Add rust-version metadata

Follow up to rust-lang/cargo#9967

This didn't make it into 1.56 with the rust-version stabilization, but
only into 1.58.

src/lib.rs

@@ -345,6 +345,8 @@ pub struct Package {
     ///
     /// This is always `None` if running with a version of Cargo older than 1.39.
     pub publish: Option<Vec<String>>,
+    /// Minimum Supported Rust Version
+    pub rust_version: Option<String>,
 }
 
 impl Package {

tests/all/Cargo.toml

@@ -14,6 +14,7 @@ homepage = "https://github.com/oli-obk/cargo_metadata/"
 documentation = "https://docs.rs/cargo_metadata/"
 links = "foo"
 publish = false
+rust-version = "1.58"
 
 [package.metadata.docs.rs]
 all-features = true

tests/test_samples.rs

@@ -155,10 +155,10 @@ struct TestObject {
 
 #[test]
 fn all_the_fields() {
-    // All the fields currently generated as of 1.49. This tries to exercise as
+    // All the fields currently generated as of 1.58. This tries to exercise as
     // much as possible.
     let ver = cargo_version();
-    let minimum = semver::Version::parse("1.49.0").unwrap();
+    let minimum = semver::Version::parse("1.58.0").unwrap();
     if ver < minimum {
         // edition added in 1.30
         // rename added in 1.31
@@ -170,6 +170,7 @@ fn all_the_fields() {
         // homepage added in 1.49
         // documentation added in 1.49
         // path added in 1.51
+        // rust-version added in 1.58
         eprintln!("Skipping all_the_fields test, cargo {} is too old.", ver);
         return;
     }
@@ -200,6 +201,7 @@ fn all_the_fields() {
     assert!(all.license_file().unwrap().ends_with("tests/all/LICENSE"));
     assert_eq!(all.publish, Some(vec![]));
     assert_eq!(all.links, Some("foo".to_string()));
+    assert_eq!(all.rust_version, Some("1.58".to_string()));
 
     assert_eq!(all.dependencies.len(), 8);
     let bitflags = all