Full support for "desugared" lambdas

[skylot]

New d8 android compiler allows to use Java 8 lambdas, but to make it work on old Android versions d8 convert (desugar) lambdas to synthetic classes and methods.
This issue continues work on lambda inlining started in #467.
Known issues:

• Lambda classes inlined
• Some lambda classes used by INSTANCE static field
• Not all arguments inlined and synthetic fields in inlined classes not removed
• lambda$* methods must be inlined
• lambdas initialized at fields declaration (with anonymous class as a field type)

Note: Inlining Kotlin lambdas change code semantic by removing extend of kotlin.jvm.internal.Lambda. To disable such inlines use flag --no-inline-kotlin-lambda in jadx-cli and option Allow to inline Kotlin Lambdas in jadx-gui.

[pubiqq]

Small sample:

Original class

package com.example;

import java.util.function.Function;

class TestJavaClass {

    private final Function<String, String> f = x -> x;

    String test() {
        return f.apply("2");
    }
}

Smali

###### Class com.example.TestJavaClass (com.example.TestJavaClass)
.class Lcom/example/TestJavaClass;
.super Ljava/lang/Object;
.source "TestJavaClass.java"


# instance fields
.field private final f:Ljava/util/function/Function;
    .annotation system Ldalvik/annotation/Signature;
        value = {
            "Ljava/util/function/Function<",
            "Ljava/lang/String;",
            "Ljava/lang/String;",
            ">;"
        }
    .end annotation
.end field


# direct methods
.method constructor <init>()V
    .registers 2

    .line 5
    invoke-direct {p0}, Ljava/lang/Object;-><init>()V

    .line 7
    new-instance v0, Lcom/example/TestJavaClass$$ExternalSyntheticLambda0;

    invoke-direct {v0}, Lcom/example/TestJavaClass$$ExternalSyntheticLambda0;-><init>()V

    iput-object v0, p0, Lcom/example/TestJavaClass;->f:Ljava/util/function/Function;

    return-void
.end method

.method static synthetic lambda$new$0(Ljava/lang/String;)Ljava/lang/String;
    .registers 1
    .param p0, "x"    # Ljava/lang/String;

    .line 7
    return-object p0
.end method


# virtual methods
.method test()Ljava/lang/String;
    .registers 3

    .line 10
    iget-object v0, p0, Lcom/example/TestJavaClass;->f:Ljava/util/function/Function;

    const-string v1, "2"

    invoke-interface {v0, v1}, Ljava/util/function/Function;->apply(Ljava/lang/Object;)Ljava/lang/Object;

    move-result-object v0

    check-cast v0, Ljava/lang/String;

    return-object v0
.end method

###### Class com.example.TestJavaClass$$ExternalSyntheticLambda0 (com.example.TestJavaClass$$ExternalSyntheticLambda0)
.class public final synthetic Lcom/example/TestJavaClass$$ExternalSyntheticLambda0;
.super Ljava/lang/Object;
.source "D8$$SyntheticClass"

# interfaces
.implements Ljava/util/function/Function;


# direct methods
.method public synthetic constructor <init>()V
    .registers 1

    invoke-direct {p0}, Ljava/lang/Object;-><init>()V

    return-void
.end method


# virtual methods
.method public final apply(Ljava/lang/Object;)Ljava/lang/Object;
    .registers 2

    check-cast p1, Ljava/lang/String;

    invoke-static {p1}, Lcom/example/TestJavaClass;->lambda$new$0(Ljava/lang/String;)Ljava/lang/String;

    move-result-object p1

    return-object p1
.end method

Decompiled class (jadx-1.5.0)

package com.example;

import java.util.function.Function;

/* loaded from: classes8.dex */
package com.example;

import java.util.function.Function;

/* loaded from: classes8.dex */
class TestJavaClass {
    private final Function<String, String> f = new Function() { // from class: com.example.TestJavaClass$$ExternalSyntheticLambda0
        @Override // java.util.function.Function
        public final Object apply(Object obj) {
            return TestJavaClass.lambda$new$0((String) obj);
        }
    };

    TestJavaClass() {
    }

    /* JADX INFO: Access modifiers changed from: package-private */
    public static /* synthetic */ String lambda$new$0(String x) {
        return x;
    }

    String test() {
        return this.f.apply("2");
    }
}