> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # Rust Bitwise OR Assignment The `|=` operator is the bitwise OR assignment operator. It performs a bitwise OR operation between a mutable left-hand operand and a right-hand operand, assigning the resulting value directly back to the left-hand operand. Mechanically, `a |= b` serves as syntactic sugar for `a = a | b`, with the strict requirement that the left-hand variable must be declared as mutable (`mut`). ```rust theme={"dark"} let mut lhs: u8 = 0b1010_0000; let rhs: u8 = 0b0000_1111; lhs |= rhs; // lhs is now 0b1010_1111 ``` ## Underlying Trait Implementation In Rust's type system, the `|=` operator is powered by the `std::ops::BitOrAssign` trait. When the compiler encounters `a |= b`, it translates the expression into a method call: `a.bitor_assign(b)`. ```rust theme={"dark"} pub trait BitOrAssign { fn bitor_assign(&mut self, rhs: Rhs); } ``` To use `|=` with custom data structures, you must implement this trait for your type. The method takes an exclusive, mutable reference to `self` (`&mut self`) and consumes the `rhs` operand. ## Type Behavior The operator exhibits specific behaviors depending on the primitive types involved: * **Integers (`u8`, `i32`, `usize`, etc.):** Performs a standard bit-level OR operation. Each bit in the result is `1` if the corresponding bit in either the LHS or RHS is `1`. Both operands must be of the exact same integer type; Rust does not implicitly coerce types during bitwise assignment. * **Booleans (`bool`):** Performs a strict logical OR operation. ## Evaluation Semantics A critical mechanical distinction of `|=` when applied to booleans is the absence of short-circuit evaluation. Unlike the standard logical OR operator (`||`), which halts evaluation if the LHS is `true`, the `|=` operator strictly evaluates both the LHS and RHS expressions before executing the assignment. ```rust theme={"dark"} let mut flag = true; // `expensive_computation()` WILL execute and be evaluated, // even though `flag` is already true. flag |= expensive_computation(); ```

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