Type date_trunc inputs, outputs, and nullability

[alexeichhorn]

Problem

date_trunc currently accepts any value type and always claims to return that same Rust type:

pub fn date_trunc<T>(part: impl IntoExpr<String>, value: impl IntoExpr<T>) -> Expr<T>

This allows invalid calls such as date_trunc("day", integer_column) to compile. DATE and TIME inputs can also receive an incorrect output type after PostgreSQL overload resolution or implicit casts.

The precision argument is fixed to required String, so a nullable precision expression is rejected even though PostgreSQL returns NULL when it is NULL.

Expected behavior

• Support the direct PostgreSQL input/result pairs:
  • TIMESTAMP → TIMESTAMP
  • TIMESTAMPTZ → TIMESTAMPTZ
  • INTERVAL → INTERVAL
• Preserve optional output when the precision or temporal value is nullable.
• Reject unrelated inputs at compile time.
• Reject DATE and TIME unless dbkit emits an explicit, stable cast with the correct output type.
• Cover required/nullable combinations for all supported base types.
• Decide whether to expose PostgreSQL's timestamptz timezone argument as part of the same typed contract.

PostgreSQL date_trunc signatures