how to use `sighash` function from `anchor-syn` crate in my on-chain code?

Question

I have on-chain code to calculate instruction discriminator as :

use anchor_syn::codegen::program::common::{sighash, SIGHASH_GLOBAL_NAMESPACE};
// use anchor_syn::hash::hash;  // module `hash` is private

pub type Discriminator = [u8; 8];

/// Calculates the discriminator for an instruction based on its name,
/// which would be found in an IDL.
pub fn ix_discriminator(name: &str) -> Discriminator {
    sighash(SIGHASH_GLOBAL_NAMESPACE, &name)
}

but it won't pass build, as show bellow:

(anchor) ❱❱❱ anchor build
   Compiling anchor-syn v0.28.0
error[E0599]: no function or associated item named `new_from_array` found for struct `Hash` in the current scope
   --> src/hash.rs:110:15
    |
12  | pub struct Hash(pub [u8; HASH_BYTES]);
    | --------------- function or associated item `new_from_array` not found for this struct
...
110 |         Hash::new_from_array(hash_result)
    |               ^^^^^^^^^^^^^^ function or associated item not found in `Hash`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `anchor-syn` due to previous error

Can anyone show me where I have done wrong? I think it should be very simple, but block me for days.

Thanks.

Answer 1

/// The function discrminator is constructed from these 8 bytes. Typically, the namespace is
/// "global" or "state"
pub fn get_function_hash(namespace: &str, name: &str) -> [u8; 8] {
    let preimage = format!("{}:{}", namespace, name);
    let mut sighash = [0u8; 8];
    sighash.copy_from_slice(
        &anchor_lang::solana_program::hash::hash(preimage.as_bytes()).to_bytes()[..8],
    );
    sighash
}