schematron: add --schematron to xmllint CLI, fix message interpolation

Add --schematron flag to xmllint following existing --relaxng/--schema
patterns. Fix value-of message interpolation to use document context for
NodeSet string values (was returning empty for element NodeSets). Add
sum(item/@attr) test exercising the XPath attribute NodeSet fix.

Author: jonwiggins

src/bin/xmllint.rs

@@ -81,6 +81,10 @@ struct Cli {
     #[arg(long, value_name = "FILE")]
     schema: Option<String>,
 
+    /// Validate against an ISO Schematron schema file.
+    #[arg(long, value_name = "FILE")]
+    schematron: Option<String>,
+
     // -- XPath -------------------------------------------------------------
     /// Evaluate an XPath expression and print the result.
     #[allow(clippy::doc_markdown)]
@@ -219,6 +223,13 @@ fn process_file(cli: &Cli, filename: &str) -> u8 {
         }
     }
 
+    if let Some(ref sch_file) = cli.schematron {
+        let code = validate_schematron_file(filename, &doc, sch_file);
+        if code > exit_code {
+            exit_code = code;
+        }
+    }
+
     // -- XPath evaluation --------------------------------------------------
     if let Some(ref expr) = cli.xpath {
         evaluate_xpath(filename, &doc, expr);
@@ -377,6 +388,28 @@ fn validate_xsd_file(filename: &str, doc: &Document, xsd_file: &str) -> u8 {
     }
 }
 
+/// Validates a document against an ISO Schematron schema file (--schematron).
+fn validate_schematron_file(filename: &str, doc: &Document, sch_file: &str) -> u8 {
+    let schema_content = match fs::read_to_string(sch_file) {
+        Ok(content) => content,
+        Err(e) => {
+            eprintln!("{sch_file}: failed to read Schematron schema: {e}");
+            return EXIT_VALIDATION_ERROR;
+        }
+    };
+
+    match xmloxide::validation::schematron::parse_schematron(&schema_content) {
+        Ok(schema) => {
+            let result = xmloxide::validation::schematron::validate_schematron(doc, &schema);
+            print_validation_result(filename, &result)
+        }
+        Err(e) => {
+            eprintln!("{sch_file}: failed to parse Schematron schema: {e}");
+            EXIT_VALIDATION_ERROR
+        }
+    }
+}
+
 /// Prints validation errors/warnings and returns the exit code.
 fn print_validation_result(filename: &str, result: &xmloxide::validation::ValidationResult) -> u8 {
     for warning in &result.warnings {

src/validation/schematron.rs

@@ -797,7 +797,7 @@ fn interpolate_message(
                         ctx.set_variable(name, val.clone());
                     }
                     if let Ok(val) = ctx.evaluate(&expr) {
-                        result.push_str(&val.to_xpath_string());
+                        result.push_str(&xpath_value_to_string(doc, &val));
                     }
                 }
             }
@@ -806,6 +806,22 @@ fn interpolate_message(
     result
 }
 
+/// Converts an `XPath` value to a string, computing string-value for
+/// node-sets using the document (unlike `to_xpath_string()` which returns
+/// empty for node-sets without document access).
+fn xpath_value_to_string(doc: &Document, val: &XPathValue) -> String {
+    match val {
+        XPathValue::NodeSet(nodes) => {
+            if let Some(&first) = nodes.first() {
+                doc.text_content(first)
+            } else {
+                String::new()
+            }
+        }
+        _ => val.to_xpath_string(),
+    }
+}
+
 // ---------------------------------------------------------------------------
 // Tests
 // ---------------------------------------------------------------------------
@@ -1524,6 +1540,39 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_validate_sum_attribute_path() {
+        // Tests that sum(child/@attr) works correctly now that
+        // attribute paths return proper NodeSets.
+        let schema = parse_schematron(
+            r#"
+            <schema xmlns="http://purl.oclc.org/dml/schematron">
+              <pattern>
+                <rule context="/order">
+                  <let name="total" value="sum(item/@price)"/>
+                  <assert test="$total = @expected">Total <value-of select="$total"/> does not match expected <value-of select="@expected"/></assert>
+                </rule>
+              </pattern>
+            </schema>
+            "#,
+        )
+        .unwrap();
+
+        let doc_pass = Document::parse_str(
+            r#"<order expected="30"><item price="10"/><item price="20"/></order>"#,
+        )
+        .unwrap();
+        let result = validate_schematron(&doc_pass, &schema);
+        assert!(result.is_valid, "sum should equal 30: {:?}", result.errors);
+
+        let doc_fail = Document::parse_str(
+            r#"<order expected="99"><item price="10"/><item price="20"/></order>"#,
+        )
+        .unwrap();
+        let result = validate_schematron(&doc_fail, &schema);
+        assert!(!result.is_valid);
+    }
+
     // ===================================================================
     // Additional edge case tests
     // ===================================================================