<<=Operator

Explanation

<<= left-shifts the left operand by the right operand's amount, in place, overloadable via std::ops::ShlAssign.

Usage examples

Left-shifting a value in place

let mut x = 1u8;
x <<= 3; // <- `<<=` left-shifts `x` in place

Bit manipulation and flags

Building up a bitmask one field at a time — shifting previously-packed bits further left to make room for the next field — is a natural fit for <<=, mutating the accumulator in place across a loop.

let field_widths = [3u8, 2, 4]; // bits per field, packed low to high

let mut mask = 0u16;
for &width in &field_widths {
    mask <<= width; // <- `<<=` shifts the accumulated mask left to make room
    mask |= (1 << width) - 1; // fill the freed low bits with 1s for this field
}

assert_eq!(mask, 0b0001_1111_1111);

Accumulating into one mut binding with <<= avoids building and discarding an intermediate value on every iteration — the same in-place-mutation case made for +=, here specialized to ShlAssign.

Explanation

Embedded support: Full

<<= shows up in embedded code anywhere a byte (or word) is being assembled one bit at a time from a hardware source that only exposes a single bit per sample. The classic case is bit-banging a serial protocol in software rather than through a dedicated peripheral: on each clock edge, the code samples a data pin, shifts the value built so far one place to the left to make room, and ORs the new bit into the now-empty low position.

Usage examples

Bit-banging a byte in over a software SPI line

fn read_bit_from_miso() -> u32 {
    // in real firmware this samples the MISO GPIO pin
    1
}

fn bitbang_receive_byte() -> u8 {
    let mut value: u8 = 0;
    for _ in 0..8 {
        value <<= 1; // <- `<<=` makes room for the next incoming bit, MSB first
        value |= read_bit_from_miso() as u8;
        // toggle_clock_pin();
    }
    value
}