Fix TIMESTAMP_DIFF/DATETIME_DIFF transpilation to Presto/Trino [CLAUDE]

[winklemad]

Presto and Trino only have date_diff(unit, ts1, ts2) — they have no TIMESTAMPDIFF, DATETIME_DIFF, or TIMESTAMP_DIFF. sqlglot maps exp.DateDiff to DATE_DIFF for Presto, but exp.TimestampDiff and exp.DatetimeDiff (produced by e.g. BigQuery TIMESTAMP_DIFF/DATETIME_DIFF, or MySQL/Snowflake TIMESTAMPDIFF) had no Presto/Trino handler and fell through to the generic function fallback:

>>> import sqlglot
>>> sqlglot.transpile("SELECT TIMESTAMP_DIFF(a, b, DAY)", read="bigquery", write="presto")
['SELECT TIMESTAMPDIFF(a, b, DAY)']   # invalid: no such function, and unit-last arg order
>>> sqlglot.transpile("SELECT DATETIME_DIFF(a, b, MONTH)", read="bigquery", write="trino")
['SELECT DATETIME_DIFF(a, b, MONTH)'] # invalid

Fix

Route all three diff nodes through DATE_DIFF(unit, expression, this) for Presto (Trino inherits Presto's generator). Since date_diff(unit, ts1, ts2) = ts2 - ts1, passing (expression, this) preserves the source's a - b semantics — the same output DATE_DIFF and DuckDB already produce. This mirrors the already-merged fixes for the same bug class in other dialects (#6126 for DuckDB, #4334 for Snowflake).

After:

['SELECT DATE_DIFF(\'DAY\', b, a)']    # presto
['SELECT DATE_DIFF(\'MONTH\', b, a)']  # trino

Tests

Added assertions to the existing BigQuery TIMESTAMP_DIFF/DATETIME_DIFF validate_all blocks (presto + trino outputs). They fail on the current code and pass with the fix. tests/dialects/test_bigquery.py → 58 passed / 1073 subtests.

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Disclosure: this fix was prepared with AI assistance (hence the [CLAUDE] tag per AGENTS.md). I reviewed every line, verified the Presto/Trino semantics against their docs, and confirmed the reproduction and tests myself.

[georgesittas]

This looks ok. I'll do follow-up cleaning up post-merge. Thank you.