clarify the meaning of the version number for accepted/removed features

compiler/rustc_feature/src/accepted.rs

@@ -42,6 +42,10 @@ declare_features! (
     // feature-group-start: accepted features
     // -------------------------------------------------------------------------
 
+    // Note that the version indicates when it got *stabilized*.
+    // When moving an unstable feature here, set the version number to
+    // `CURRENT RUSTC VERSION` with ` ` replaced by `_`.
+
     /// Allows `#[target_feature(...)]` on aarch64 platforms
     (accepted, aarch64_target_feature, "1.61.0", Some(44839)),
     /// Allows using the `efiapi` ABI.

compiler/rustc_feature/src/lib.rs

@@ -31,6 +31,10 @@ use std::num::NonZero;
 #[derive(Debug, Clone)]
 pub struct Feature {
     pub name: Symbol,
+    /// For unstable features: the version the feature was added in.
+    /// For accepted features: the version the feature got stabilized in.
+    /// For removed features we are inconsistent; sometimes this is the
+    /// version it got added, sometimes the version it got removed.
     pub since: &'static str,
     issue: Option<NonZero<u32>>,
 }

compiler/rustc_feature/src/removed.rs

@@ -32,6 +32,12 @@ declare_features! (
     // feature-group-start: removed features
     // -------------------------------------------------------------------------
 
+    // Note that the version indicates when it got *removed*.
+    // When moving an unstable feature here, set the version number to
+    // `CURRENT RUSTC VERSION` with ` ` replaced by `_`.
+    // (But not all features below do this properly; many indicate the
+    // version they got originally added in.)
+
     /// Allows using the `amdgpu-kernel` ABI.
     (removed, abi_amdgpu_kernel, "1.77.0", Some(51575), None),
     (removed, advanced_slice_patterns, "1.0.0", Some(62254),