String slices

[xunilrj]

Description

This PR introduces string slices.

The basic usage is at test/src/ir_generation/tests/str_slice.sw:

let a: str = "ABC";

Before this PR a would be of type str[3], a string array.

Now both string slices and string arrays exist. This PR contains a new intrinsic that converts from string literals to arrays.

let a: str = "ABC";
let b: str[3] = __to_str_array("ABC");

Runtime conversions can be done using

let a = "abcd";
let b: str[4] = a.try_as_str_array().unwrap();
let c = from_str_array(b);

string slices to string arrays can fail, so they return Option<str[N]>; and because of this try_as_str_array lives in std. The inverse, from_str_array only fails if alloc fails and lives in core.

At this PR string slices are forbidden at configurable, storage, const, and main arguments and returns. The reason for these limitations is the internal structure of string slices having a ptr.

The optimized IR for initializing the slice is:

v0 = const string<3> "abc"
v1 = ptr_to_int v0 to u64, !2
v2 = get_local ptr { u64, u64 }, __anon_0, !2
v3 = const u64 0
v4 = get_elem_ptr v2, ptr u64, v3
store v1 to v4, !2

v5 = const u64 1
v6 = get_elem_ptr v2, ptr u64, v5
v7 = const u64 3
store v7 to v6, !2

v8 = get_local ptr slice, __anon_1, !2
mem_copy_bytes v8, v2, 16

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[anton-trunov]

LGTM!

[IGI-111]