Add test verifying that changing comments recompiles dependents

Author: yaahc

tests/run-make/rdr-ignores-comments/after.rs

@@ -0,0 +1,8 @@
+#![crate_name = "foo"]
+#![crate_type = "lib"]
+// bbb
+pub struct Foo;
+
+impl Foo {
+    pub fn bar(self: Foo) {}
+}

tests/run-make/rdr-ignores-comments/before.rs

@@ -0,0 +1,8 @@
+#![crate_name = "foo"]
+#![crate_type = "lib"]
+// aaa
+pub struct Foo;
+
+impl Foo {
+    pub fn bar(self: Foo) {}
+}

tests/run-make/rdr-ignores-comments/rmake.rs

@@ -0,0 +1,25 @@
+// tests that we can reuse an existing rlib instead of rebuilding
+// when changin a non-doc comments in a file
+use run_make_support::{rfs, rustc};
+use std::path::Path;
+use std::hash::{Hash, Hasher};
+
+fn main() {
+    let before = check_and_hash("before.rs");
+    let after = check_and_hash("after.rs");
+    dbg!(before, after);
+    assert_eq!(before, after);
+}
+
+fn check_and_hash<P>(filename: P) -> u64
+where
+    P: AsRef<Path>,
+{
+    rfs::rename(filename, "foo.rs");
+    rustc().input("foo.rs").emit("metadata").run();
+    // hash the output
+    let bytes = rfs::read("libfoo.rmeta");
+    let mut hasher = std::hash::DefaultHasher::new();
+    bytes.hash(&mut hasher);
+    hasher.finish()
+}