Auto merge of rust-lang#88988 - Mark-Simulacrum:avoid-into-ok, r=nagisa

Avoid codegen for Result::into_ok in lang_start

This extra codegen seems to be the cause for the regressions in max-rss on rust-lang#86034. While LLVM will certainly optimize the dead code away, avoiding it's generation in the first place seems good, particularly when it is so simple.

rust-lang#86034 produced this [diff](https://gist.github.com/Mark-Simulacrum/95c7599883093af3b960c35ffadf4dab#file-86034-diff) for a simple `fn main() {}`. With this PR, that diff [becomes limited to just a few extra IR instructions](https://gist.github.com/Mark-Simulacrum/95c7599883093af3b960c35ffadf4dab#file-88988-from-pre-diff) -- no extra functions.

Note that these are pre-optimization; LLVM surely will eliminate this during optimization. However, that optimization can end up generating more work and bump memory usage, and this eliminates that.

Author: bors

library/std/src/rt.rs

@@ -60,10 +60,10 @@ fn lang_start<T: crate::process::Termination + 'static>(
     argc: isize,
     argv: *const *const u8,
 ) -> isize {
-    lang_start_internal(
+    let Ok(v) = lang_start_internal(
         &move || crate::sys_common::backtrace::__rust_begin_short_backtrace(main).report(),
         argc,
         argv,
-    )
-    .into_ok()
+    );
+    v
 }