Move specialized execute method code into an external phase (#51954)

This change moves almost all of the customized code required to decorate 
the execute method into an external phase. This external phase operates 
only on the ir tree after the user tree has generated the initial ir tree from 
user specified code. The external phase uses ir nodes to create the 
necessary customized code instead of writing asm directly. This is a first 
example of modifying the ir tree to customize it specifically for Elasticsearch 
and leaves the ir nodes in a more generic state.

Another change required for this was to remove the notion of auto-return 
from the ir tree completely. The user tree is now responsible for generating 
appropriate ir tree nodes to support auto-return. This is the first example of 
divergent user and ir trees as the user tree is intended to be higher level 
while the ir tree is supposed to provide lower level translation into asm.

Relates to #49869
Relates to #51841

modules/lang-painless/src/main/java/org/elasticsearch/painless/Compiler.java

@@ -211,6 +211,7 @@ ScriptRoot compile(Loader loader, String name, String source, CompilerSettings s
         SClass root = Walker.buildPainlessTree(scriptClassInfo, name, source, settings, painlessLookup, null);
         ScriptRoot scriptRoot = root.analyze(painlessLookup, settings);
         ClassNode classNode = root.writeClass();
+        ScriptInjectionPhase.phase(scriptRoot, classNode);
         Map<String, Object> statics = classNode.write();
 
         try {
@@ -240,8 +241,9 @@ ScriptRoot compile(Loader loader, String name, String source, CompilerSettings s
     byte[] compile(String name, String source, CompilerSettings settings, Printer debugStream) {
         ScriptClassInfo scriptClassInfo = new ScriptClassInfo(painlessLookup, scriptClass);
         SClass root = Walker.buildPainlessTree(scriptClassInfo, name, source, settings, painlessLookup, debugStream);
-        root.analyze(painlessLookup, settings);
+        ScriptRoot scriptRoot = root.analyze(painlessLookup, settings);
         ClassNode classNode = root.writeClass();
+        ScriptInjectionPhase.phase(scriptRoot, classNode);
         classNode.write();
 
         return classNode.getBytes();

modules/lang-painless/src/main/java/org/elasticsearch/painless/MethodWriter.java

@@ -502,7 +502,7 @@ public void invokeMethodCall(PainlessMethod painlessMethod) {
             // method since java 8 did not check, but java 9 and 10 do
             if (painlessMethod.javaMethod.getDeclaringClass().isInterface()) {
                 visitMethodInsn(Opcodes.INVOKESTATIC, type.getInternalName(),
-                        painlessMethod.javaMethod.getName(), painlessMethod.methodType.toMethodDescriptorString(), true);
+                        painlessMethod.javaMethod.getName(), method.getDescriptor(), true);
             } else {
                 invokeStatic(type, method);
             }

modules/lang-painless/src/main/java/org/elasticsearch/painless/Scope.java

@@ -87,9 +87,9 @@ public boolean isFinal() {
     public static class FunctionScope extends Scope {
 
         protected final Class<?> returnType;
-        protected final Set<String> usedVariables = new HashSet<>();
 
         public FunctionScope(Class<?> returnType) {
+            super(new HashSet<>());
             this.returnType = Objects.requireNonNull(returnType);
         }
 
@@ -128,10 +128,6 @@ public Class<?> getReturnType() {
         public String getReturnCanonicalTypeName() {
             return PainlessLookupUtility.typeToCanonicalTypeName(returnType);
         }
-
-        public Set<String> getUsedVariables() {
-            return Collections.unmodifiableSet(usedVariables);
-        }
     }
 
     /**
@@ -150,6 +146,7 @@ public static class LambdaScope extends Scope {
         protected final Set<Variable> captures = new HashSet<>();
 
         protected LambdaScope(Scope parent, Class<?> returnType) {
+            super(parent.usedVariables);
             this.parent = parent;
             this.returnType = returnType;
         }
@@ -180,6 +177,8 @@ public Variable getVariable(Location location, String name) {
                 variable = parent.getVariable(location, name);
                 variable = new Variable(variable.getType(), variable.getName(), true);
                 captures.add(variable);
+            } else {
+                usedVariables.add(name);
             }
 
             return variable;
@@ -213,6 +212,7 @@ public static class BlockScope extends Scope {
         protected final Scope parent;
 
         protected BlockScope(Scope parent) {
+            super(parent.usedVariables);
             this.parent = parent;
         }
 
@@ -238,6 +238,8 @@ public Variable getVariable(Location location, String name) {
 
             if (variable == null) {
                 variable = parent.getVariable(location, name);
+            } else {
+                usedVariables.add(name);
             }
 
             return variable;
@@ -263,9 +265,10 @@ public static FunctionScope newFunctionScope(Class<?> returnType) {
     }
 
     protected final Map<String, Variable> variables = new HashMap<>();
+    protected final Set<String> usedVariables;
 
-    protected Scope() {
-        // do nothing
+    protected Scope(Set<String> usedVariables) {
+        this.usedVariables = usedVariables;
     }
 
     /**
@@ -312,4 +315,13 @@ public boolean isInternalVariableDefined(String name) {
     public Variable getInternalVariable(Location location, String name) {
         return getVariable(location, "#" + name);
     }
+
+    /**
+     * Returns the set of variables used within a top-level {@link FunctionScope}
+     * including local variables no longer in scope upon completion of writing a
+     * function to ASM bytecode.
+     */
+    public Set<String> getUsedVariables() {
+        return Collections.unmodifiableSet(usedVariables);
+    }
 }