add support for path references

src/main/java/org/nixos/idea/imports/NixFilePathReferenceContributor.kt

@@ -0,0 +1,63 @@
+package org.nixos.idea.imports
+
+import com.intellij.codeInsight.lookup.LookupElement
+import com.intellij.openapi.util.TextRange
+import com.intellij.openapi.util.io.FileUtil
+import com.intellij.openapi.vfs.LocalFileSystem
+import com.intellij.openapi.vfs.VirtualFile
+import com.intellij.openapi.vfs.VirtualFileSystem
+import com.intellij.patterns.PlatformPatterns
+import com.intellij.psi.PsiElement
+import com.intellij.psi.PsiManager
+import com.intellij.psi.PsiReference
+import com.intellij.psi.PsiReferenceBase
+import com.intellij.psi.PsiReferenceContributor
+import com.intellij.psi.PsiReferenceProvider
+import com.intellij.psi.PsiReferenceRegistrar
+import com.intellij.util.ProcessingContext
+import org.nixos.idea.psi.impl.NixExprStdPathMixin
+
+class NixFilePathReferenceContributor: PsiReferenceContributor() {
+    override fun registerReferenceProviders(registrar: PsiReferenceRegistrar) {
+        registrar.registerReferenceProvider(
+            PlatformPatterns.psiElement(NixExprStdPathMixin::class.java),
+            object : PsiReferenceProvider() {
+                override fun getReferencesByElement(
+                    element: PsiElement,
+                    context: ProcessingContext
+                ): Array<PsiReference> {
+                    val it = element as? NixExprStdPathMixin ?: return emptyArray()
+                    return arrayOf(NixImportReferenceImpl(it))
+                }
+            }
+        )
+    }
+}
+
+private class NixImportReferenceImpl(key: NixExprStdPathMixin) : PsiReferenceBase<NixExprStdPathMixin>(key) {
+    override fun resolve(): PsiElement? {
+        val path = element.containingFile.parent?.virtualFile?.path ?: return null
+        val fs = LocalFileSystem.getInstance()
+        val file = resolvePath(fs, path, element.text) ?: return null
+
+        val project = element.project
+        val psiFile = PsiManager.getInstance(project).findFile(file)
+
+        return psiFile
+    }
+
+    override fun getVariants(): Array<out LookupElement> = LookupElement.EMPTY_ARRAY
+
+    override fun calculateDefaultRangeInElement(): TextRange = TextRange.from(0, element.textLength)
+}
+
+private fun resolvePath(fs: VirtualFileSystem, cwd: String, target: String): VirtualFile? {
+    val resolved = FileUtil.join(cwd, target)
+    val resolvedFile = fs.findFileByPath(resolved) ?: return null
+
+    if (resolvedFile.isDirectory) {
+        return resolvedFile.findChild("default.nix")
+    }
+
+    return resolvedFile
+}

src/main/java/org/nixos/idea/psi/impl/NixExprStdPathMixin.kt

@@ -0,0 +1,10 @@
+package org.nixos.idea.psi.impl
+
+import com.intellij.lang.ASTNode
+import com.intellij.psi.PsiReference
+import com.intellij.psi.impl.source.resolve.reference.ReferenceProvidersRegistry
+
+open class NixExprStdPathMixin(node: ASTNode): NixExprPathImpl(node) {
+    override fun getReferences(): Array<PsiReference> =
+        ReferenceProvidersRegistry.getReferencesFromProviders(this)
+}

src/main/lang/Nix.bnf

@@ -191,7 +191,9 @@ expr_uri ::= URI
 ;{ extends("expr_lookup_path|expr_std_path")=expr_path }
 expr_path ::= expr_lookup_path | expr_std_path
 expr_lookup_path ::= SPATH
-expr_std_path ::= PATH_SEGMENT (PATH_SEGMENT | antiquotation)* PATH_END
+expr_std_path ::= PATH_SEGMENT (PATH_SEGMENT | antiquotation)* PATH_END {
+  mixin="org.nixos.idea.psi.impl.NixExprStdPathMixin"
+}
 expr_parens ::= LPAREN expr recover_parens RPAREN { pin=1 }
 expr_attrs ::= [ REC | LET ] LCURLY recover_set (bind recover_set)* RCURLY { pin=2 }
 expr_list ::= LBRAC recover_list (expr_select recover_list)* RBRAC { pin=1 }

src/main/resources/META-INF/plugin.xml

@@ -38,6 +38,10 @@
         <searcher forClass="com.intellij.find.usages.api.UsageSearchParameters"
                   implementationClass="org.nixos.idea.lang.references.NixUsageSearcher"/>
 
+        <psi.referenceContributor
+                language="Nix"
+                implementation="org.nixos.idea.imports.NixFilePathReferenceContributor" />
+
         <projectConfigurable
                 parentId="build"
                 displayName="NixIDEA Settings"