
+          - --features unstable auto-install
           - --all-features
     steps:
       - uses: actions/checkout@v1

color-spantrace/Cargo.toml

@@ -11,6 +11,12 @@ repository = { workspace = true }
 readme = { workspace = true }
 rust-version = { workspace = true }
 
+[features]
+default = ["auto-install", "track-caller"]
+auto-install = []
+track-caller = []
+unstable = []
+
 [dependencies]
 tracing-error = "0.2.0"
 tracing-core = "0.1.21"

color-spantrace/src/lib.rs

@@ -61,7 +61,7 @@
 //! [`color-backtrace`]: https://github.com/athre0z/color-backtrace
 #![doc(html_root_url = "https://docs.rs/color-spantrace/0.2.0")]
 #![cfg_attr(
-    nightly,
+    all(nightly, feature = "unstable"),
     feature(rustdoc_missing_doc_code_examples),
     warn(rustdoc::missing_doc_code_examples)
 )]

eyre/Cargo.toml

@@ -16,6 +16,7 @@ rust-version = { workspace = true }
 default = ["auto-install", "track-caller"]
 auto-install = []
 track-caller = []
+unstable = []
 
 [dependencies]
 indenter = { workspace = true }

eyre/src/lib.rs

@@ -316,7 +316,7 @@
 //! [`color-backtrace`]: https://github.com/athre0z/color-backtrace
 #![doc(html_root_url = "https://docs.rs/eyre/0.6.8")]
 #![cfg_attr(
-    nightly,
+    all(nightly, feature = "unstable"),
     feature(rustdoc_missing_doc_code_examples),
     warn(rustdoc::missing_doc_code_examples)
 )]

eyre/tests/compiletest.rs

@@ -1,4 +1,4 @@
-#[rustversion::attr(not(nightly), ignore)]
+#[cfg_attr(any(not(nightly), not(feature = "unstable")), ignore)]
 #[cfg_attr(miri, ignore)]
 #[test]
 fn ui() {