Move likely/unlikely argument outside of invisible unsafe block

[dtolnay]

The previous likely!/unlikely! macros were unsound because it permits the caller's expr to contain arbitrary unsafe code.

pub fn huh() -> bool {
    likely!(std::ptr::read(&() as *const () as *const bool))
}

Before: compiles cleanly.
After:

error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
   |
70 |     likely!(std::ptr::read(&() as *const () as *const bool))
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
   |
   = note: consult the function's documentation for information on how to avoid undefined behavior

[rust-highfive]

r? @varkor

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[Mark-Simulacrum]

@bors r+ rollup

[bors]

📌 Commit afb8170 has been approved by Mark-Simulacrum