fix: Error when mutating comptime variables in non-comptime code (#6003)

# Description

## Problem\*

Resolves #5982

## Summary\*

Issues an error when mutating comptime globals in a non-comptime
context. Previously we either allowed the code if the method is unused,
or panicked during monomorphization.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_frontend/src/elaborator/statements.rs

@@ -249,6 +249,13 @@ impl<'context> Elaborator<'context> {
                 } else {
                     if let Some(definition) = self.interner.try_definition(ident.id) {
                         mutable = definition.mutable;
+
+                        if definition.comptime && !self.in_comptime_context() {
+                            self.push_err(ResolverError::MutatingComptimeInNonComptimeContext {
+                                name: definition.name.clone(),
+                                span: ident.location.span,
+                            });
+                        }
                     }
 
                     let typ = self.interner.definition_type(ident.id).instantiate(self.interner).0;

compiler/noirc_frontend/src/hir/resolution/errors.rs

@@ -128,6 +128,8 @@ pub enum ResolverError {
     QuoteInRuntimeCode { span: Span },
     #[error("Comptime-only type `{typ}` cannot be used in runtime code")]
     ComptimeTypeInRuntimeCode { typ: String, span: Span },
+    #[error("Comptime variable `{name}` cannot be mutated in a non-comptime context")]
+    MutatingComptimeInNonComptimeContext { name: String, span: Span },
 }
 
 impl ResolverError {
@@ -522,6 +524,13 @@ impl<'a> From<&'a ResolverError> for Diagnostic {
                     *span,
                 )
             },
+            ResolverError::MutatingComptimeInNonComptimeContext { name, span } => {
+                Diagnostic::simple_error(
+                    format!("Comptime variable `{name}` cannot be mutated in a non-comptime context"),
+                    format!("`{name}` mutated here"),
+                    *span,
+                )
+            },
         }
     }
 }