Add toolchains support to Gradle plugin

[3flex]

This will need some tweaking to make sure it works properly with incremental builds and build cache.

This toolchain support does what the Kotlin Gradle Plugin does, but for detekt. If a toolchain is configured for the project (either using java or kotlin) that will then set both jdkTarget and jdkHome defaults on any Detekt or DetektCreateBaseline tasks.

See here for more on the KGP design goals.

In Kotlin, toolchain support affects only the Kotlin/JVM -jdk-home option’s value and additionally sets the -jvm-target value if it was not set explicitly by the user.

Notably this means that detekt itself will still run on the JDK that Gradle runs on - if Gradle is running on Java 8, then so will detekt. This means that despite what I said, I expect this PR will have no effect on #3396.

Closes #4120

[BraisGabin]

I'm not sure what this PR will provide to our users. Let me explain what I think it will provide and you tell me if I'm wrong.

If we run detekt-cli defining --jdk-home the creation of BindingContext will have the information related with the classes inside the JDK. For example: java.time.Instant. But, weren't we adding those classes already?

And then, if we run detekt using the plugin the --jdk-home will be informed automatically taking that information from the configuration of the kotlin gradle plugin. For that reason we don't need to add any new configuration to out plugin, right?

And your last sentence means that the --jdk-home only works to provide classes information. That jdk will not be used to execute anything, right?

[3flex]

weren't we adding those classes already?

There's special handling in the Kotlin compiler that takes care of it. detekt calls this to make sure it's configured: https://github.com/JetBrains/kotlin/blob/v1.7.10/compiler/cli/src/org/jetbrains/kotlin/cli/jvm/config/JvmContentRoots.kt#L91-L111

The problem is, if JDK home isn't provided, the compiler will use the JRE it's currently running in to populate the JDK class roots. When detekt is run from Gradle, that will be the JDK Gradle is running under. If the Kotlin compilation is using a different JDK version set through a toolchain, there's a mismatch between detekt's JDK home and the Kotlin compilation JDK home, which affects detekt's ability to analyse the project correctly.

if we run detekt using the plugin the --jdk-home will be informed automatically taking that information from the configuration of the kotlin gradle plugin. For that reason we don't need to add any new configuration to out plugin, right?

Are you asking if the value will be set by default? Yes, it will be - if there's a toolchain setup. If not, detekt will run using the JDK that Gradle is running under.

That jdk will not be used to execute anything, right?
Correct. detekt will still run using the JDK that Gradle runs.

---

For an example, you can run Gradle with Java 8, add java.util.Optional.stream (added in Java 9) to ForbiddenMethodCall, and setup a toolchain with:

java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(11))
    }
}

Then call the forbidden method somewhere in the project. assemble works, because Java 11 is applied to the compilation using the toolchain. detektMain works, but analysis is not correct as it uses Java 8 to setup the JDK roots, so it throws these warnings:

\C:\Users\3flex\IdeaProjects\toolchains-demo\src\main\kotlin\Main.kt:7:18: error: type mismatch: inferred type is Unit but Stream<out TypeVariable(R)!>! was expected
        .flatMap { it.stream() }
                 ^
\C:\Users\3flex\IdeaProjects\toolchains-demo\src\main\kotlin\Main.kt:7:23: error: unresolved reference: stream
        .flatMap { it.stream() }
                      ^
The BindingContext was created with 2 issues. Run detekt with --debug to see the error messages.

The forbidden method call is not picked up as an error.

When using this PR, and the toolchain's JDK path passed to detekt, it works correctly:

C:\Users\3flex\IdeaProjects\toolchains-demo\src\main\kotlin\Main.kt:7:23: The method `java.util.Optional.stream` has been forbidden in the detekt config. [ForbiddenMethodCall]

Gradle is still running Java 8, so detekt itself also runs on Java 8 but now performs analysis with JDK 11 passed as jdkHome, so the issue is flagged as expected.

[BraisGabin]

Thanks for the explanation. All clear now :)

[cortinico]

Code looks great to me 👍

[cortinico]

This needs an update to the CLI page on the website as well

[3flex]

On this page https://detekt.dev/docs/gettingstarted/cli? Isn't that auto generated now? Or have I misunderstood...

That said, it needs to be added to the Grade docs since they're manually maintained, will do that before merging.

[cortinico]

Isn't that auto generated now?

Yes you're right 👍 I forgot we autogenerate that 🚀