Rollup merge of #113716 - DianQK:add-no_builtins-to-function, r=pnkfelix

Add the `no-builtins` attribute to functions when `no_builtins` is applied at the crate level.

**When `no_builtins` is applied at the crate level, we should add the `no-builtins` attribute to each function to ensure it takes effect in LTO.**

This is also the reason why no_builtins does not take effect in LTO as mentioned in #35540.

Now, `#![no_builtins]` should be similar to `-fno-builtin` in clang/gcc, see https://clang.godbolt.org/z/z4j6Wsod5.

Next, we should make `#![no_builtins]` participate in LTO again. That makes sense, as LTO also takes into consideration function-level instruction optimizations, such as the MachineOutliner. More importantly, when a user writes a large `#![no_builtins]` crate, they would like this crate to participate in LTO as well.

We should also add a function-level no_builtins attribute to allow users to have more control over it. This is similar to Clang's `__attribute__((no_builtin))` feature, see https://clang.godbolt.org/z/Wod6KK6eq. Before implementing this feature, maybe we should discuss whether to support more fine-grained control, such as `__attribute__((no_builtin("memcpy")))`.

Related discussions:
- #109821
- #35540

Next (a separate pull request?):
- [ ] Revert #35637
- [ ] Add a function-level `no_builtin` attribute?

1 files changed, 2 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/middle/codegen_fn_attrs.rs b/compiler/rustc_middle/src/middle/codegen_fn_attrs.rs
index c4601a1fb41..02fd6ed7ba6 100644
--- a/compiler/rustc_middle/src/middle/codegen_fn_attrs.rs
+++ b/compiler/rustc_middle/src/middle/codegen_fn_attrs.rs
@@ -100,6 +100,8 @@ bitflags! {
         const REALLOCATOR               = 1 << 18;
         /// `#[rustc_allocator_zeroed]`: a hint to LLVM that the function only allocates zeroed memory.
         const ALLOCATOR_ZEROED          = 1 << 19;
+        /// `#[no_builtins]`: indicates that disable implicit builtin knowledge of functions for the function.
+        const NO_BUILTINS               = 1 << 20;
     }
 }