Add more benchmarks of Vec::dedup

They are for more specific cases than old benches.
Also, better usage of blackbox

library/alloc/benches/vec.rs

@@ -658,13 +658,16 @@ fn random_sorted_fill(mut seed: u32, buf: &mut [u32]) {
     buf.sort();
 }
 
-fn bench_vec_dedup_old(b: &mut Bencher, sz: usize) {
+// Measures performance of slice dedup impl.
+// "Old" implementation of Vec::dedup
+fn bench_dedup_slice_truncate(b: &mut Bencher, sz: usize) {
     let mut template = vec![0u32; sz];
     b.bytes = std::mem::size_of_val(template.as_slice()) as u64;
     random_sorted_fill(0x43, &mut template);
 
     let mut vec = template.clone();
     b.iter(|| {
+        let vec = black_box(&mut vec);
         let len = {
             let (dedup, _) = vec.partition_dedup();
             dedup.len()
@@ -672,59 +675,143 @@ fn bench_vec_dedup_old(b: &mut Bencher, sz: usize) {
         vec.truncate(len);
 
         black_box(vec.first());
+        let vec = black_box(vec);
         vec.clear();
         vec.extend_from_slice(&template);
     });
 }
 
-fn bench_vec_dedup_new(b: &mut Bencher, sz: usize) {
+// Measures performance of Vec::dedup on random data.
+fn bench_vec_dedup_random(b: &mut Bencher, sz: usize) {
     let mut template = vec![0u32; sz];
     b.bytes = std::mem::size_of_val(template.as_slice()) as u64;
     random_sorted_fill(0x43, &mut template);
 
     let mut vec = template.clone();
     b.iter(|| {
+        let vec = black_box(&mut vec);
         vec.dedup();
         black_box(vec.first());
+        let vec = black_box(vec);
+        vec.clear();
+        vec.extend_from_slice(&template);
+    });
+}
+
+// Measures performance of Vec::dedup when there is no items removed
+fn bench_vec_dedup_none(b: &mut Bencher, sz: usize) {
+    let mut template = vec![0u32; sz];
+    b.bytes = std::mem::size_of_val(template.as_slice()) as u64;
+    template.chunks_exact_mut(2).for_each(|w| {
+        w[0] = black_box(0);
+        w[1] = black_box(5);
+    });
+
+    let mut vec = template.clone();
+    b.iter(|| {
+        let vec = black_box(&mut vec);
+        vec.dedup();
+        black_box(vec.first());
+        // Unlike other benches of `dedup`
+        // this doesn't reinitialize vec
+        // because we measure how effecient dedup is
+        // when no memory written
+    });
+}
+
+// Measures performance of Vec::dedup when there is all items removed
+fn bench_vec_dedup_all(b: &mut Bencher, sz: usize) {
+    let mut template = vec![0u32; sz];
+    b.bytes = std::mem::size_of_val(template.as_slice()) as u64;
+    template.iter_mut().for_each(|w| {
+        *w = black_box(0);
+    });
+
+    let mut vec = template.clone();
+    b.iter(|| {
+        let vec = black_box(&mut vec);
+        vec.dedup();
+        black_box(vec.first());
+        let vec = black_box(vec);
         vec.clear();
         vec.extend_from_slice(&template);
     });
 }
 
 #[bench]
-fn bench_dedup_old_100(b: &mut Bencher) {
-    bench_vec_dedup_old(b, 100);
+fn bench_dedup_slice_truncate_100(b: &mut Bencher) {
+    bench_dedup_slice_truncate(b, 100);
 }
 #[bench]
-fn bench_dedup_new_100(b: &mut Bencher) {
-    bench_vec_dedup_new(b, 100);
+fn bench_dedup_random_100(b: &mut Bencher) {
+    bench_vec_dedup_random(b, 100);
 }
 
 #[bench]
-fn bench_dedup_old_1000(b: &mut Bencher) {
-    bench_vec_dedup_old(b, 1000);
-}
-#[bench]
-fn bench_dedup_new_1000(b: &mut Bencher) {
-    bench_vec_dedup_new(b, 1000);
+fn bench_dedup_none_100(b: &mut Bencher) {
+    bench_vec_dedup_none(b, 100);
 }
 
 #[bench]
-fn bench_dedup_old_10000(b: &mut Bencher) {
-    bench_vec_dedup_old(b, 10000);
-}
-#[bench]
-fn bench_dedup_new_10000(b: &mut Bencher) {
-    bench_vec_dedup_new(b, 10000);
+fn bench_dedup_all_100(b: &mut Bencher) {
+    bench_vec_dedup_all(b, 100);
 }
 
 #[bench]
-fn bench_dedup_old_100000(b: &mut Bencher) {
-    bench_vec_dedup_old(b, 100000);
+fn bench_dedup_slice_truncate_1000(b: &mut Bencher) {
+    bench_dedup_slice_truncate(b, 1000);
 }
 #[bench]
-fn bench_dedup_new_100000(b: &mut Bencher) {
-    bench_vec_dedup_new(b, 100000);
+fn bench_dedup_random_1000(b: &mut Bencher) {
+    bench_vec_dedup_random(b, 1000);
+}
+
+#[bench]
+fn bench_dedup_none_1000(b: &mut Bencher) {
+    bench_vec_dedup_none(b, 1000);
+}
+
+#[bench]
+fn bench_dedup_all_1000(b: &mut Bencher) {
+    bench_vec_dedup_all(b, 1000);
+}
+
+#[bench]
+fn bench_dedup_slice_truncate_10000(b: &mut Bencher) {
+    bench_dedup_slice_truncate(b, 10000);
+}
+#[bench]
+fn bench_dedup_random_10000(b: &mut Bencher) {
+    bench_vec_dedup_random(b, 10000);
+}
+
+#[bench]
+fn bench_dedup_none_10000(b: &mut Bencher) {
+    bench_vec_dedup_none(b, 10000);
+}
+
+#[bench]
+fn bench_dedup_all_10000(b: &mut Bencher) {
+    bench_vec_dedup_all(b, 10000);
+}
+
+#[bench]
+fn bench_dedup_slice_truncate_100000(b: &mut Bencher) {
+    bench_dedup_slice_truncate(b, 100000);
+}
+#[bench]
+fn bench_dedup_random_100000(b: &mut Bencher) {
+    bench_vec_dedup_random(b, 100000);
+}
+
+#[bench]
+fn bench_dedup_none_100000(b: &mut Bencher) {
+    bench_vec_dedup_none(b, 100000);
+}
+
+#[bench]
+fn bench_dedup_all_100000(b: &mut Bencher) {
+    bench_vec_dedup_all(b, 100000);
 }
 
 #[bench]