feat(rules): add preferSchemaUnion diagnostic and code fix for Effect v4

.changeset/add-prefer-schema-union.md

@@ -0,0 +1,22 @@
+---
+"@effect/tsgo": minor
+---
+
+Add `preferSchemaUnion` diagnostic (`TS377130`) and code fix for Effect v4 to suggest composing Schema values with `Schema.Union` instead of unioning their extracted `Type` properties.
+
+### Example
+
+```ts
+// Before
+export const Circle = Schema.Struct({ kind: Schema.Literal("circle"), radius: Schema.Number })
+export const Square = Schema.Struct({ kind: Schema.Literal("square"), side: Schema.Number })
+
+export type Shape = typeof Circle.Type | typeof Square.Type
+
+// After applying code fix
+export const Circle = Schema.Struct({ kind: Schema.Literal("circle"), radius: Schema.Number })
+export const Square = Schema.Struct({ kind: Schema.Literal("square"), side: Schema.Number })
+
+export const Shape = Schema.Union([Circle, Square])
+export type Shape = typeof Shape.Type
+```

README.md

@@ -151,6 +151,7 @@ Some diagnostics are off by default or have a default severity of suggestion, bu
     <tr><td><a href="https://github.com/Effect-TS/tsgo/blob/main/docs/rules/new-schema-class.md"><code>newSchemaClass</code></a></td><td>Suggests using Schema make instead of new for Schema classes</td></tr>
     <tr><td><a href="https://github.com/Effect-TS/tsgo/blob/main/docs/rules/option-match-to-from-option.md"><code>optionMatchToFromOption</code></a></td><td>Suggests Effect.fromOption when Option.match or an Option tag conditional only converts Some to Effect.succeed and None to Effect.fail</td></tr>
     <tr><td><a href="https://github.com/Effect-TS/tsgo/blob/main/docs/rules/prefer-schema-type-property.md"><code>preferSchemaTypeProperty</code></a></td><td>Disallows Schema.Schema.Type&lt;typeof X&gt; in favor of typeof X.Type</td></tr>
+    <tr><td><a href="https://github.com/Effect-TS/tsgo/blob/main/docs/rules/prefer-schema-union.md"><code>preferSchemaUnion</code></a></td><td>Suggests composing Schema values with Schema.Union instead of unioning their extracted Type properties</td></tr>
     <tr><td><a href="https://github.com/Effect-TS/tsgo/blob/main/docs/rules/prefer-succeed-some-or-none.md"><code>preferSucceedSomeOrNone</code></a></td><td>Suggests using Effect.succeedNone or Effect.succeedSome instead of wrapping Option.none or Option.some with Effect.succeed</td></tr>
     <tr><td><a href="https://github.com/Effect-TS/tsgo/blob/main/docs/rules/prefer-typed-schema-decoder.md"><code>preferTypedSchemaDecoder</code></a></td><td>Suggests typed Schema decoders when the input is assignable to the schema&#39;s Encoded type</td></tr>
     <tr><td><a href="https://github.com/Effect-TS/tsgo/blob/main/docs/rules/provide-layer-succeed-to-provide-service.md"><code>provideLayerSucceedToProvideService</code></a></td><td>Suggests providing inline Layer.succeed and Layer.effect services directly</td></tr>

_packages/tsgo/src/metadata.json

@@ -2232,6 +2232,29 @@
         ]
       }
     },
+    {
+      "name": "preferSchemaUnion",
+      "group": "style",
+      "description": "Suggests composing Schema values with Schema.Union instead of unioning their extracted Type properties",
+      "defaultSeverity": "off",
+      "fixable": true,
+      "supportedEffect": [
+        "v4"
+      ],
+      "codes": [
+        377130
+      ],
+      "preview": {
+        "sourceText": "import { Schema } from \"effect\"\n\nexport const Circle = Schema.Struct({ kind: Schema.Literal(\"circle\"), radius: Schema.Number })\nexport const Square = Schema.Struct({ kind: Schema.Literal(\"square\"), side: Schema.Number })\n\nexport type Shape = typeof Circle.Type | typeof Square.Type\n",
+        "diagnostics": [
+          {
+            "start": 242,
+            "end": 281,
+            "text": "This type alias unions decoded Effect Schema types. Prefer Schema.Union([...]) and derive the type from the resulting schema. effect(preferSchemaUnion)"
+          }
+        ]
+      }
+    },
     {
       "name": "preferSucceedSomeOrNone",
       "group": "style",

docs/rules/prefer-schema-union.md

@@ -0,0 +1,66 @@
+<!-- This file is generated by internal/rules/rules_json_test.go. Do not edit it manually. -->
+
+# `preferSchemaUnion`
+
+Suggests composing Schema values with Schema.Union instead of unioning their extracted Type properties
+
+| Property | Value |
+| --- | --- |
+| Category | Style |
+| Default severity | `off` |
+| Fixable | Yes |
+| Effect versions | v4 |
+| Diagnostic codes | `TS377130` |
+| Language Service name | `preferSchemaUnion` |
+| Oxlint name | `effecttsgo/prefer-schema-union` |
+
+## Preview
+
+```ts
+import { Schema } from "effect"
+
+export const Circle = Schema.Struct({ kind: Schema.Literal("circle"), radius: Schema.Number })
+export const Square = Schema.Struct({ kind: Schema.Literal("square"), side: Schema.Number })
+
+export type Shape = typeof Circle.Type | typeof Square.Type
+/**
+                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ effecttsgo(prefer-schema-union): This type alias unions decoded Effect Schema types. Prefer Schema.Union([...]) and derive the type from the resulting schema.
+*/
+```
+
+## Language Service Configuration
+
+See the [Language Service setup guide](../../README.md#installation) for installation instructions.
+
+```jsonc
+{
+  "$schema": "./node_modules/@effect/tsgo/schema.json",
+  "compilerOptions": {
+    "plugins": [
+      {
+        "name": "@effect/language-service",
+        "diagnosticSeverity": {
+          "preferSchemaUnion": "warning"
+        }
+      }
+    ]
+  }
+}
+```
+
+## Oxlint Configuration
+
+See the [Oxlint setup guide](../README.md#oxlint-setup) for installation and patching instructions.
+
+```json
+{
+  "$schema": "./node_modules/@effect/tsgo/oxlint-schema.json",
+  "options": {
+    "typeAware": true
+  },
+  "plugins": ["effecttsgo"],
+  "rules": {
+    "effecttsgo/prefer-schema-union": "warn"
+  }
+}
+```

internal/diagnostics/effectDiagnosticMessages.json

@@ -506,5 +506,9 @@
     "This module reference imports `{0}`, which is obsolete in Effect v4. In Effect v4, Schema is provided directly by `Schema` from `effect` (or `effect/Schema`). effect(obsoleteSchemaImport)": {
         "category": "Warning",
         "code": 377128
+    },
+    "This type alias unions decoded Effect Schema types. Prefer Schema.Union([...]) and derive the type from the resulting schema. effect(preferSchemaUnion)": {
+        "category": "Suggestion",
+        "code": 377130
     }
 }

internal/fixables/fixables.go

@@ -39,6 +39,7 @@ var All = []fixable.Fixable{
 	UnnecessaryArrowBlockFix,
 	UnnecessaryTypeofTypeFix,
 	PreferSchemaTypePropertyFix,
+	PreferSchemaUnionFix,
 	EffectMapVoidFix,
 	UnnecessaryFailYieldableErrorFix,
 	ClassSelfMismatchFix,