test: add edge case tests for diagnostics module

[mattvaux]

Summary

Adds 7 edge case tests for the diagnostics module, expanding coverage from 1 to 8 tests. Covers empty diagnostics, multiple diagnostics, severity handling, extend/merge operations, notes formatting, and duplicate filtering.

---

Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

⚠️ Note:
To keep maintainer workload sustainable, we generally do not accept PRs that
are only minor wording, grammar, formatting, or style changes.
Such PRs may be closed without detailed review.

---

Why is this change needed?

The diagnostics module is core infrastructure used across the compiler, but it had minimal test coverage—only one basic test. Critical methods like is_empty(), has_errors(), extend(), merge(), and get_diagnostics_without_duplicates() lacked tests, creating risk of regressions.

---

What was the behavior or documentation before?

Only one test (test_diagnostics) verified basic formatting of a single error diagnostic.

---

What is the behavior or documentation after?

8 tests covering empty diagnostics, multiple diagnostics, severity handling (warnings vs errors), extend/merge operations, notes formatting, and duplicate filtering.

---

Related issue or discussion (if any)

None.

---

Additional context

The diagnostics module is used throughout the Cairo compiler. These tests validate edge cases and help prevent regressions. All tests pass.

[reviewable-StarkWare]

This change is 

[orizi]

@orizi made 3 comments.
Reviewable status: 0 of 1 files reviewed, 3 unresolved discussions (waiting on @mattvaux).

---

crates/cairo-lang-diagnostics/src/diagnostics_test.rs line 42 at r1 (raw file):

    fn format(&self, _db: &dyn Database) -> String {
        self.message.clone()
    }

as not really used.

Suggestion:

    file_id: FileId<'db>,
    severity: Severity,
    notes: Vec<DiagnosticNote<'db>>,
}
impl<'db> SimpleDiag<'db> {
    fn new(file_id: FileId<'db>) -> Self {
        Self { file_id, severity: Severity::Error, notes: vec![] }
    }

    fn with_severity(file_id: FileId<'db>, severity: Severity) -> Self {
        Self { file_id, severity, notes: vec![] }
    }

    fn with_notes(file_id: FileId<'db>, notes: Vec<DiagnosticNote<'db>>) -> Self {
        Self { file_id, severity: Severity::Error, notes }
    }
}
impl<'db> DiagnosticEntry<'db> for SimpleDiag<'db> {
    fn format(&self, _db: &dyn Database) -> String {
        "Simple diagnostic.".into()
    }

---

crates/cairo-lang-diagnostics/src/diagnostics_test.rs line 48 at r1 (raw file):

            file_id: self.file_id,
            span: TextSpan::new(TextOffset::START, TextWidth::new_for_testing(6).as_offset()),
        }

make the location somewhat configurable instead - to check duplications.

Code quote:

        SpanInFile {
            file_id: self.file_id,
            span: TextSpan::new(TextOffset::START, TextWidth::new_for_testing(6).as_offset()),
        }

---

crates/cairo-lang-diagnostics/src/diagnostics_test.rs line 208 at r1 (raw file):

    let formatted = diagnostics.build().format(&db_val);
    assert!(formatted.contains("note:"));
    assert!(formatted.contains("This is a note."));

Suggestion:

    let formatted = diagnostics.build().format(&db_val);
    assert_eq!(diagnostics.build().format(&db_val), indoc! {"
    ...
    "});