Avoid underflow in default ranges before a BOM

crates/ruff_db/src/diagnostic/render.rs

@@ -710,7 +710,11 @@ impl<'r> RenderableAnnotation<'r> {
     /// lifetime parameter here refers to the lifetime of the resolver that
     /// created the given `ResolvedAnnotation`.
     fn new(snippet_start: TextSize, ann: &'_ ResolvedAnnotation<'r>) -> RenderableAnnotation<'r> {
-        let range = ann.range - snippet_start;
+        // This should only ever saturate if a BOM is present _and_ the annotation range points
+        // before the BOM (i.e. at offset 0). In Ruff this typically results from the use of
+        // `TextRange::default()` for a diagnostic range instead of a range relative to file
+        // contents.
+        let range = ann.range.checked_sub(snippet_start).unwrap_or(ann.range);
         RenderableAnnotation {
             range,
             message: ann.message,

crates/ruff_db/src/diagnostic/render/full.rs

@@ -512,6 +512,27 @@ print()
         ");
     }
 
+    #[test]
+    fn bom_with_default_range() {
+        let mut env = TestEnvironment::new();
+        env.add("example.py", "\u{feff}import foo");
+        env.format(DiagnosticFormat::Full);
+
+        let mut diagnostic = env.err().build();
+        let span = env.path("example.py").with_range(TextRange::default());
+        let annotation = Annotation::primary(span);
+        diagnostic.annotate(annotation);
+
+        insta::assert_snapshot!(env.render(&diagnostic), @r"
+        error[test-diagnostic]: main diagnostic message
+         --> example.py:1:1
+          |
+        1 | import foo
+          | ^
+          |
+        ");
+    }
+
     /// We previously rendered this correctly, but the header was falling back to 1:1 for ranges
     /// pointing to the final newline in a file. Like Ruff, we now use the offset of the first
     /// character in the nonexistent final line in the header.