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Use String.visualWidth (backed by StringInfo.LengthInTextElements) instead of String.length when advancing the column counter in WriterModel.update. Combining characters attach to a preceding base character and have no visual width; counting them as separate code units caused the formatter to over-estimate line length and make wrong line-break decisions. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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Summary
Fixes #2945 — Unicode strings containing combining characters (e.g. diacritics like U+036E, U+0312, U+036B) caused incorrect column tracking in the formatter's
WriterModel, leading to wrong line-break decisions.Root Cause
WriterModel.updateadvanced the column counter usingString.length s, which counts UTF-16 code units. Combining characters have no visual advance — they attach to the preceding base character. A string like"l\u036e\u0312\u036b"hasString.length = 4but visually occupies only 1 column. This caused Fantomas to over-estimate line width and make wrong line-break decisions.Fix
String.visualWidthinUtils.fs/Utils.fsi, backed bySystem.Globalization.StringInfo.LengthInTextElements, which counts grapheme clusters rather than UTF-16 code units.WriterModel.update(Write/WriteTriviacases inContext.fs) to useString.visualWidthinstead ofString.length.StringInfoon every token write (which is the common case for typical F# source code).Trade-offs
The fast path means no overhead for ASCII tokens (the common case).
StringInfois only allocated for tokens with non-ASCII characters.Test Status
dotnet build fantomas.sln— ✅ Build succeededdotnet test src/Fantomas.Core.Tests/— ✅ 2838 passed, 0 failed, 11 skippedTwo new tests added:
String.visualWidth counts grapheme clusters not UTF-16 code units, 2945(UtilsTests.fs)string with Unicode combining characters should not affect formatting decisions, 2945(StringTests.fs)