Add back taint models for append and copy

This is needed when they are used with string arguments.

Author: owen-mc

go/ql/lib/semmle/go/frameworks/Stdlib.qll

@@ -44,6 +44,30 @@ import semmle.go.frameworks.stdlib.TextTabwriter
 import semmle.go.frameworks.stdlib.TextTemplate
 import semmle.go.frameworks.stdlib.Unsafe
 
+/**
+ * A model of the built-in `append` function, which propagates taint from its arguments to its
+ * result.
+ */
+private class AppendFunction extends TaintTracking::FunctionModel {
+  AppendFunction() { this = Builtin::append() }
+
+  override predicate hasTaintFlow(FunctionInput inp, FunctionOutput outp) {
+    inp.isParameter(_) and outp.isResult()
+  }
+}
+
+/**
+ * A model of the built-in `copy` function, which propagates taint from its second argument
+ * to its first.
+ */
+private class CopyFunction extends TaintTracking::FunctionModel {
+  CopyFunction() { this = Builtin::copy() }
+
+  override predicate hasTaintFlow(FunctionInput inp, FunctionOutput outp) {
+    inp.isParameter(1) and outp.isParameter(0)
+  }
+}
+
 /** Provides a class for modeling functions which convert strings into integers. */
 module IntegerParser {
   /**