Resolve https://github.com/rust-lang/rust/pull/124870#issuecomment-21… · rust-lang/rust@22668e8

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//!

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//! [^extern_fn]: this remains true for any argument/return types and any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`)

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//!

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//! Under some conditions the above types `T` are also null pointer optimized when wrapped in a [`Result`][result_repr].

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//!

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//! [`Box<U>`]: ../../std/boxed/struct.Box.html

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//! [`num::NonZero*`]: crate::num

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//! [`ptr::NonNull<U>`]: crate::ptr::NonNull

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//! [function call ABI]: ../primitive.fn.html#abi-compatibility

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//! [result_repr]: crate::result::Result#Representation

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//!

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//! This is called the "null pointer optimization" or NPO.

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//!