Dereferencing operator index []

[ironcev]

Description

This PR implements index operator [] for references. The overall effort related to references is tracked in #5063.

[] is defined for references using this recursive definition: <reference>[<index>] := (*<reference>)[<index>].

This eliminates the need for the dereferencing operator * when working with references to arrays:

let array = [1, 2, 3];

let r = &&&array;

assert((***r)[0] == r[0]);

let r = &&&[ &&[1, 2, 3], &&[4, 5, 6] ];
assert(r[0][2] == 3);
assert(r[1][0] == 4);

Additionally, the PR fixes two previously existing issues (see below screenshots):

• the misleading error message on type not implementing the "index" method when indexing a non-indexable type.
• the broken error span and expression text when indexing a non-indexable type in reassignments.

Before:

After:

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.

[IGI-111]

Though we'll want to formalize it eventually with something like Deref it's appropriate to hardcode it for builtin types at this time.

[ironcev]

Though we'll want to formalize it eventually with something like Deref it's appropriate to hardcode it for builtin types at this time.

Yes, agree. In this case it will be Deref and Index traits. Once we introduce them, extending the current solution will be straightforward. The sam will be with the upcoming . dereferencing operator for field and element access and method calls.