PsiReference in yaml file works differently when i use it thought ReferenceInjector and when i use PsiReferenceContributor.

I can't reproduce "it works with Injector, but not with Contributor" (and didn't expect it would make a difference).

Actually, highlighting unresolved references must be performed by the plugin itself. There is no "automatic highlighting" unless some other inspection is - by accident - also processing your references and adding highlights for them. You can use Annotator or Inspector for such highlighting, see https://plugins.jetbrains.com/docs/intellij/syntax-highlighting-and-error-highlighting.html#annotator.

Notes: Please upgrade id 'org.jetbrains.intellij' version '0.6.5' to 0.7.2 in Gradle

The actual resolving should occur _inside_ com.icthh.xm.demoplugin.LinkTypeKeyReference#resolve method, not in PsiReferenceProvider, also some cleanup. You can use getVariants() here to quickly visually test your reference is working at expected range.

class YamlPsiReferenceContributor: PsiReferenceContributor() {
    override fun registerReferenceProviders(registrar: PsiReferenceRegistrar) {
        registrar.registerReferenceProvider(
            getPattern(),
            object : PsiReferenceProvider() {
                override fun getReferencesByElement(
                    element: PsiElement,
                    context: ProcessingContext
                ): Array<PsiReference> {
                    return arrayOf(LinkTypeKeyReference(element))
                }
            },
            HIGHER_PRIORITY
        )
    }

    private fun getPattern() = psiElement<YAMLScalar>().withParent(
        psiElement<YAMLKeyValue>().withName("link").withParent(
            psiElement<YAMLMapping>().withParent(
                psiElement<YAMLSequenceItem>().withParent(
                    psiElement<YAMLSequence>().withParent(
                        psiElement<YAMLKeyValue>().withName("list")
                    )
                )
            )
        )
    )

    inline fun <reified T : PsiElement> psiElement() = psiElement(T::class.java)

}

class LinkTypeKeyReference(target:PsiElement):
    PsiReferenceBase<PsiElement>(target),
    EmptyResolveMessageProvider {
    override fun resolve() = findReferenceTarget(element)
    override fun getVariants(): Array<Any> {
        return arrayOf("1","2","3")
    }

    override fun getUnresolvedMessagePattern(): String = "Link with value $value not found"
}