Function lookup_trap_code

crates/environ/src/trap_encoding.rs:291–313

Decodes the provided trap information section and attempts to find the trap code corresponding to the `offset` specified. The `section` provided is expected to have been built by `TrapEncodingBuilder` above. Additionally the `offset` should be a relative offset within the text section of the compilation image.

(section: &[u8], offset: usize)

/// `TrapEncodingBuilder` above. Additionally the `offset` should be a relative
/// offset within the text section of the compilation image.
pub fn lookup_trap_code(section: &[u8], offset: usize) -> Option<CompiledTrap> {
    let (offsets, traps) = parse(section)?;

    // The `offsets` table is sorted in the trap section so perform a binary
    // search of the contents of this section to find whether `offset` is an
    // entry in the section. Note that this is a precise search because trap pcs
    // should always be precise as well as our metadata about them, which means
    // we expect an exact match to correspond to a trap opcode.
    //
    // Once an index is found within the `offsets` array then that same index is
    // used to lookup from the `traps` list of bytes to get the trap code byte
    // corresponding to this offset.
    let offset = u32::try_from(offset).ok()?;
    let index = offsets
        .binary_search_by_key(&offset, |val| val.get(LittleEndian))
        .ok()?;
    debug_assert!(index < traps.len());
    let byte = *traps.get(index)?;

    let trap = CompiledTrap::from_u8(byte);
    debug_assert!(trap.is_some(), "missing mapping for {byte}");
    trap
}

fn parse(section: &[u8]) -> Option<(&[U32<LittleEndian>], &[u8])> {
    let mut section = Bytes(section);