test(freshness_checksum): different fingeprint methods are compatible

This ensures that users can switch between different fingerprint
freshness methods and still correctly rebuild stuff.

Author: Xaeroxe

tests/testsuite/freshness_checksum.rs

@@ -53,6 +53,45 @@ fn checksum_actually_uses_checksum() {
         .run();
 }
 
+#[cargo_test(nightly, reason = "requires -Zchecksum-hash-algorithm")]
+fn checksum_build_compatible_with_mtime_build() {
+    let p = project()
+        .file("src/main.rs", "mod a; fn main() {}")
+        .file("src/a.rs", "")
+        .build();
+
+    p.cargo("check -Zchecksum-freshness")
+        .masquerade_as_nightly_cargo(&["checksum-freshness"])
+        .with_stderr_data(str![[r#"
+[CHECKING] foo v0.0.1 ([ROOT]/foo)
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+    p.cargo("check")
+        .with_stderr_data(str![[r#"
+[CHECKING] foo v0.0.1 ([ROOT]/foo)
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+    p.cargo("check -Zchecksum-freshness")
+        .masquerade_as_nightly_cargo(&["checksum-freshness"])
+        .with_stderr_data(str![[r#"
+[CHECKING] foo v0.0.1 ([ROOT]/foo)
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+    p.cargo("check")
+        .with_stderr_data(str![[r#"
+[CHECKING] foo v0.0.1 ([ROOT]/foo)
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}
+
 #[cargo_test(nightly, reason = "requires -Zchecksum-hash-algorithm")]
 fn same_size_different_content() {
     let p = project()