fix: diagnostic renderer byte-slices UTF-8 in truncation

amterp · amterp · commit ea38df934d7d · 2026-03-09T22:26:45.000-05:00
line[:117] slices by byte offset, which can cut mid-character
for multi-byte UTF-8 strings. Convert to []rune before
truncating to ensure clean character boundaries.
diff --git a/core/diagnostic_render.go b/core/diagnostic_render.go
@@ -171,9 +171,10 @@ func (r *DiagnosticRenderer) renderSourceLine(lines []string, lineIdx, lineNum,
 	}
 
 	line := lines[lineIdx]
-	// Truncate long lines
-	if len(line) > 120 {
-		line = line[:117] + "..."
+	// Truncate long lines (use runes to avoid splitting multi-byte UTF-8 characters)
+	runes := []rune(line)
+	if len(runes) > 120 {
+		line = string(runes[:117]) + "..."
 	}
 
 	gutter := fmt.Sprintf("%*d", gutterWidth, lineNum)

Original file line number	Diff line number	Diff line change
`@@ -171,9 +171,10 @@ func (r *DiagnosticRenderer) renderSourceLine(lines []string, lineIdx, lineNum,`
`171`	`171`	`}`
`172`	`172`
`173`	`173`	`line := lines[lineIdx]`
`174`		`- // Truncate long lines`
`175`		`- if len(line) > 120 {`
`176`		`- line = line[:117] + "..."`
	`174`	`+ // Truncate long lines (use runes to avoid splitting multi-byte UTF-8 characters)`
	`175`	`+ runes := []rune(line)`
	`176`	`+ if len(runes) > 120 {`
	`177`	`+ line = string(runes[:117]) + "..."`
`177`	`178`	`}`
`178`	`179`
`179`	`180`	`gutter := fmt.Sprintf("%*d", gutterWidth, lineNum)`