Auto merge of #117148 - dtolnay:sinceversion, r=cjgillot

Store #[stable] attribute's `since` value in structured form

Followup to rust-lang/rust#116773 (review).

Prior to this PR, if you wrote an improper `since` version in a `stable` attribute, such as `#[stable(feature = "foo", since = "wat.0")]`, rustc would emit a diagnostic saying **_'since' must be a Rust version number, such as "1.31.0"_** and then throw out the whole `stable` attribute as if it weren't there. This strategy had 2 problems, both fixed in this PR:

1. If there was also a `#[deprecated]` attribute on the same item, rustc would want to enforce that the stabilization version is older than the deprecation version. This involved reparsing the `stable` attribute's `since` version, with a diagnostic **_invalid stability version found_** if it failed to parse. Of course this diagnostic was unreachable because an invalid `since` version would have already caused the `stable` attribute to be thrown out. This PR deletes that unreachable diagnostic.

2. By throwing out the `stable` attribute when `since` is invalid, you'd end up with a second diagnostic saying **_function has missing stability attribute_** even though your function is not missing a stability attribute. This PR preserves the `stable` attribute even when `since` cannot be parsed, avoiding the misleading second diagnostic.

Followups I plan to try next:

- Do the same for the `since` value of `#[deprecated]`.

- See whether it makes sense to also preserve `stable` and/or `unstable` attributes when they contain an invalid `feature`. What redundant/misleading diagnostics can this eliminate? What problems arise from not having a usable feature name for some API, in the situation that we're already failing compilation, so not concerned about anything that happens in downstream code?

clippy_utils/src/qualify_min_const_fn.rs

@@ -5,6 +5,7 @@
 
 use crate::msrvs::Msrv;
 use hir::LangItem;
+use rustc_attr::{Since, CURRENT_RUSTC_VERSION};
 use rustc_const_eval::transform::check_consts::ConstCx;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
@@ -370,19 +371,24 @@ fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId, msrv: &Msrv) -> bool {
                 // function could be removed if `rustc` provided a MSRV-aware version of `is_const_fn`.
                 // as a part of an unimplemented MSRV check https://github.com/rust-lang/rust/issues/65262.
 
-                // HACK(nilstrieb): CURRENT_RUSTC_VERSION can return versions like 1.66.0-dev. `rustc-semver`
-                // doesn't accept the `-dev` version number so we have to strip it off.
-                let short_version = since
-                    .as_str()
-                    .split('-')
-                    .next()
-                    .expect("rustc_attr::StabilityLevel::Stable::since` is empty");
+                let const_stab_rust_version = match since {
+                    Since::Version(version) => RustcVersion::new(
+                        u32::from(version.major),
+                        u32::from(version.minor),
+                        u32::from(version.patch),
+                    ),
+                    Since::Current => {
+                        // HACK(nilstrieb): CURRENT_RUSTC_VERSION can return versions like 1.66.0-dev.
+                        // `rustc-semver` doesn't accept the `-dev` version number so we have to strip it off.
+                        let short_version = CURRENT_RUSTC_VERSION.split('-').next().unwrap();
+                        RustcVersion::parse(short_version).unwrap_or_else(|err| {
+                            panic!("`rustc_attr::StabilityLevel::Stable::since` is ill-formatted: `{CURRENT_RUSTC_VERSION}`, {err:?}")
+                        })
+                    },
+                    Since::Err => return false,
+                };
 
-                let since = rustc_span::Symbol::intern(short_version);
-
-                msrv.meets(RustcVersion::parse(since.as_str()).unwrap_or_else(|err| {
-                    panic!("`rustc_attr::StabilityLevel::Stable::since` is ill-formatted: `{since}`, {err:?}")
-                }))
+                msrv.meets(const_stab_rust_version)
             } else {
                 // Unstable const fn with the feature enabled.
                 msrv.current().is_none()