Add phantom attribute.

[ilyalesokhin-starkware]

This change is 

[orizi]

Reviewed 2 of 6 files at r1.
Reviewable status: 2 of 6 files reviewed, 2 unresolved discussions (waiting on @ilyalesokhin-starkware)

---

crates/cairo-lang-semantic/src/expr/compute.rs line 805 at r1 (raw file):

                    CannotCreateInstancesOfPhantomTypes,
                );
            }

.

Suggestion:

            if let Some(attr) = concrete_enum_id.find_attr(db, PHANTOM_ATTR)? {
                ctx.diagnostics.report(
                    syntax,
                    CannotCreateInstancesOfPhantomTypes,
                );
            }

---

crates/cairo-lang-semantic/src/expr/compute.rs line 1927 at r1 (raw file):

        ctx.diagnostics
            .report_by_ptr(attr.id_stable_ptr.untyped(), CannotCreateInstancesOfPhantomTypes);
    }

the error should appear at the usage - not the attr.

Suggestion:

    if let Some(attr) = concrete_struct_id.find_attr(db, PHANTOM_ATTR)? {
        ctx.diagnostics
            .report(ctor_syntax, CannotCreateInstancesOfPhantomTypes);
    }

[ilyalesokhin-starkware]

Reviewable status: 1 of 6 files reviewed, 2 unresolved discussions (waiting on @orizi)

---

crates/cairo-lang-semantic/src/expr/compute.rs line 805 at r1 (raw file):

Previously, orizi wrote…

.

Done.

---

crates/cairo-lang-semantic/src/expr/compute.rs line 1927 at r1 (raw file):

Previously, orizi wrote…

the error should appear at the usage - not the attr.

Done.

[orizi]

Reviewed 3 of 6 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ilyalesokhin-starkware)