test: demonstrate old lockfile compat matrix

tests/testsuite/lockfile_compat.rs

@@ -1161,3 +1161,137 @@ fn v4_and_git_url_encoded_tag() {
 fn v4_and_git_url_encoded_rev() {
     v4_and_git_url_encoded("rev", create_tag)
 }
+
+#[cargo_test]
+fn with_msrv() {
+    let cksum = Package::new("bar", "0.1.0").publish();
+    let v2_lockfile = format!(
+        r#"# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "bar"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "{cksum}"
+
+[[package]]
+name = "foo"
+version = "0.0.1"
+dependencies = [
+ "bar",
+]
+"#
+    );
+
+    let v1_lockfile = format!(
+        r#"# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "bar"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "foo"
+version = "0.0.1"
+dependencies = [
+ "bar 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[metadata]
+"checksum bar 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "{cksum}"
+"#
+    );
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.0.1"
+
+            [dependencies]
+            bar = "0.1.0"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    let cases = [
+        // v1
+        ("1.37", None, 3),
+        ("1.37", Some(1), 1),
+        ("1.37", Some(2), 2),
+        ("1.37", Some(3), 3),
+        // v2 introduced
+        ("1.38", None, 3),
+        ("1.38", Some(1), 1),
+        ("1.38", Some(2), 2),
+        ("1.38", Some(3), 3),
+        // v2 became the default
+        ("1.41", None, 3),
+        ("1.41", Some(1), 1),
+        ("1.41", Some(2), 2),
+        ("1.41", Some(3), 3),
+        // v3 introduced
+        ("1.47", None, 3),
+        ("1.47", Some(1), 1),
+        ("1.47", Some(2), 2),
+        ("1.47", Some(3), 3),
+        // v3 became the default
+        ("1.53", None, 3),
+        ("1.53", Some(1), 1),
+        ("1.53", Some(2), 2),
+        ("1.53", Some(3), 3),
+    ];
+
+    for (msrv, existing_lockfile, expected_version) in cases {
+        // Clean previous lockfile.
+        _ = std::fs::remove_file(p.root().join("Cargo.lock"));
+
+        p.change_file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+                rust-version = "{msrv}"
+
+                [dependencies]
+                bar = "0.1.0"
+                "#,
+            ),
+        );
+
+        if let Some(existing_lockfile) = existing_lockfile {
+            let existing_lockfile = match existing_lockfile {
+                1 => v1_lockfile.as_str().into(),
+                2 => v2_lockfile.as_str().into(),
+                v => std::borrow::Cow::from(format!("version = {v}")),
+            };
+            p.change_file("Cargo.lock", &existing_lockfile);
+        }
+
+        p.cargo("fetch").run();
+
+        let lock = p.read_lockfile();
+        let toml = lock.parse::<toml::Table>().unwrap();
+        // get `version = <n>` from Cargo.lock
+        let version_field = toml.get("version").and_then(|v| v.as_integer());
+
+        let actual_version = if let Some(ver) = version_field {
+            ver
+        } else if lock.find("\nchecksum = ").is_some() {
+            2
+        } else {
+            1
+        };
+
+        assert_eq!(
+            expected_version, actual_version,
+            "msrv: {msrv}, existing lockfile: {existing_lockfile:?}"
+        );
+    }
+}