Fixes Self not found in match statement. (#4459)

## Description

`Self` was not working when qualifying an enum variant in a match
statement.

The solution was to add the implementing type declaration as an
impl_trait namespace symbol named "Self".

Closes #4263.

## Checklist

- [x] I have linked to any relevant issues.
- [x] 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).
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

Co-authored-by: IGI-111 <[email protected]>
Co-authored-by: Joshua Batty <[email protected]>

sway-core/src/semantic_analysis/ast_node/declaration/impl_trait.rs

@@ -305,6 +305,17 @@ impl ty::TyImplTrait {
             .with_help_text("")
             .with_type_annotation(type_engine.insert(decl_engine, TypeInfo::Unknown));
 
+        // Insert implementing type decl as `Self` symbol.
+        let self_decl: Option<ty::TyDecl> = match type_engine.get(implementing_for.type_id) {
+            TypeInfo::Enum(r) => Some(r.into()),
+            TypeInfo::Struct(r) => Some(r.into()),
+            _ => None,
+        };
+        if let Some(self_decl) = self_decl {
+            ctx.namespace
+                .insert_symbol(Ident::new_no_span("Self".to_string()), self_decl);
+        }
+
         // type check the items inside of the impl block
         let mut new_items = vec![];
 

test/src/e2e_vm_tests/test_programs/should_pass/language/match_expressions_with_self/src/main.sw

@@ -18,11 +18,22 @@ impl Eq for Initialized {
     }
 }
 
+impl Initialized {
+    fn foo(self) -> bool {
+        match self {
+            Self::True(_) => true,
+            Self::False(_) => false,
+        }
+    }
+}
+
 fn main() -> u64 {
     let a = Initialized::True;
     let b = Initialized::False;
     let c = a == b;
     assert(c == false);
 
+    assert(a.foo());
+
     1
 }