Don't dirty shadow.rs unless contents have changed

Stop spurious rebuilds for any project that uses shadow-rs - even if no
source has changed, because the shadow.rs file is blindly clobbered on
every build, cargo thinks something has changed due to the directive to
rebuild if shadow.rs changes. Instead, check if any of the new contents
are actually different, ignoring the "Generation time" header.

src/lib.rs

@@ -327,6 +327,17 @@ pub(crate) fn get_std_env() -> BTreeMap<String, String> {
     env_map
 }
 
+fn is_same_contents<P1: AsRef<Path>, P2: AsRef<Path>>(p1: P1, p2: P2) -> SdResult<bool> {
+    let c1 = std::fs::read_to_string(p1.as_ref())?;
+    let c2 = std::fs::read_to_string(p2.as_ref())?;
+
+    // Skip the auto-generated header comment which always changes
+    let c1 = c1.split('\n').skip_while(|line| line.starts_with("//")).collect::<Vec<_>>();
+    let c2 = c2.split('\n').skip_while(|line| line.starts_with("//")).collect::<Vec<_>>();
+
+    Ok(c1 == c2)
+}
+
 /// `shadow-rs` configuration.
 ///
 /// This struct encapsulates the configuration for the `shadow-rs` build process. It allows for fine-grained control over
@@ -474,9 +485,10 @@ impl Shadow {
                 path.join(DEFINE_SHADOW_RS)
             }
         };
+        let tmp_out = out.with_file_name(format!(".{DEFINE_SHADOW_RS}.tmp"));
 
         let mut shadow = Shadow {
-            f: File::create(out)?,
+            f: File::create(&tmp_out)?,
             map: Default::default(),
             std_env: Default::default(),
             deny_const,
@@ -502,6 +514,10 @@ impl Shadow {
 
         shadow.write_all()?;
 
+        if !is_same_contents(&tmp_out, &out).unwrap_or(false) {
+            std::fs::rename(tmp_out, out)?;
+        }
+
         // handle hook
         if let Some(h) = builder.get_hook() {
             shadow.hook(h.hook_inner())?