Don't use the full pathname of a custom target json in fingerprint hashes.

This makes it easier to get reproducible builds for custom targets.

Author: shkoo

src/cargo/core/compiler/compile_kind.rs

@@ -182,14 +182,20 @@ impl CompileTarget {
     /// See [`CompileKind::fingerprint_hash`].
     pub fn fingerprint_hash(&self) -> u64 {
         let mut hasher = StableHasher::new();
-        self.name.hash(&mut hasher);
-        if self.name.ends_with(".json") {
-            // This may have some performance concerns, since it is called
-            // fairly often. If that ever seems worth fixing, consider
-            // embedding this in `CompileTarget`.
-            if let Ok(contents) = fs::read_to_string(self.name) {
+        match self
+            .name
+            .ends_with(".json")
+            .then(|| fs::read_to_string(self.name))
+        {
+            Some(Ok(contents)) => {
+                // This may have some performance concerns, since it is called
+                // fairly often. If that ever seems worth fixing, consider
+                // embedding this in `CompileTarget`.
                 contents.hash(&mut hasher);
             }
+            _ => {
+                self.name.hash(&mut hasher);
+            }
         }
         hasher.finish()
     }

src/cargo/core/compiler/context/compilation_files.rs

@@ -587,10 +587,12 @@ fn compute_metadata(
     unit.mode.hash(&mut hasher);
     cx.lto[unit].hash(&mut hasher);
 
-    // Artifacts compiled for the host should have a different metadata
-    // piece than those compiled for the target, so make sure we throw in
-    // the unit's `kind` as well
-    unit.kind.hash(&mut hasher);
+    // Artifacts compiled for the host should have a different
+    // metadata piece than those compiled for the target, so make sure
+    // we throw in the unit's `kind` as well.  Use `fingerprint_hash`
+    // so that the StableHash doesn't change based on the pathnames
+    // of the custom target JSON spec files.
+    unit.kind.fingerprint_hash().hash(&mut hasher);
 
     // Finally throw in the target name/kind. This ensures that concurrent
     // compiles of targets in the same crate don't collide.

tests/testsuite/custom_target.rs

@@ -234,3 +234,42 @@ fn changing_spec_relearns_crate_types() {
         )
         .run();
 }
+
+#[cargo_test]
+fn custom_target_ignores_filepath() {
+    // Changing the path of the .json file will not trigger a rebuild.
+    if !is_nightly() {
+        // Requires features no_core, lang_items
+        return;
+    }
+    let p = project()
+        .file(
+            "src/lib.rs",
+            &"
+                __MINIMAL_LIB__
+
+                pub fn foo() -> u32 {
+                    42
+                }
+            "
+            .replace("__MINIMAL_LIB__", MINIMAL_LIB),
+        )
+        .file("b/custom-target.json", SIMPLE_SPEC)
+        .file("a/custom-target.json", SIMPLE_SPEC)
+        .build();
+
+    // Should build the library the first time.
+    p.cargo("build --lib --target a/custom-target.json")
+        .with_stderr(
+            "\
+[..]Compiling foo v0.0.1 ([..])
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]
+",
+        )
+        .run();
+
+    // But not the second time, even though the path to the custom target is dfferent.
+    p.cargo("build --lib --target b/custom-target.json")
+        .with_stderr("[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]")
+        .run();
+}