Markdown links with custom actions (see https://github.com/dkandalov/live-plugin)

plugin.kts

import com.intellij.codeInsight.navigation.actions.GotoDeclarationHandler
import com.intellij.lang.LanguageAnnotators
import com.intellij.lang.LanguageExtensionPoint
import com.intellij.lang.annotation.Annotator
import com.intellij.lang.annotation.HighlightSeverity.INFORMATION
import com.intellij.openapi.application.ApplicationManager
import com.intellij.openapi.editor.Editor
import com.intellij.openapi.editor.colors.CodeInsightColors.INACTIVE_HYPERLINK_ATTRIBUTES
import com.intellij.openapi.extensions.DefaultPluginDescriptor
import com.intellij.openapi.util.TextRange
import com.intellij.patterns.PsiElementPattern
import com.intellij.psi.PsiElement
import com.intellij.psi.PsiReferenceBase
import com.intellij.psi.PsiReferenceProvider
import com.intellij.psi.SyntheticElement
import com.intellij.psi.impl.FakePsiElement
import com.intellij.psi.impl.source.resolve.reference.PsiReferenceRegistrarImpl
import com.intellij.psi.impl.source.resolve.reference.ReferenceProvidersRegistry
import com.intellij.util.ProcessingContext
import liveplugin.show

val markdown = com.intellij.lang.Language.findLanguageByID("Markdown")!!

val psiPattern = object : PsiElementPattern.Capture<PsiElement>(PsiElement::class.java) {
    override fun accepts(o: Any?, context: ProcessingContext?) = (o as? PsiElement)?.text == "hello"
}
val psiReferenceProvider = object : PsiReferenceProvider() {
    override fun getReferencesByElement(element: PsiElement, context: ProcessingContext) =
        arrayOf(PsiReferenceTo(PsiElementWithAction(element, "action text") {
            show("hello from element: $element")
        }))
}
(ReferenceProvidersRegistry.getInstance().getRegistrar(markdown) as PsiReferenceRegistrarImpl)
    .registerReferenceProvider(psiPattern, psiReferenceProvider, 1.0, pluginDisposable)

class PsiReferenceTo(private val ref: PsiElement) : PsiReferenceBase<PsiElement>(ref.parent, true) {
    override fun resolve(): PsiElement = ref
}

class PsiElementWithAction(
    private val _parent: PsiElement,
    private val _text: String,
    private val action: () -> Unit
) : FakePsiElement(), SyntheticElement {
    override fun navigate(requestFocus: Boolean) = action()
    override fun getParent() = _parent
    override fun getPresentableText() = _text
    override fun getName() = _text
    override fun getTextRange(): TextRange = _parent.textRange
}

if (false) {
    fun annotator() = Annotator { element, holder ->
        val text = element.text
        text.indices
            .filter { i -> text.substring(i, minOf(i + 5, text.length)) == "hello" }
            .forEach { i ->
                holder.newAnnotation(INFORMATION, "Hello!")
                    .range(TextRange(i, i + 5))
                    .textAttributes(INACTIVE_HYPERLINK_ATTRIBUTES)
                    .create()
            }
    }
    val pluginDescriptor = DefaultPluginDescriptor("hello-world")
    ApplicationManager.getApplication().extensionArea.getExtensionPoint(LanguageAnnotators.EP_NAME)
        .registerExtension(
            LanguageExtensionPoint(markdown.id, annotator()).also { it.pluginDescriptor = pluginDescriptor },
            pluginDescriptor,
            pluginDisposable
        )
    LanguageAnnotators.INSTANCE.clearCache(markdown)

    val declarationHandler = GotoDeclarationHandler { element: PsiElement?, _: Int, _: Editor ->
        val i = element?.text?.indexOf("hello")
        if (i == null || i == -1) arrayOf()
        else arrayOf(PsiElementWithAction(element, "foooo") {
            show("hello")
        })
    }
    ApplicationManager.getApplication().extensionArea.getExtensionPoint(GotoDeclarationHandler.EP_NAME)
        .registerExtension(declarationHandler, pluginDisposable)
}