docs: move style guide into a separate document

Code contribution guidelines
are rearranged into a list of steps to follow.

STYLE.md

@@ -0,0 +1,98 @@
+# Coding conventions
+
+We format the code using `rustfmt`. Run `cargo fmt` prior to committing the code.
+Run `scripts/clippy.sh` to check the code for common mistakes with [Clippy].
+
+[Clippy]: https://doc.rust-lang.org/clippy/
+
+## SQL
+
+Multi-line SQL statements should be formatted using string literals,
+for example
+```
+    sql.execute(
+        "CREATE TABLE messages (
+id INTEGER PRIMARY KEY AUTOINCREMENT,
+text TEXT DEFAULT '' NOT NULL -- message text
+) STRICT",
+    )
+    .await?;
+```
+
+Do not use macros like [`concat!`](https://doc.rust-lang.org/std/macro.concat.html)
+or [`indoc!](https://docs.rs/indoc).
+Do not escape newlines like this:
+```
+    sql.execute(
+        "CREATE TABLE messages ( \
+id INTEGER PRIMARY KEY AUTOINCREMENT, \
+text TEXT DEFAULT '' NOT NULL \
+) STRICT",
+    )
+    .await?;
+```
+Escaping newlines
+is prone to errors like this if space before backslash is missing:
+```
+"SELECT foo\
+ FROM bar"
+```
+Literal above results in `SELECT fooFROM bar` string.
+This style also does not allow using `--` comments.
+
+---
+
+Declare new SQL tables with [`STRICT`](https://sqlite.org/stricttables.html) keyword
+to make SQLite check column types.
+
+Declare primary keys with [`AUTOINCREMENT`](https://www.sqlite.org/autoinc.html) keyword.
+This avoids reuse of the row IDs and can avoid dangerous bugs
+like forwarding wrong message because the message was deleted
+and another message took its row ID.
+
+Declare all new columns as `NOT NULL`
+and set the `DEFAULT` value if it is optional so the column can be skipped in `INSERT` statements.
+Dealing with `NULL` values both in SQL and in Rust is tricky and we try to avoid it.
+If column is already declared without `NOT NULL`, use `IFNULL` function to provide default value when selecting it.
+Use `HAVING COUNT(*) > 0` clause
+to [prevent aggregate functions such as `MIN` and `MAX` from returning `NULL`](https://stackoverflow.com/questions/66527856/aggregate-functions-max-etc-return-null-instead-of-no-rows).
+
+Don't delete unused columns too early, but maybe after several months/releases, unused columns are
+still used by older versions, so deleting them breaks downgrading the core or importing a backup in
+an older version. Also don't change the column type, consider adding a new column with another name
+instead. Finally, never change column semantics, this is especially dangerous because the `STRICT`
+keyword doesn't help here.
+
+## Errors
+
+Delta Chat core mostly uses [`anyhow`](https://docs.rs/anyhow/) errors.
+When using [`Context`](https://docs.rs/anyhow/latest/anyhow/trait.Context.html),
+capitalize it but do not add a full stop as the contexts will be separated by `:`.
+For example:
+```
+.with_context(|| format!("Unable to trash message {msg_id}"))
+```
+
+All errors should be handled in one of these ways:
+- With `if let Err() =` (incl. logging them into `warn!()`/`err!()`).
+- With `.log_err().ok()`.
+- Bubbled up with `?`.
+
+`backtrace` feature is enabled for `anyhow` crate
+and `debug = 1` option is set in the test profile.
+This allows to run `RUST_BACKTRACE=1 cargo test`
+and get a backtrace with line numbers in resultified tests
+which return `anyhow::Result`.
+
+## Logging
+
+For logging, use `info!`, `warn!` and `error!` macros.
+Log messages should be capitalized and have a full stop in the end. For example:
+```
+info!(context, "Ignoring addition of {added_addr:?} to {chat_id}.");
+```
+
+Format anyhow errors with `{:#}` to print all the contexts like this:
+```
+error!(context, "Failed to set selfavatar timestamp: {err:#}.");
+```