Optimize Vec::retain

[oxalica]

Use copy_non_overlapping instead of swap to reduce memory writes, like what we've done in #44355 and String::retain.
#48065 already tried to do this optimization but it is reverted in #67300 due to bad codegen of DrainFilter::drop.

This PR re-implement the drop-then-move approach. I did a benchmark on small-no-drop, small-need-drop, large-no-drop elements with different predicate functions. It turns out that the new implementation is >20% faster in average for almost all cases. Only 2/24 cases are slower by 3% and 5%. See the link above for more detail.

I think regression in may-panic cases is due to drop-guard preventing some optimization. If it's permitted to leak elements when predicate function of element's drop panic, the new implementation should be almost always faster than current one.
I'm not sure if we should leak on panic, since there is indeed an issue (#52267) complains about it before.

[rust-highfive]

r? @kennytm

(rust-highfive has picked a reviewer for you, use r? to override)

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

configure: rust.channel         := nightly
configure: rust.debug-assertions := True
configure: llvm.assertions      := True
configure: dist.missing-tools   := True
configure: build.configure-args := ['--enable-sccache', '--disable-manage-submodu ...
configure: writing `config.toml` in current directory
configure: 
configure: run `python /checkout/x.py --help`
configure: 
---
Diff in /checkout/library/alloc/src/vec/mod.rs at line 1411:
             }
         }
 
-        let mut guard = BackshiftOnDrop {
-            v: self,
-            processed_len: 0,
-            deleted_cnt: 0,
-            original_len: len,
+        let mut guard =
+        let mut guard =
Running `"/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/rustfmt" "--config-path" "/checkout" "--edition" "2018" "--unstable-features" "--skip-children" "--check" "/checkout/library/alloc/src/vec/mod.rs"` failed.
+            BackshiftOnDrop { v: self, processed_len: 0, deleted_cnt: 0, original_len: len };
 
         let mut del = 0usize;
         for i in 0..len {
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.
Build completed unsuccessfully in 0:00:17

[m-ou-se]

Thanks for working on this!

I have a few comments:

[oxalica]

@m-ou-se Fixed. Also updated the benchmark result.

[m-ou-se]

@bors r+

[bors]

📌 Commit 2a11c57 has been approved by m-ou-se

[m-ou-se]

Thanks for working on this!

[bors]

⌛ Testing commit 2a11c57 with merge 1efd804...

[bors]

☀️ Test successful - checks-actions
Approved by: m-ou-se
Pushing 1efd804 to master...