Provide some code style for rust

docs/contributing/code-style/rust.md

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+---
+
+# Rust
+
+We use `rustfmt` to format our code. This tool is part of the Rust toolchain and will format your
+code according to the Rust style guide. We also use `clippy` to catch common mistakes and improve
+the quality of our code.
+
+This document covers some additional guidelines that are not enforced by `rustfmt` or `clippy`. And
+will generally allow you to write more idiomatic Rust code.
+
+## Avoid panics
+
+Panics are forbidden in the Bitwarden codebase outside of tests. Errors should be handled gracefully
+and returned to the caller. `clippy` will forbid you from using `unwrap`. While `expect` is allowed,
+it should be used sparingly and should always provide a helpful message indicating why it can never
+occur.
+
+Some scenarios where `expect` are allowed are:
+
+- Calling libraries that guarantee that the allowed inputs never results in Err or None.
+- Operating on slices or arrays where the index is guaranteed to be within bounds.
+
+## Discourage match
+
+Rust often provide good alternatives to doing match on `Option`s and `Result`s.
+
+### if let
+
+Use [`if let`](https://doc.rust-lang.org/rust-by-example/flow_control/if_let.html) when you only
+care about one arm of a match.
+
+```rust
+// Bad
+match result {
+  Ok(value) => outer.append(value),
+  _ => (),
+}
+
+// Good
+if let Ok(value) = result {
+  outer.append(value);
+}
+```
+
+### Options
+
+We use `clippy` to enforce general guidelines for working with
+[`Option`](https://doc.rust-lang.org/std/option/enum.Option.html)s. Generally reach for methods
+like:
+
+#### map
+
+Use [`map`](https://doc.rust-lang.org/std/option/enum.Option.html#method.map) when you want to
+transform the value inside the `Option`.
+
+```rust
+// Bad
+match Some(0) {
+  Some(x) => Some(x + 1),
+  None => None,
+};
+
+// Good
+Some(0).map(|x| x + 1);
+```
+
+#### and_then
+
+Use [`and_then`](https://doc.rust-lang.org/std/option/enum.Option.html#method.and_then) when you
+want to chain multiple operations that return an `Option`.
+
+```rust
+func(x: i32) -> Option<i32>;
+
+// Bad
+match Some(0) {
+  Some(x) => func(x + 1),
+  None => None,
+};
+
+// Good
+Some(0).and_then(|x| func(x + 1));
+```
+
+#### unwrap_or
+
+Use [`unwrap_or`](https://doc.rust-lang.org/std/option/enum.Option.html#method.unwrap_or) when you
+want to provide a default value.
+
+```rust
+// Bad
+match Some(0) {
+  Some(x) => x,
+  None => 1,
+};
+
+// Good
+Some(0).unwrap_or(1);
+```
+
+#### ok_or
+
+Use [`ok_or`](https://doc.rust-lang.org/std/option/enum.Option.html#method.ok_or) when you want to
+convert an `Option` to a `Result`.
+
+```rust
+// Bad
+match Some(0) {
+  Some(x) => x,
+  None => Err("error"),
+};
+
+// Good
+foo.ok_or("error");
+```
+
+### Results
+
+When working on [`Result`](https://doc.rust-lang.org/std/result/enum.Result.html)s it might be
+tempting to use match to get the value out of the result. Instead, use the `?` operator to propagate
+the error.
+
+#### map_err
+
+Use [`map_err`](https://doc.rust-lang.org/std/result/enum.Result.html#method.map_err) when you want
+to transform the error inside the `Result`.
+
+```rust
+// Bad
+match result {
+  Ok(value) => value,
+  Err(_) => return Err("Another error"),
+}
+
+// Good
+do_something().map_err(|e| "Another error")?
+```