CompileError::ContractIdConstantNotAConstDecl (#6842)

## Description
In contract_helpers.rs, replaced the ContractIdConstantNotAConstDecl
error with an Internal compiler error since this represents an
impossible state that should never occur in normal operation.
In error.rs, removed the now unused ContractIdConstantNotAConstDecl
error variant and its corresponding span implementation.
The changes make sense because:
The error was only used in one place where it represented an impossible
state
If that state is ever reached, it indicates a bug in the compiler itself
rather than user error
Using an internal compiler error will make it clearer that this is a
compiler bug that needs to be fixed if it ever occurs
fix #6841 

## Checklist

- [x] 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).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [ ] 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.
- [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 <igi-111@protonmail.com>
Co-authored-by: Sophie Dankel <47993817+sdankel@users.noreply.github.com>
Co-authored-by: jjcnn <38888011+jjcnn@users.noreply.github.com>
Co-authored-by: Joshua Batty <joshpbatty@gmail.com>

Author: 5 people

sway-core/src/semantic_analysis/namespace/contract_helpers.rs

@@ -99,15 +99,13 @@ fn bind_contract_id_in_root_module(
     let type_check_ctx =
         TypeCheckContext::from_namespace(&mut namespace, &mut symbol_ctx, engines, experimental);
     // Typecheck the const declaration. This will add the binding in the supplied namespace
-    match TyAstNode::type_check(handler, type_check_ctx, &ast_node)
-        .unwrap()
-        .content
-    {
-        TyAstNodeContent::Declaration(_) => Ok(namespace.root()),
-        _ => Err(
-            handler.emit_err(CompileError::ContractIdConstantNotAConstDecl {
-                span: const_item_span,
-            }),
-        ),
+    let type_checked = TyAstNode::type_check(handler, type_check_ctx, &ast_node).unwrap();
+    if let TyAstNodeContent::Declaration(_) = type_checked.content {
+        Ok(namespace.root())
+    } else {
+        Err(handler.emit_err(CompileError::Internal(
+            "Contract ID declaration did not typecheck to a declaration, which should be impossible",
+            const_item_span,
+        )))
     }
 }

sway-error/src/error.rs

@@ -917,10 +917,6 @@ pub enum CompileError {
     ContinueOutsideLoop { span: Span },
     /// This will be removed once loading contract IDs in a dependency namespace is refactored and no longer manual:
     /// https://github.com/FuelLabs/sway/issues/3077
-    #[error("Contract ID is not a constant item.")]
-    ContractIdConstantNotAConstDecl { span: Span },
-    /// This will be removed once loading contract IDs in a dependency namespace is refactored and no longer manual:
-    /// https://github.com/FuelLabs/sway/issues/3077
     #[error("Contract ID value is not a literal.")]
     ContractIdValueNotALiteral { span: Span },
 
@@ -1231,7 +1227,6 @@ impl Spanned for CompileError {
             IntrinsicIncorrectNumTArgs { span, .. } => span.clone(),
             BreakOutsideLoop { span } => span.clone(),
             ContinueOutsideLoop { span } => span.clone(),
-            ContractIdConstantNotAConstDecl { span } => span.clone(),
             ContractIdValueNotALiteral { span } => span.clone(),
             RefMutableNotAllowedInMain { span, .. } => span.clone(),
             InitializedRegisterReassignment { span, .. } => span.clone(),