feat(sqlite): no_tx migration support

[AlexTMjugador]

Context, problem statement and description

SQLite includes several SQL statements that are useful during migrations but must be executed outside of a transaction to take effect, such as PRAGMA foreign_keys = ON|OFF or VACUUM. Additionally, advanced migrations may want more precise control over how statements are grouped into transactions or savepoints to achieve the desired atomicity for different parts of the migration.

While SQLx already supports marking migrations to run outside explicit transactions through a -- no-transaction comment, this feature is currently only implemented for PgConnection's Migrate, leaving SQLite and MySQL without this capability. Although it's possible to work around this limitation by implementing custom migration logic instead of executing Migrator#run, this comes at the cost of significantly reduced developer ergonomics: code that relies on the default migration logic, such as #[sqlx::test] or cargo sqlx database setup, won't support these migrations.

These changes extend SqliteConnection's Migrate implementation to support no_tx migrations in the same way as PostgreSQL, addressing this feature gap. I also considered implementing the same functionality for MySQL, but since I haven't found a practical use case for it yet, and every non-transaction-friendly statement I could think about in MySQL triggers implicit commits anyway, I determined it wasn't necessary at this time and could be considered an overreach.

Does your PR solve an issue?

To my knowledge, this PR doesn't directly address any previously reported issue in this repository. I believe I may be the first person that happens to both attempt using SQLx with SQLite migrations containing "advanced" SQL statements and have the motivation to improve things at the source :)

Is this a breaking change?

Technically yes, as SQLite migrations that begin with a -- no-transaction comment will now be executed outside of a transaction, which represents a significant behavioral change. However, I doubt this will cause substantial disruption in practice for several reasons:

• Users who needed to execute migrations outside of a transaction in SQLite already had alternative approaches that remain unaffected by this change.
• It's highly unlikely that someone would accidentally include a -- no-transaction comment without intending to disable the automatic migration transaction. Since this functionality didn't work previously, users likely either settled for removing the comment or implementing alternative solutions, as mentioned above. This change could therefore be considered a bug fix, and Hyrum's law is less likely to apply to behaviors not useful to achieve any outcome.

[nk9]

I've updated sqlx in my project to 0.9.0-alpha.1 and put -- no-transaction at the top of my migration file, but I'm still getting cannot start a transaction within a transaction with DATABASE_URL=sqlite:/path/to/db.sqlite sqlx migrate run. I have the latest sqlx-cli (0.8.6), and there doesn't seem to be an alpha available for that.

Is a newer version of the CLI necessary to take adavantage of this new feature?

Update: It is, and can be installed today using cargo install sqlx-cli --version '0.9.0-alpha.1'