Clear out memoized hashes before building crates

Build script updates during execution can change the memoized hash of a
`Fingerprint`, and while previously we cleared out a single build
script's memoized hash we forgot to clear out everything that depended
on it as well. This commit pessimistically clears out all `Fingerprint`
memoized hashes just before building to ensure that during the build
everything has the most up-to-date view of the world, and when build
scripts change fingerprints everything that depends on them won't have
run yet.

Closes #7362

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

@@ -147,6 +147,16 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
             super::compile(&mut self, &mut queue, &mut plan, unit, exec, force_rebuild)?;
         }
 
+        // Now that we've got the full job queue and we've done all our
+        // fingerprint analysis to determine what to run, bust all the memoized
+        // fingerprint hashes to ensure that during the build they all get the
+        // most up-to-date values. In theory we only need to bust hashes that
+        // transitively depend on a dirty build script, but it shouldn't matter
+        // that much for performance anyway.
+        for fingerprint in self.fingerprints.values() {
+            fingerprint.clear_memoized();
+        }
+
         // Now that we've figured out everything that we're going to do, do it!
         queue.execute(&mut self, &mut plan)?;
 

src/cargo/core/compiler/fingerprint.rs

@@ -301,7 +301,6 @@ pub fn prepare_target<'a, 'cfg>(
             // hobble along if it happens to return `Some`.
             if let Some(new_local) = (gen_local)(&deps, None)? {
                 *fingerprint.local.lock().unwrap() = new_local;
-                *fingerprint.memoized_hash.lock().unwrap() = None;
             }
 
             write_fingerprint(&loc, &fingerprint)
@@ -604,6 +603,16 @@ impl Fingerprint {
         }
     }
 
+    /// For performance reasons fingerprints will memoize their own hash, but
+    /// there's also internal mutability with its `local` field which can
+    /// change, for example with build scripts, during a build.
+    ///
+    /// This method can be used to bust all memoized hashes just before a build
+    /// to ensure that after a build completes everything is up-to-date.
+    pub fn clear_memoized(&self) {
+        *self.memoized_hash.lock().unwrap() = None;
+    }
+
     fn hash(&self) -> u64 {
         if let Some(s) = *self.memoized_hash.lock().unwrap() {
             return s;

tests/testsuite/freshness.rs

@@ -2049,3 +2049,74 @@ fn move_target_directory_with_path_deps() {
         .with_stderr("[FINISHED] [..]")
         .run();
 }
+
+#[cargo_test]
+fn rerun_if_changes() {
+    let p = project()
+        .file(
+            "build.rs",
+            r#"
+                fn main() {
+                    println!("cargo:rerun-if-env-changed=FOO");
+                    if std::env::var("FOO").is_ok() {
+                        println!("cargo:rerun-if-env-changed=BAR");
+                    }
+                }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("build").run();
+    p.cargo("build").with_stderr("[FINISHED] [..]").run();
+
+    p.cargo("build -v")
+        .env("FOO", "1")
+        .with_stderr(
+            "\
+[COMPILING] foo [..]
+[RUNNING] `[..]build-script-build`
+[RUNNING] `rustc [..]
+[FINISHED] [..]
+",
+        )
+        .run();
+    p.cargo("build")
+        .env("FOO", "1")
+        .with_stderr("[FINISHED] [..]")
+        .run();
+
+    p.cargo("build -v")
+        .env("FOO", "1")
+        .env("BAR", "1")
+        .with_stderr(
+            "\
+[COMPILING] foo [..]
+[RUNNING] `[..]build-script-build`
+[RUNNING] `rustc [..]
+[FINISHED] [..]
+",
+        )
+        .run();
+    p.cargo("build")
+        .env("FOO", "1")
+        .env("BAR", "1")
+        .with_stderr("[FINISHED] [..]")
+        .run();
+
+    p.cargo("build -v")
+        .env("BAR", "2")
+        .with_stderr(
+            "\
+[COMPILING] foo [..]
+[RUNNING] `[..]build-script-build`
+[RUNNING] `rustc [..]
+[FINISHED] [..]
+",
+        )
+        .run();
+    p.cargo("build")
+        .env("BAR", "2")
+        .with_stderr("[FINISHED] [..]")
+        .run();
+}