Fix ComposePreviewPublic to not look into non-public methods (#85)

`ComposePreviewPublic` was looking into private/internal methods and could have false positives if they used `PreviewParameter`s. This patch changes that behavior and adds a test to prevent regressions.

Author: mrmans0n

rules/common/src/main/kotlin/com/twitter/compose/rules/ComposePreviewPublic.kt

@@ -15,11 +15,12 @@ class ComposePreviewPublic : ComposeKtVisitor {
     override fun visitComposable(function: KtFunction, autoCorrect: Boolean, emitter: Emitter) {
         // We only want previews
         if (!function.isPreview) return
-        // ...that if it is public, none of it's params is tagged as preview
-        if (function.isPublic && function.valueParameters.none { it.isPreviewParameter }) return
-        // ...and if it isn't public, all params are tagged as preview
-        if (!function.isPublic && function.valueParameters.all { it.isPreviewParameter }) return
+        // We only care about public methods
+        if (!function.isPublic) return
+        // If the method is public, none of it's params should be tagged as preview
+        if (function.valueParameters.none { it.isPreviewParameter }) return
 
+        // If we got here, it's a public method in a @Preview composable with a @PreviewParameter parameter
         emitter.report(function, ComposablesPreviewShouldNotBePublic, true)
         if (autoCorrect) {
             function.addModifier(KtTokens.PRIVATE_KEYWORD)

rules/detekt/src/test/kotlin/com/twitter/compose/rules/detekt/ComposePreviewPublicCheckTest.kt

@@ -57,6 +57,24 @@ class ComposePreviewPublicCheckTest {
         }
     }
 
+    @Test
+    fun `passes when a non-public preview composable uses preview params`() {
+        @Language("kotlin")
+        val code =
+            """
+            @Preview
+            @Composable
+            private fun MyComposable(@PreviewParameter(User::class) user: User) {
+            }
+            @Preview
+            @Composable
+            internal fun MyComposable(@PreviewParameter(User::class) user: User) {
+            }
+            """.trimIndent()
+        val errors = rule.lint(code)
+        assertThat(errors).isEmpty()
+    }
+
     @Test
     fun `passes when a private preview composable uses preview params`() {
         @Language("kotlin")

rules/ktlint/src/test/kotlin/com/twitter/compose/rules/ktlint/ComposePreviewPublicCheckTest.kt

@@ -51,6 +51,23 @@ class ComposePreviewPublicCheckTest {
         )
     }
 
+    @Test
+    fun `passes when a non-public preview composable uses preview params`() {
+        @Language("kotlin")
+        val code =
+            """
+            @Preview
+            @Composable
+            private fun MyComposable(@PreviewParameter(User::class) user: User) {
+            }
+            @Preview
+            @Composable
+            internal fun MyComposable(@PreviewParameter(User::class) user: User) {
+            }
+            """.trimIndent()
+        ruleAssertThat(code).hasNoLintViolations()
+    }
+
     @Test
     fun `autofix makes private the public preview`() {
         @Language("kotlin")