hint.rs.html -- source
Description: Source to the Rust file `src/libcore/hint.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2018 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    #![stable(feature = "core_hint", since = "1.27.0")]

    //! Hints to compiler that affects how code should be emitted or optimized.

    use intrinsics;

    /// Informs the compiler that this point in the code is not reachable, enabling
    /// further optimizations.
    ///
    /// # Safety
    ///
    /// Reaching this function is completely *undefined behavior* (UB). In
    /// particular, the compiler assumes that all UB must never happen, and
    /// therefore will eliminate all branches that reach to a call to
    /// `unreachable_unchecked()`.
    ///
    /// Like all instances of UB, if this assumption turns out to be wrong, i.e. the
    /// `unreachable_unchecked()` call is actually reachable among all possible
    /// control flow, the compiler will apply the wrong optimization strategy, and
    /// may sometimes even corrupt seemingly unrelated code, causing
    /// difficult-to-debug problems.
    ///
    /// Use this function only when you can prove that the code will never call it.
    ///
    /// The [`unreachable!()`] macro is the safe counterpart of this function, which
    /// will panic instead when executed.
    ///
    /// [`unreachable!()`]: ../macro.unreachable.html
    ///
    /// # Example
    ///
    /// ```
    /// fn div_1(a: u32, b: u32) -> u32 {
    ///     use std::hint::unreachable_unchecked;
    ///
    ///     // `b.saturating_add(1)` is always positive (not zero),
    ///     // hence `checked_div` will never return None.
    ///     // Therefore, the else branch is unreachable.
    ///     a.checked_div(b.saturating_add(1))
    ///         .unwrap_or_else(|| unsafe { unreachable_unchecked() })
    /// }
    ///
    /// assert_eq!(div_1(7, 0), 7);
    /// assert_eq!(div_1(9, 1), 4);
    /// assert_eq!(div_1(11, std::u32::MAX), 0);
    /// ```
    #[inline]
    #[stable(feature = "unreachable", since = "1.27.0")]
    pub unsafe fn unreachable_unchecked() -> ! {
        intrinsics::unreachable()
    }

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