Allow Bazel's own build to use Java 11 language features

[cushon]

Bazel's build now requires a minimum of JDK 11, but it's still configured to use Java 8 language features.

We should enable Java 11 language features for Bazel.

e.g.

 git diff
diff --git a/src/main/java/com/google/devtools/build/lib/vfs/DigestHashFunction.java b/src/main/java/com/google/devtools/build/lib/vfs/DigestHashFunction.java
index 8d5fd508f8..e760bb31d0 100644
--- a/src/main/java/com/google/devtools/build/lib/vfs/DigestHashFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/vfs/DigestHashFunction.java
@@ -187,7 +187,7 @@ public class DigestHashFunction {

   private static boolean supportsClone(MessageDigest toCheck) {
     try {
-      toCheck.clone();
+      var unused = toCheck.clone();
       return true;
     } catch (CloneNotSupportedException e) {
       return false;

$ bazel build //src:bazel
...
src/main/java/com/google/devtools/build/lib/vfs/DigestHashFunction.java:190: warning: as of release 10, 'var' is a restricted local variable type and cannot be used for type declarations or as the element type of an array
      var unused = toCheck.clone();
          ^

[linzhp]

After checking out latest master and run bazel build //src:bazel, I still got:

src/main/java/com/google/devtools/build/lib/vfs/DigestHashFunction.java:190: error: cannot find symbol
      var unused = toCheck.clone();
      ^
  symbol:   class var
  location: class DigestHashFunction

My local javac -version is "javac 11.0.11". What should I do to compile Java 11 features in Bazel source code?

[cushon]

Are you using bazel 5.0.0?

[linzhp]

Ah, Bazel 5.0 fixed the issue. I was on Bazel 4.2.2. Didn't realize that Bazel now requires 5.0 to build itself 3 days after its released.

[damingerdai]

but i also meet this issue with bazel 5.0
https://github.com/damingerdai/jobs/runs/7263067500?check_suite_focus=true