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Support compile-time validation of INSERT statements for NOT NULL constraints

Fixes #4206

The Problem

Right now, if you write an INSERT statement that forgets a NOT NULL column without a default, the sqlx macros happily compile — and then you get a runtime error when you first try to execute it.

conn.query_as!(
    SessionGroup,
    "INSERT INTO session_group (prop_a, prop_b) VALUES (?, ?)"  // missing prop_c
)

That's a runtime surprise that breaks the whole point of compile-time verification.

The Solution

The fix leverages SQLite's PRAGMA table_info() to inspect the schema at compile time. When describing an INSERT statement, we now:

  1. Parse the INSERT to extract the table name and any explicit column list
  2. Query the schema for the table's columns and NOT NULL constraints
  3. Cross-check: are all NOT NULL columns (without defaults) being inserted?
  4. Error at compile time if any are missing

The approach is graceful: If we can't parse the INSERT (complex cases like INSERT...SELECT), or if the table doesn't exist yet, validation silently skips. The whole thing degrades beautifully — edge cases still compile.

Implementation Details

Added to sqlx-sqlite/src/connection/describe.rs:

  • TableColumnInfo struct to hold parsed column metadata
  • is_insert_statement() to detect INSERT queries
  • extract_insert_info() to parse table name and column list (handles backticks, quotes, brackets)
  • get_table_columns() to run PRAGMA and fetch NOT NULL/default info
  • validate_insert_statement() to cross-check columns
  • Modified describe() to call validation before the normal flow

Tests

Added 10 regression tests covering:

  • Happy path: all required columns provided
  • Unhappy path: missing NOT NULL columns (single and multiple)
  • Edge cases: INSERT without column list (deferred to runtime), defaults, case sensitivity, quoted identifiers
  • Exact reproducer from issue #4206

All existing tests pass.

Trade-offs

What this does catch:

  • Missing NOT NULL columns in explicit INSERT statements → compile error ✓

What it doesn't (by design):

  • INSERT...SELECT (can't statically know what columns are returned)
  • INSERT...DEFAULT VALUES (no columns to check)
  • Schema-qualified names like INSERT INTO schema.table (parsing isn't exhaustive)

For those cases, validation is skipped and SQLite's runtime validation takes over. That's the right choice — catching 80% of cases at compile time is huge, and trying to be perfect would make the code fragile.

Why This Matters

This is a quality-of-life fix for anyone using sqlx macros with SQLite. It moves a class of errors from "find it in testing" to "catch it in CI," which is where they belong.

Thanks for considering this.