[6.4.0] Also apply NestedSet optimizations to Depset (#19492)

If the construction of a `NestedSet` involves only a single non-empty
transitive set, this set may be returned directly as an optimization.
This commit checks for this case when constructing a `Depset` and reuses
the corresponding `Depset` if possible.

Since `Depset`s that are direct elements of providers are usually
unwrapped into their `NestedSet`, this optimization only applies to
instances stored in `struct`s as well as reduces allocations.

Realizes an optimization proposed by @brentleyjones in
https://bazelbuild.slack.com/archives/CA31HN1T3/p1693487067454409?thread_ts=1693484609.534579&cid=CA31HN1T3.

Closes #19379.

Commit
216fce5

PiperOrigin-RevId: 563679197
Change-Id: I542bbf56784832768ca3bbbd550cc78bfb981ffe

Co-authored-by: Fabian Meumertzheim <fabian@meumertzhe.im>

src/main/java/com/google/devtools/build/lib/collect/nestedset/Depset.java

@@ -392,7 +392,16 @@ static Depset fromDirectAndTransitive(
     if (builder.isEmpty()) {
       return builder.getOrder().emptyDepset();
     }
-    return new Depset(type, builder.build());
+    NestedSet<Object> set = builder.build();
+    // If the nested set was optimized to one of the transitive elements, reuse the corresponding
+    // depset.
+    for (Depset x : transitive) {
+      if (x.getSet() == set) {
+        return x;
+      }
+    }
+
+    return new Depset(type, set);
   }
 
   /** An exception thrown when validation fails on the type of elements of a nested set. */

src/test/java/com/google/devtools/build/lib/collect/nestedset/DepsetTest.java

@@ -417,4 +417,17 @@ public void testEmptyDepsetInternedPerOrder() throws Exception {
     assertThat(ev.lookup("stable1")).isNotSameInstanceAs(ev.lookup("preorder1"));
     assertThat(ev.lookup("stable2")).isNotSameInstanceAs(ev.lookup("preorder2"));
   }
+
+  @Test
+  public void testSingleNonEmptyTransitiveAndNoDirectsUnwrapped() throws Exception {
+    ev.exec(
+        "inner = depset([1, 2, 3])", "outer = depset(transitive = [depset(), inner, depset()])");
+    assertThat(ev.lookup("outer")).isSameInstanceAs(ev.lookup("inner"));
+  }
+
+  @Test
+  public void testSingleNonEmptyTransitiveAndMatchingDirectUnwrapped() throws Exception {
+    ev.exec("inner = depset([1])", "outer = depset([1], transitive = [depset(), inner, depset()])");
+    assertThat(ev.lookup("outer")).isSameInstanceAs(ev.lookup("inner"));
+  }
 }