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Mention possible future rejections

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Co-authored-by: Josh Triplett <josh@joshtriplett.org>
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Smittyvb 2021-11-04 11:31:03 -04:00 • committed by GitHub
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src/doc/unstable-book/src/library-features/asm.md
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@ -886,6 +886,6 @@ The compiler performs some additional checks on options:
- The set of memory locations that you may access is the intersection of those allowed by the `asm!` blocks you entered and exited.
- You cannot assume that an `asm!` block will appear exactly once in the output binary. The compiler is allowed to instantiate multiple copies of the `asm!` block, for example when the function containing it is inlined in multiple places.
- On x86, inline assembly must not end with an instruction prefix (such as `LOCK`) that would apply to instructions generated by the compiler.
- The compiler is currently unable to detect this due to the way inline assembly is compiled.
- The compiler is currently unable to detect this due to the way inline assembly is compiled, but may catch and reject this in the future.
> **Note**: As a general rule, the flags covered by `preserves_flags` are those which are *not* preserved when performing a function call.