Auto merge of rust-lang#122975 - DianQK:simplify_ub_check, r=saethlin

Eliminate `UbChecks` for non-standard libraries

 The purpose of this PR is to allow other passes to treat `UbChecks` as constants in MIR for optimization after rust-lang#122629.

r? RalfJung

Author: bors

alloc/src/lib.rs

@@ -176,6 +176,7 @@
 // Language features:
 // tidy-alphabetical-start
 #![cfg_attr(bootstrap, feature(associated_type_bounds))]
+#![cfg_attr(not(bootstrap), rustc_preserve_ub_checks)]
 #![cfg_attr(not(test), feature(coroutine_trait))]
 #![cfg_attr(test, feature(panic_update_hook))]
 #![cfg_attr(test, feature(test))]

core/src/intrinsics.rs

@@ -2686,12 +2686,14 @@ pub const unsafe fn typed_swap<T>(x: *mut T, y: *mut T) {
     unsafe { ptr::swap_nonoverlapping(x, y, 1) };
 }
 
-/// Returns whether we should perform some UB-checking at runtime. This evaluate to the value of
-/// `cfg!(debug_assertions)` during monomorphization.
-///
-/// This intrinsic is evaluated after monomorphization, which is relevant when mixing crates
-/// compiled with and without debug_assertions. The common case here is a user program built with
-/// debug_assertions linked against the distributed sysroot which is built without debug_assertions.
+/// Returns whether we should perform some UB-checking at runtime. This eventually evaluates to
+/// `cfg!(debug_assertions)`, but behaves different from `cfg!` when mixing crates built with different
+/// flags: if the crate has debug assertions enabled or carries the `#[rustc_preserve_ub_checks]`
+/// attribute, evaluation is delayed until monomorphization (or until the call gets inlined into
+/// a crate that does not delay evaluation further); otherwise it can happen any time.
+///
+/// The common case here is a user program built with debug_assertions linked against the distributed
+/// sysroot which is built without debug_assertions but with `#[rustc_preserve_ub_checks]`.
 /// For code that gets monomorphized in the user crate (i.e., generic functions and functions with
 /// `#[inline]`), gating assertions on `ub_checks()` rather than `cfg!(debug_assertions)` means that
 /// assertions are enabled whenever the *user crate* has debug assertions enabled. However if the

core/src/lib.rs

@@ -94,6 +94,7 @@
 ))]
 #![no_core]
 #![rustc_coherence_is_core]
+#![cfg_attr(not(bootstrap), rustc_preserve_ub_checks)]
 //
 // Lints:
 #![deny(rust_2021_incompatible_or_patterns)]

std/src/lib.rs

@@ -221,6 +221,7 @@
 //
 #![cfg_attr(not(feature = "restricted-std"), stable(feature = "rust1", since = "1.0.0"))]
 #![cfg_attr(feature = "restricted-std", unstable(feature = "restricted_std", issue = "none"))]
+#![cfg_attr(not(bootstrap), rustc_preserve_ub_checks)]
 #![doc(
     html_playground_url = "https://play.rust-lang.org/",
     issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/",