feat(index): Read 'rust_version'

Author: epage

src/cargo/sources/registry/index.rs

@@ -814,6 +814,7 @@ impl IndexSummary {
             features2,
             yanked,
             links,
+            rust_version: _,
             v,
         } = serde_json::from_slice(line)?;
         let v = v.unwrap_or(1);

src/cargo/sources/registry/mod.rs

@@ -287,6 +287,14 @@ pub struct RegistryPackage<'a> {
     /// Added early 2018 (see <https://github.com/rust-lang/cargo/pull/4978>),
     /// can be `None` if published before then.
     links: Option<InternedString>,
+    /// Required version of rust
+    ///
+    /// Corresponds to `package.rust-version`.
+    ///
+    /// Added in 2023 (see <https://github.com/rust-lang/crates.io/pull/6267>),
+    /// can be `None` if published before then or if not set in the manifest.
+    #[allow(dead_code)]
+    rust_version: Option<InternedString>,
     /// The schema version for this entry.
     ///
     /// If this is None, it defaults to version 1. Entries with unknown