Add and use WriteNode.writesElementPreUpdate

go/ql/lib/semmle/go/controlflow/ControlFlowGraph.qll

@@ -188,9 +188,7 @@ module ControlFlow {
      * initialized.
      */
     predicate writesElement(DataFlow::Node base, DataFlow::Node index, DataFlow::Node rhs) {
-      exists(DataFlow::Node b |
+      exists(DataFlow::Node b | this.writesElementPreUpdate(b, index, rhs) |
-        this.writesElementInsn(b.asInstruction(), index.asInstruction(), rhs.asInstruction())
-      |
         this.isInitialization() and base = b
         or
         not this.isInitialization() and
@@ -198,6 +196,21 @@ module ControlFlow {
       )
     }
 
+    /**
+     * Holds if this node sets the value of element `index` on `base` (or its implicit dereference)
+     * to `rhs`.
+     *
+     * For example, for the assignment `xs[i] = v`, `base` is the post-update node of the data-flow
+     * node corresponding to `xs` or (if `xs` is a pointer) the implicit dereference `*xs`, `index`
+     * is the data-flow node corresponding to `i`, and `rhs` is the data-flow node corresponding to
+     * `base`. If this `WriteNode` corresponds to the initialization of an array/slice/map then
+     * there is no need for a post-update node and `base` is the array/slice/map literal being
+     * initialized.
+     */
+    predicate writesElementPreUpdate(DataFlow::Node base, DataFlow::Node index, DataFlow::Node rhs) {
+      this.writesElementInsn(base.asInstruction(), index.asInstruction(), rhs.asInstruction())
+    }
+
     private predicate writesElementInsn(
       IR::Instruction base, IR::Instruction index, IR::Instruction rhs
     ) {

go/ql/lib/semmle/go/frameworks/stdlib/NetHttp.qll

@@ -52,13 +52,7 @@ module NetHttp {
 
     MapWrite() {
       this.getType().hasQualifiedName("net/http", "Header") and
-      exists(Write write, DataFlow::Node base |
+      any(Write write).writesElementPreUpdate(this, index, rhs)
-        write.writesElement(base, index, rhs) and
-        // The following line works because `Http::HeaderWrite::Range` extends
-        // `DataFlow::ExprNode`, which is incompatible with
-        // `DataFlow::PostUpdateNode`.
-        this = [base, base.(DataFlow::PostUpdateNode).getPreUpdateNode()]
-      )
     }
 
     override DataFlow::Node getName() { result = index }