fixes #15848 [backport:1.2] (#17959)

tests/stylecheck/tusages.nim

@@ -0,0 +1,23 @@
+discard """
+  cmd: "nim c --styleCheck:error --styleCheck:usages $file"
+  errormsg: "'BAD_STYLE' should be: 'BADSTYLE'"
+  line: 20
+"""
+
+import strutils
+
+proc BADSTYLE(c: char) = discard
+
+proc toSnakeCase(s: string): string =
+  result = newStringOfCap(s.len + 3)
+  for i in 0..<s.len:
+    if s[i] in {'A'..'Z'}:
+      if i > 0 and s[i-1] in {'a'..'z'}:
+        result.add '_'
+      result.add toLowerAscii(s[i])
+    else:
+      result.add s[i]
+    BAD_STYLE(s[i])
+
+echo toSnakeCase("fooBarBaz Yes")
+