`Id::from_name` should be const

What problem does this solve?

I want to be able to efficiently store constants for the Ids computed from const strings.

This is great when working with hybrid code-data workflows, where you want to quickly spawn an object in a scripting-like way without having to recompute the hash. It also adds some level of typo-resistance, as the name only needs to be typed in a single location.

What solution would you like?

Add const to Id::from_name.

How could this be implemented?

Unfortunately, it's not that simple.

Even when accepting only a String in from_name, adding const to the function below results in a number of compiler errors.

    pub fn from_name(name: String) -> Self {
        // Algorithm adopted from <https://cp-algorithms.com/string/string-hashing.html>

        let mut value = 0;
        let mut p_pow = 1;

        name.bytes().for_each(|byte| {
            value = (value + (byte as u64 + 1) * p_pow) % HASH_M;
            p_pow = (p_pow * HASH_P) % HASH_M;
        });

        Self::new(value)
    }

error[E0015]: cannot perform deref coercion on `std::string::String` in constant functions
    --> src\identifier.rs:64:9
     |
64   |         name.bytes().for_each(|byte| {
     |         ^^^^^^^^^^^^
     |
     = note: attempting to deref into `str`
note: deref defined here
    --> C:\Users\alice\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\alloc\src\string.rs:2531:5
     |
2531 |     type Target = str;
     |     ^^^^^^^^^^^
note: impl defined here, but it is not `const`
    --> C:\Users\alice\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\alloc\src\string.rs:2530:1
     |
2530 | impl ops::Deref for String {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^
     = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

error[E0015]: cannot call non-const fn `core::str::<impl str>::bytes` in constant functions
  --> src\identifier.rs:64:14
   |
64 |         name.bytes().for_each(|byte| {
   |              ^^^^^^^
   |
   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

error[E0658]: mutable references are not allowed in constant functions
  --> src\identifier.rs:64:31
   |
64 |           name.bytes().for_each(|byte| {
   |  _______________________________^
65 | |             value = (value + (byte as u64 + 1) * p_pow) % HASH_M;
66 | |             p_pow = (p_pow * HASH_P) % HASH_M;
67 | |         });
   | |_________^
   |
   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information

error[E0015]: cannot call non-const fn `<std::str::Bytes<'_> as Iterator>::for_each::<{closure@src\identifier.rs:64:31: 64:37}>` in constant functions
  --> src\identifier.rs:64:22
   |
64 |           name.bytes().for_each(|byte| {
   |  ______________________^
65 | |             value = (value + (byte as u64 + 1) * p_pow) % HASH_M;
66 | |             p_pow = (p_pow * HASH_P) % HASH_M;
67 | |         });
   | |__________^
   |
   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants

error[E0493]: destructor of `std::string::String` cannot be evaluated at compile-time
  --> src\identifier.rs:58:28
   |
58 |     pub const fn from_name(name: String) -> Self {
   |                            ^^^^ the destructor for this type cannot be evaluated in constant functions
...
70 |     }
   |     - value is dropped here

Some errors have detailed explanations: E0015, E0493, E0658.
For more information about an error, try `rustc --explain E0015`.
error: could not compile `leafwing_manifest` (lib) due to 5 previous errors

[Optional] What alternatives have you considered?

Swapping to a global append-only store of Names <-> Ids would work, but require unsafe and be more technically complex. If this approach was chosen, we may want to swap to an atomically incrementing identifier scheme, rather than a hash-derived one.

Leafwing-Studios / leafwing_manifest