Spreadsheet+LibSyntax: Never insert spans directly

Function `CellSyntaxHighlighter::rehighlight()` direct inserted spans
to TextDocument `m_span` vector missing out important reordering and
merging operation carried out by `TextDocument::set_spans()`.

This caused overlapping spans for a cell with only a `=` symbol
(one for the actual token and one for the highlighting) to
miscalculate `start` and `end` value and a `length` value (of
`size_t` type) with a `0-1` substraction (result: 18446744073709551615)
causing `Utf32View::substring_view()` to fail the
`!Checked<size_t>::addition_would_overflow(offset, length)` assertion

This remove the possibility to directly alter `TextDocument`'s spans
thus forcing the utilization of `HighlighterClient::do_set_spans()`
interface function.

Proper refactor have been applied to
`CellSyntaxHighlighter::rehighlight()` function

3 files changed, 1 insertions, 3 deletions

diff --git a/Userland/Libraries/LibGUI/TextEditor.h b/Userland/Libraries/LibGUI/TextEditor.h
index dfabda73cb..55ddc8946a 100644
--- a/Userland/Libraries/LibGUI/TextEditor.h
+++ b/Userland/Libraries/LibGUI/TextEditor.h
@@ -306,7 +306,6 @@ private:
     virtual void document_did_update_undo_stack() override;
 
     // ^Syntax::HighlighterClient
-    virtual Vector<TextDocumentSpan>& spans() final { return document().spans(); }
     virtual Vector<TextDocumentSpan> const& spans() const final { return document().spans(); }
     virtual void set_span_at_index(size_t index, TextDocumentSpan span) final { document().set_span_at_index(index, move(span)); }
     virtual Vector<GUI::TextDocumentFoldingRegion>& folding_regions() final { return document().folding_regions(); };
diff --git a/Userland/Libraries/LibSyntax/Highlighter.h b/Userland/Libraries/LibSyntax/Highlighter.h
index c67777c1b8..8ba81ea77b 100644
--- a/Userland/Libraries/LibSyntax/Highlighter.h
+++ b/Userland/Libraries/LibSyntax/Highlighter.h
@@ -125,7 +125,6 @@ public:
     }
 
 private:
-    virtual Vector<GUI::TextDocumentSpan>& spans() override { return m_spans; }
     virtual Vector<GUI::TextDocumentSpan> const& spans() const override { return m_spans; }
     virtual void set_span_at_index(size_t index, GUI::TextDocumentSpan span) override { m_spans.at(index) = move(span); }
 
diff --git a/Userland/Libraries/LibSyntax/HighlighterClient.h b/Userland/Libraries/LibSyntax/HighlighterClient.h
index 3491ca8995..a535a60e85 100644
--- a/Userland/Libraries/LibSyntax/HighlighterClient.h
+++ b/Userland/Libraries/LibSyntax/HighlighterClient.h
@@ -17,9 +17,9 @@ class HighlighterClient {
 public:
     virtual ~HighlighterClient() = default;
 
-    virtual Vector<GUI::TextDocumentSpan>& spans() = 0;
     virtual Vector<GUI::TextDocumentSpan> const& spans() const = 0;
     virtual void set_span_at_index(size_t index, GUI::TextDocumentSpan span) = 0;
+    virtual void clear_spans() { do_set_spans({}); };
 
     virtual Vector<GUI::TextDocumentFoldingRegion>& folding_regions() = 0;
     virtual Vector<GUI::TextDocumentFoldingRegion> const& folding_regions() const = 0;