me: fix regression in enum column type diffing

This regression was reported by users in
prisma/prisma#16180

It was introduced on b6400f1 by the
same change that caused the _other_ regression that was fixed in
eb39236.

Symptoms: enum migrations became non-idempotent in many cases, depending
on the ordering of the migrations in the Prisma schema.

In that refactoring, we started storing enum ids instead of enum names
in `ColumnTypeFamily::Enum(_)`. That makes the `ColumntypeFamily` type
smaller, easier to deal with with regards to ownership (`EnumId` is
`Copy`), and ready for multi-schema, because the name can be ambiguous
and does not carry schema information, whereas the `EnumId` does. But
more relevant to this commit: it makes comparing two
`ColumnTypeFamily` inaccurate _when the types come from two different
schemas_, like in sql_schema_differ.

That in turn changed the signature of `column_type_family_as_enum()`,
leading to the differ comparing enum ids instead of enum names, which
directly causes the false positives in diffing, because the same enum
has a different ID in each of the two schemas. The code that caused the
regression does not appear in b6400f1,
it uses code that was changed there.

This is also a problem with the generated implementation of PartialEq
for ColumnTypeFamily. Since it would be a larger refactoring, we only
deal with this where relevant (sql_schema_differ) and leave removing the
dangerous PartialEq implementation to another PR. This is the issue:
#3373.

How could this have been caught?

- Tests that check that migrations are idempotent with multiple enums,
  all used in model columns _not defined in alphabetical order_ (sad but true).
  - This chimes with the theme of us not testing with large enough
    Prisma schemas.
- migrations-ci could have caught this. We are neglecting it at the
  moment.

migration-engine/connectors/sql-migration-connector/src/sql_schema_differ/sql_schema_differ_flavour/postgres.rs

@@ -54,7 +54,10 @@ impl SqlSchemaDifferFlavour for PostgresFlavour {
 
     fn column_type_change(&self, columns: Pair<ColumnWalker<'_>>) -> Option<ColumnTypeChange> {
         // Handle the enum cases first.
-        match columns.map(|col| col.column_type_family().as_enum()).into_tuple() {
+        match columns
+            .map(|col| col.column_type_family_as_enum().map(|e| e.name()))
+            .into_tuple()
+        {
             (Some(previous_enum), Some(next_enum)) if previous_enum == next_enum => return None,
             (Some(_), Some(_)) => return Some(ColumnTypeChange::NotCastable),
             (None, Some(_)) | (Some(_), None) => return Some(ColumnTypeChange::NotCastable),

migration-engine/migration-engine-tests/tests/single_migration_tests/mysql/enums/enum_fields.prisma

@@ -0,0 +1,32 @@
+// tags=mysql
+
+datasource mydb {
+    provider = "mysql"
+    url = env("TEST_DATABASE_URL")
+}
+
+model MyTable {
+    id Int @id
+    k Kind? @default(error)
+    r Result @default(succeeded)
+}
+
+enum Result {
+  succeeded
+  failed
+}
+
+enum Kind {
+  info
+  warning
+  error
+}
+// Expected Migration:
+// -- CreateTable
+// CREATE TABLE `MyTable` (
+//     `id` INTEGER NOT NULL,
+//     `k` ENUM('info', 'warning', 'error') NULL DEFAULT 'error',
+//     `r` ENUM('succeeded', 'failed') NOT NULL DEFAULT 'succeeded',
+// 
+//     PRIMARY KEY (`id`)
+// ) DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

migration-engine/migration-engine-tests/tests/single_migration_tests/postgres/enums/enum_fields.prisma

@@ -0,0 +1,51 @@
+// tags=postgres
+// exclude=cockroachdb
+
+datasource mydb {
+    provider = "postgresql"
+    url = env("TEST_DATABASE_URL")
+}
+
+model MyTable {
+    id Int @id
+    a Activity @default(SLEEPING)
+    k Kind? @default(error)
+    r Result @default(succeeded)
+}
+
+enum Result {
+  succeeded
+  failed
+}
+
+enum Activity {
+  DANCING
+  SLEEPING
+  READING
+}
+
+enum Kind {
+  info
+  warning
+  error
+}
+
+// Expected Migration:
+// -- CreateEnum
+// CREATE TYPE "Result" AS ENUM ('succeeded', 'failed');
+// 
+// -- CreateEnum
+// CREATE TYPE "Activity" AS ENUM ('DANCING', 'SLEEPING', 'READING');
+// 
+// -- CreateEnum
+// CREATE TYPE "Kind" AS ENUM ('info', 'warning', 'error');
+// 
+// -- CreateTable
+// CREATE TABLE "MyTable" (
+//     "id" INTEGER NOT NULL,
+//     "a" "Activity" NOT NULL DEFAULT 'SLEEPING',
+//     "k" "Kind" DEFAULT 'error',
+//     "r" "Result" NOT NULL DEFAULT 'succeeded',
+// 
+//     CONSTRAINT "MyTable_pkey" PRIMARY KEY ("id")
+// );