ktlint-ruleset-template/src/main/kotlin/yourpkgname/NoVarRule.kt

package yourpkgname

import com.github.shyiko.ktlint.core.Rule
import org.jetbrains.kotlin.com.intellij.lang.ASTNode
import org.jetbrains.kotlin.com.intellij.psi.impl.source.tree.LeafPsiElement
import org.jetbrains.kotlin.com.intellij.psi.util.PsiTreeUtil.getNonStrictParentOfType
import org.jetbrains.kotlin.psi.KtStringTemplateEntry

class NoVarRule : Rule("no-var") {

    override fun visit(
        node: ASTNode,
        autoCorrect: Boolean,
        emit: (offset: Int, errorMessage: String, canBeAutoCorrected: Boolean) -> Unit
    ) {
        if (node is LeafPsiElement && node.textMatches("var") &&
                getNonStrictParentOfType(node, KtStringTemplateEntry::class.java) == null) {
            emit(node.startOffset, "Unexpected var, use val instead", false)
        }
    }
}