Fix #637: recognise rho-walking references as void-attr usages

src/main/resources/org/eolang/lints/misc/unused-void-attr.xsl

@@ -3,7 +3,7 @@
  * SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
  * SPDX-License-Identifier: MIT
 -->
-<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:eo="https://www.eolang.org" id="unused-void-attr" version="2.0">
+<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:eo="https://www.eolang.org" xmlns:xs="http://www.w3.org/2001/XMLSchema" id="unused-void-attr" version="2.0">
   <xsl:import href="/org/eolang/parser/_funcs.xsl"/>
   <xsl:import href="/org/eolang/funcs/lineno.xsl"/>
   <xsl:import href="/org/eolang/funcs/escape.xsl"/>
@@ -14,7 +14,18 @@
       <xsl:for-each select="//o[@base='∅']">
         <xsl:variable name="attr" select="@name"/>
         <xsl:variable name="formation" select=".."/>
-        <xsl:if test="not($attr='φ') and not(eo:atom($formation)) and not($formation//o[contains(@base, concat('ξ.', $attr))]/ancestor::o[eo:abstract(.)][1][generate-id()=$formation/generate-id()]) and not($formation//o[eo:atom(.)])">
+        <xsl:variable name="pattern" select="concat('^ξ(\.ρ)*\.', $attr, '(\..*)?$')"/>
+        <xsl:variable name="usages" as="element()*">
+          <xsl:for-each select="$formation//o[matches(@base, $pattern)]">
+            <xsl:variable name="rhos" select="replace(@base, concat('^ξ((\.ρ)*)\.', $attr, '.*$'), '$1')"/>
+            <xsl:variable name="hops" select="string-length($rhos) - string-length(translate($rhos, 'ρ', ''))" as="xs:integer"/>
+            <xsl:variable name="anchor" select="ancestor::o[eo:abstract(.)][position() = $hops + 1]"/>
+            <xsl:if test="exists($anchor) and generate-id($anchor) = generate-id($formation)">
+              <xsl:sequence select="."/>
+            </xsl:if>
+          </xsl:for-each>
+        </xsl:variable>
+        <xsl:if test="not($attr='φ') and not(eo:atom($formation)) and empty($usages) and not($formation//o[eo:atom(.)])">
           <xsl:element name="defect">
             <xsl:variable name="line" select="eo:lineno(@line)"/>
             <xsl:attribute name="line">

src/test/resources/org/eolang/lints/packs/single/unused-void-attr/catches-usage-as-nested-rho-ref.yaml → src/test/resources/org/eolang/lints/packs/single/unused-void-attr/allows-usage-as-nested-rho-ref.yaml

@@ -4,8 +4,7 @@
 sheets:
   - /org/eolang/lints/misc/unused-void-attr.xsl
 asserts:
-  - /defects[count(defect[@severity='warning'])=1]
-  - /defects/defect[@line='2']
+  - /defects[count(defect[@severity='warning'])=0]
 input: |
   # Foo
   [x] > foo

src/test/resources/org/eolang/lints/packs/single/unused-void-attr/allows-void-attr-used-in-nested-formation.yaml

@@ -0,0 +1,17 @@
+# SPDX-FileCopyrightText: Copyright (c) 2016-2026 Objectionary.com
+# SPDX-License-Identifier: MIT
+---
+# Regression test for objectionary/lints#637.
+# The void attribute "widths" of "tabular" is referenced from inside the
+# nested "[i] > inner" formation as "widths.at i", which the parser turns
+# into "ξ.ρ.widths.at". The lint must recognise this rho-walking reference
+# as a real usage and not flag "widths" as unused.
+sheets:
+  - /org/eolang/lints/misc/unused-void-attr.xsl
+asserts:
+  - /defects[count(defect[contains(., '"widths"')])=0]
+input: |
+  # Foo.
+  [widths] > tabular
+    [i] > inner
+      widths.at i > @