Rollup merge of #144519 - clarfonthey:const-system-time, r=jhpratt

Constify `SystemTime` methods

This is separated out from rust-lang/rust#143949 due to the fact that it touches nontrivial system code. While the same arithmetic methods on `Instant` could be made const, since that type explicitly cannot be constructed at const-time, we don't bother. However, due to the fact that `SystemTime::UNIX_EPOCH` exists, it can be useful to create other anchors at other points in time, and thus these methods should be possible to use in const context.

> Side comment: I would honestly like to just move every single implementation which is a thin wrapper over `Duration` or some integer clock into their own module which is tested on all targets, so that we don't have to worry about code for lower-tier targets not being tested. However, the comments in `std::sys_common` pointing out the desire to move things to `std::sys::common` confused me, particularly due to the fact that `std::sys::common` is taken from the hellish asterisk-import of `std::sys::pal::common` and others.
>
> I think that, for trivial types like this, we should just have a platform-independent module that can be tested on all platforms, to both ensure that the code is properly tested regardless of whether higher-tier platforms needed, and to avoid the extreme mental gymnastics required to determine where in the code it lies.
>
> However, since I'm not on any of the teams maintaining this code and am not involved, I'll settle for just copy-pasting like everyone else has been doing. I just want to express how I'm not happy about that.

**Reviewer note: please only pay attention to the last commit of this PR, until its blocker is merged.**

Since this depends on the previous PR:
`@rustbot` blocked

1 files changed, 6 insertions, 0 deletions

diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 0c537530647..25355efd20e 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -281,9 +281,11 @@
 #![feature(cfg_target_thread_local)]
 #![feature(cfi_encoding)]
 #![feature(char_max_len)]
+#![feature(const_trait_impl)]
 #![feature(core_float_math)]
 #![feature(decl_macro)]
 #![feature(deprecated_suggestion)]
+#![feature(derive_const)]
 #![feature(doc_cfg)]
 #![feature(doc_cfg_hide)]
 #![feature(doc_masked)]
@@ -329,6 +331,10 @@
 #![feature(bstr_internals)]
 #![feature(char_internals)]
 #![feature(clone_to_uninit)]
+#![feature(const_cmp)]
+#![feature(const_ops)]
+#![feature(const_option_ops)]
+#![feature(const_try)]
 #![feature(core_intrinsics)]
 #![feature(core_io_borrowed_buf)]
 #![feature(drop_guard)]