Hint upgrading for future edition keys

Author: kornelski

src/cargo/core/features.rs

@@ -82,6 +82,11 @@ impl FromStr for Edition {
         match s {
             "2015" => Ok(Edition::Edition2015),
             "2018" => Ok(Edition::Edition2018),
+            s if s.parse().map_or(false, |y: u16| y > 2020 && y < 2050) => bail!(
+                "this version of Cargo is older than the `{}` edition, \
+                 and only supports `2015` and `2018` editions.",
+                s
+            ),
             s => bail!(
                 "supported edition values are `2015` or `2018`, but `{}` \
                  is unknown",

tests/testsuite/package.rs

@@ -1086,6 +1086,38 @@ Caused by:
         .run();
 }
 
+#[cargo_test]
+fn test_edition_from_the_future() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"[package]
+                edition = "2038"
+                name = "foo"
+                version = "99.99.99"
+                authors = []
+            "#,
+        )
+        .file("src/main.rs", r#""#)
+        .build();
+
+    p.cargo("build")
+        .with_status(101)
+        .with_stderr(
+            "\
+error: failed to parse manifest at `[..]`
+
+Caused by:
+  failed to parse the `edition` key
+
+Caused by:
+  this version of Cargo is older than the `2038` edition, and only supports `2015` and `2018` editions.
+"
+            .to_string(),
+        )
+        .run();
+}
+
 #[cargo_test]
 fn do_not_package_if_src_was_modified() {
     let p = project()