sqlx-core 0.5.9

Core of SQLx, the rust SQL toolkit. Not intended to be used directly.
Documentation 
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use crate::describe::Describe;
use crate::error::Error;
use crate::sqlite::connection::explain::explain;
use crate::sqlite::statement::VirtualStatement;
use crate::sqlite::type_info::DataType;
use crate::sqlite::{Sqlite, SqliteColumn, SqliteConnection};
use either::Either;
use futures_core::future::BoxFuture;
use std::convert::identity;

pub(super) fn describe<'c: 'e, 'q: 'e, 'e>(
    conn: &'c mut SqliteConnection,
    query: &'q str,
) -> BoxFuture<'e, Result<Describe<Sqlite>, Error>> {
    Box::pin(async move {
        // describing a statement from SQLite can be involved
        // each SQLx statement is comprised of multiple SQL statements

        let statement = VirtualStatement::new(query, false);

        let mut columns = Vec::new();
        let mut nullable = Vec::new();
        let mut num_params = 0;

        let mut statement = statement?;

        // we start by finding the first statement that *can* return results
        while let Some((stmt, ..)) = statement.prepare(&mut conn.handle)? {
            num_params += stmt.bind_parameter_count();

            let mut stepped = false;

            let num = stmt.column_count();
            if num == 0 {
                // no columns in this statement; skip
                continue;
            }

            // next we try to use [column_decltype] to inspect the type of each column
            columns.reserve(num);

            // as a last resort, we explain the original query and attempt to
            // infer what would the expression types be as a fallback
            // to [column_decltype]

            // if explain.. fails, ignore the failure and we'll have no fallback
            let (fallback, fallback_nullable) = match explain(conn, stmt.sql()).await {
                Ok(v) => v,
                Err(err) => {
                    log::debug!("describe: explain introspection failed: {}", err);

                    (vec![], vec![])
                }
            };

            for col in 0..num {
                let name = stmt.column_name(col).to_owned();

                let type_info = if let Some(ty) = stmt.column_decltype(col) {
                    ty
                } else {
                    // if that fails, we back up and attempt to step the statement
                    // once *if* its read-only and then use [column_type] as a
                    // fallback to [column_decltype]
                    if !stepped && stmt.read_only() {
                        stepped = true;
                        let _ = conn.worker.step(stmt).await;
                    }

                    let mut ty = stmt.column_type_info(col);

                    if ty.0 == DataType::Null {
                        if let Some(fallback) = fallback.get(col).cloned() {
                            ty = fallback;
                        }
                    }

                    ty
                };

                // check explain
                let col_nullable = stmt.column_nullable(col)?;
                let exp_nullable = fallback_nullable.get(col).copied().and_then(identity);

                nullable.push(exp_nullable.or(col_nullable));

                columns.push(SqliteColumn {
                    name: name.into(),
                    type_info,
                    ordinal: col,
                });
            }
        }

        Ok(Describe {
            columns,
            parameters: Some(Either::Right(num_params)),
            nullable,
        })
    })
}