Crate aliri_braid

Expand description

Improve and strengthen your strings

Strongly-typed APIs reduce errors and confusion over passing around un-typed strings. Braid helps in that endeavor by making it painless to create wrappers around your string values, ensuring that you use them in the right way every time.

Examples of the documentation and implementations provided for braids are available below and in the aliri_braid_examples crate documentation.

§Usage

A braid is created by attaching #[braid] to a struct definition. The macro will take care of automatically updating the representation of the struct to wrap a string and generate the borrowed form of the strong type.

use aliri_braid::braid;

#[braid]
pub struct DatabaseName;

Braids of custom string types are also supported, so long as they implement a set of expected traits. If not specified, the type named String in the current namespace will be used. See the section on custom string types for more information.

use aliri_braid::braid;
use compact_str::CompactString as String;

#[braid]
pub struct UserId;

Once created, braids can be passed around as strongly-typed, immutable strings.

fn take_strong_string(n: DatabaseName) {}
fn borrow_strong_string(n: &DatabaseNameRef) {}

let owned = DatabaseName::new(String::from("mongo"));
borrow_strong_string(&owned);
take_strong_string(owned);

A braid can also be untyped for use in stringly-typed interfaces.

fn take_raw_string(s: String) {}
fn borrow_raw_str(s: &str) {}

let owned = DatabaseName::new(String::from("mongo"));
borrow_raw_str(owned.as_str());
take_raw_string(owned.take());

By default, the name of the borrowed form will be the same as the owned form with Ref appended to the end.

#[braid]
pub struct DatabaseName;

let owned = DatabaseName::from_static("mongo");
let borrowed = DatabaseNameRef::from_static("mongo");

If the name ends with Buf, however, then the borrowed form will drop the Buf, similar to the relationship between PathBuf and Path.

#[braid]
pub struct DatabaseNameBuf;

let owned = DatabaseNameBuf::from_static("mongo");
let borrowed = DatabaseName::from_static("mongo");

If the name ends with String, then the borrowed form will use Str.

#[braid]
pub struct ConnectionString;

let owned = ConnectionString::from_static("mysql://…");
let borrowed = ConnectionStr::from_static("mysql://…");

If a different name is desired, this behavior can be overridden by specifying the name of the reference type to create using the ref parameter.

#[braid(ref_name = "TempDb")]
pub struct DatabaseNameBuf;

let owned = DatabaseNameBuf::from_static("mongo");
let borrowed = TempDb::from_static("mongo");
let to_owned: DatabaseNameBuf = borrowed.to_owned();

A default doc comment is added to the borrowed form that refers back to the owned form. If a custom doc comment is desired, the ref_doc parameter allows supplying custom documentation.

#[braid(ref_doc = "A temporary reference to a database name")]
pub struct DatabaseName;

Attributes added to the braid will be applied to both the owned and borrowed forms with the exception of /// and #[doc = ""] attributes. To add an attribute to only the owned form, use the owned_attr parameter. Similarly, use ref_attr to add an attribute to only the borrowed form.

use aliri_braid::braid;

#[braid(
   owned_attr(must_use = "database name should always be used"),
   ref_attr(must_use = "created a reference, but never used it"),
)]
#[cfg(not(feature = "nightly"))]
pub struct DatabaseName;

§Extensibility

The types created by the braid macro are placed in the same module where declared. This means additional functionality, including mutations, can be implemented easily.

As a basic example, here is a type built to hold Amazon ARNs. The type has been extended to support some mutation and introspection.

#[braid]
pub struct AmazonArnBuf;

impl AmazonArnBuf {
    /// Append an ARN segment
    pub fn add_segment(&mut self, segment: &str) {
        self.0.push_str(":");
        self.0.push_str(segment);
    }
}

impl AmazonArn {
    /// Returns an iterator of all ARN segments
    pub fn get_segments(&self) -> std::str::Split<char> {
        self.0.split(':')
    }

    /// Returns the service segment of the ARN
    pub fn get_service(&self) -> &str {
        self.get_segments().nth(2).unwrap_or("")
    }
}

§Encapsulation

Because code within the same module where the braid is defined are allowed to access the internal value, you can use a module in order to more strictly enforce encapsulation and limit accessibility that might otherwise violate established invariants. This may be particularly desired when the wrapped type requires validation.

mod amazon_arn {
    #[aliri_braid::braid]
    pub struct AmazonArnBuf;

    /* Additional impls that need access to the inner values */
}

pub use amazon_arn::{AmazonArnBuf, AmazonArn};

let x = AmazonArnBuf::from_static("arn:aws:iam::123456789012:user/Development");
assert_eq!("iam", x.get_service());

§Soundness

This crate ensures that the from_str implementation provided for wrapping borrowed str slices does not extend lifetimes.

In the example below, we verify that the borrowed DatabaseNameRef is unable to escape the lifetime of data. The following code snippet will fail to compile, because data will go out of scope and be dropped at the end of the block creating ex_ref.

let ex_ref = {
    let data = DatabaseName::new("test string");
    DatabaseNameRef::from_str(data.as_str())
}; // `data` is dropped at this point

// Which means that `ex_ref` would be invalid if allowed.
println!("{}", ex_ref);

§Validation

Types can be configured to only contain certain values. This can be used to strongly enforce domain type boundaries, thus making invalid values unrepresentable.

For example, if you wanted to have a username type that did not accept the root user, you have a few options:

Pass the username around as a string, validate that it isn’t root at known entry points.
Create a username type and allow creation from a raw string, then validate it just after creation.
Create a strong username type that requires the value to be validated prior to being creatable.

Braided strings give the strongest, third guarantee. The other two methods require constant vigilance to ensure that an unexpected root value doesn’t sneak in through other backdoors.

By default, Rust’s module system allows items within the same module to have access to each other’s non-public members. If not handled properly, this can lead to unintentionally violating invariants. Thus, for the strongest guarantees, it is recommended to use the module system to further control access to the interior values held by the braided type as described in the section on encapsulation.

As a convenience, from_static functions are provided that accept &'static str. For fallible braids and the owned form of normalized braids, this function will panic if the value is not valid. For borrowed form of normalized braids, the function will panic if the value is not normalized.

#[derive(Debug, PartialEq, Eq)]
pub struct InvalidUsername;
// Error implementation elided

#[braid(validator)]
pub struct NonRootUsername;

impl aliri_braid::Validator for NonRootUsername {
    type Error = InvalidUsername;
    fn validate(s: &str) -> Result<(), Self::Error> {
        if s.is_empty() || s.eq_ignore_ascii_case("root") {
            Err(InvalidUsername)
        } else {
            Ok(())
        }
    }
}

assert!(NonRootUsername::new("".to_string()).is_err());
assert!(NonRootUsername::new("root".to_string()).is_err());
assert!(NonRootUsername::new("nobody".to_string()).is_ok());

NonRootUsername::from_static("nobody");

assert!(NonRootUsernameRef::from_str("").is_err());
assert!(NonRootUsernameRef::from_str("root").is_err());
assert!(NonRootUsernameRef::from_str("nobody").is_ok());

NonRootUsernameRef::from_static("nobody");

Foreign validators can also be used by specifying the name of the type that implements the validation logic.

#[braid(validator = "UsernameValidator")]
pub struct NonRootUsername;

pub struct UsernameValidator;

impl aliri_braid::Validator for UsernameValidator {
    /* … */
}

assert!(NonRootUsername::new("".to_string()).is_err());
assert!(NonRootUsername::new("root".to_string()).is_err());
assert!(NonRootUsername::new("nobody".to_string()).is_ok());

NonRootUsername::from_static("nobody");

assert!(NonRootUsernameRef::from_str("").is_err());
assert!(NonRootUsernameRef::from_str("root").is_err());
assert!(NonRootUsernameRef::from_str("nobody").is_ok());

NonRootUsernameRef::from_static("nobody");

Note: Validator::Error is expected to implement From<Infallible>. If you haven’t implemented this trait, you’ll receive an error of the following form:

the trait bound `MyError: std::convert::From<std::convert::Infallible>` is not satisfied
the trait `std::convert::From<std::convert::Infallible>` is not implemented for `MyError`

In order to assist in implementing this trait trivially, use the from_infallible!() helper macro:

pub struct InvalidUsername;

aliri_braid::from_infallible!(InvalidUsername);

This expands to the following code:

struct InvalidUsername;
impl From<core::convert::Infallible> for InvalidUsername {
    #[inline(always)]
    fn from(x: core::convert::Infallible) -> Self {
        match x {}
    }
}

§Normalization

Braided strings can also have enforced normalization, which is carried out at the creation boundary. In this case, the .from_str() function on the borrowed form will return a Cow<Borrowed>, which can be inspected to determine whether normalization and conversion to an owned value was required. In cases where the incoming value is expected to already be normalized, the .from_normalized_str() function can be used. This function will return an error if the value required normalization.

Note that when implementing Validator for a braided type, the validate method must ensure that the value is already in normalized form and return an error if it is not.

When using serde to deserialze directly to the borrowed form, care must be taken, as only already normalized values will be able to be deserialized. If normalization is expected, deserialize into the owned form or Cow<Borrowed>.

Here is a toy example where the value must not be empty and must be composed of ASCII characters, but that is also normalized to use lowercase ASCII letters.

use std::borrow::Cow;

#[derive(Debug, PartialEq, Eq)]
pub struct InvalidHeaderName;
// Error implementation elided

#[braid(normalizer)]
pub struct HeaderName;

impl aliri_braid::Validator for HeaderName {
    type Error = InvalidHeaderName;
    fn validate(s: &str) -> Result<(), Self::Error> {
        if s.is_empty() || !s.is_ascii() || s.as_bytes().iter().any(|&b| b'A' <= b && b <= b'Z') {
            Err(InvalidHeaderName)
        } else {
            Ok(())
        }
    }
}

impl aliri_braid::Normalizer for HeaderName {
    fn normalize(s: &str) -> Result<Cow<str>, Self::Error> {
        if s.is_empty() || !s.is_ascii() {
            Err(InvalidHeaderName)
        } else if s.as_bytes().iter().any(|&b| b'A' <= b && b <= b'Z') {
            Ok(Cow::Owned(s.to_ascii_lowercase()))
        } else {
            Ok(Cow::Borrowed(s))
        }
    }
}

assert!(HeaderName::new("".to_string()).is_err());
assert_eq!("mixedcase", HeaderName::new("MixedCase".to_string()).unwrap().as_str());
assert_eq!("lowercase", HeaderName::new("lowercase".to_string()).unwrap().as_str());

assert_eq!("mixedcase", HeaderName::from_static("MixedCase").as_str());
assert_eq!("lowercase", HeaderName::from_static("lowercase").as_str());

assert!(HeaderNameRef::from_str("").is_err());
assert_eq!("mixedcase", HeaderNameRef::from_str("MixedCase").unwrap().as_str());
assert_eq!("lowercase", HeaderNameRef::from_str("lowercase").unwrap().as_str());

assert!(HeaderNameRef::from_normalized_str("").is_err());
assert!(HeaderNameRef::from_normalized_str("MixedCase").is_err());
assert_eq!("lowercase", HeaderNameRef::from_normalized_str("lowercase").unwrap().as_str());

assert_eq!("lowercase", HeaderNameRef::from_static("lowercase").as_str());

§Unchecked creation

Where necessary for efficiency, it is possible to bypass the validations on creation through the use of the .new_unchecked() or from_str_unchecked() functions. These functions are marked as unsafe, as they require the caller to assert that they are fulfilling the implicit contract that the value be both valid and in normal form. If either of these constraints are violated, undefined behavior could result when downstream consumers depend on these constraints being upheld.

NonRootUsername::new_unchecked("");
NonRootUsernameRef::from_str_unchecked("nobody");

If you find violations of your guarantees, you can look specifically for uses of unsafe.

unsafe {
    NonRootUsername::new_unchecked(String::from(""));
    NonRootUsernameRef::from_str_unchecked("root");
}

§Provided trait impls

By default, the following traits will be automatically implemented.

For the Owned type

Additionally, unvalidated owned types implement

Validated and normalized owned types will instead implement

When normalized, the above conversions will normalize values.

For the Borrowed type

Additionally, unvalidated borrowed types implement

core::convert::From<&str>

Validated and normalize borrowed types will instead implement

core::convert::TryFrom<&str>

For Cow<'static, Borrowed>

core::convert::From<Owned>

For Cow<Borrowed>

core::convert::From<&Borrowed>

For Box<Borrowed>

core::convert::From<Owned>

The above conversion will fail if the value is not already normalized.

Types that are not normalized will additionally implement

core::borrow::Borrow<str>

Borrow<str> cannot be implemented for normalized braids because equality and hashing of equivalent braid values will have differing results for equality, which violates the contract implied by the Borrow trait.

Deref to a str is explicitly not implemented. This means that an explicit call is required to treat a value as an untyped string, whether .as_str(), .to_string(), or .into_string()

§Omitting `Clone`

For some types, it may be desirable to prevent arbitrary cloning of a type. In that case, the clone parameter can be used to prevent automatically deriving Clone.

#[braid(clone = "omit")]
pub struct Sensitive;

assert_not_impl_any!(Sensitive: Clone);

§Custom `Display`, `Debug`, and `PartialOrd`/`Ord` implementations

By default, the implementations of Display, Debug PartialOrd, and Ord provided by a braid delegate directly to the underlying String or str types. If a custom implementation is desired, the automatic derivation of these traits can be controlled by the display, debug, and ord parameters. Both of these parameters accept one of impl, owned, or omit. By default, the impl derivation mode is used.

The modes have the following effects:

impl: Format the owned and reference type transparently as the underlying string (slice) type.
owned: Automatically provide an owned implementation that transparently delegates to the implementation of the borrowed form. The consumer must provide their custom implementation on the borrowed form.
omit: No implementations are provided for the owned or borrowed forms. These must be implemented by the consumer if they are desired.

Note: Omitting a PartialOrd and Ord implementation will make the braid unable to be used as a key in a BTreeMap or BTreeSet.

As an example:

use std::fmt;
#[braid(clone = "omit", display = "owned", debug = "owned")]
pub struct Sensitive;

impl fmt::Debug for SensitiveRef {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.write_str("SENSITIVE")
    }
}

impl fmt::Display for SensitiveRef {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         f.write_str("SENSITIVE DISPLAY")
    }
}

let owned = Sensitive::from_static("secret value");
assert_eq!("SENSITIVE", format!("{:?}", owned));
assert_eq!("SENSITIVE DISPLAY", format!("{}", owned));
assert_eq!("secret value", owned.as_str());

let borrowed: &SensitiveRef = &owned;
assert_eq!("SENSITIVE", format!("{:?}", borrowed));
assert_eq!("SENSITIVE DISPLAY", format!("{}", borrowed));
assert_eq!("secret value", borrowed.as_str());

§Serde

Serialize and Deserialize implementations from the serde crate can be automatically generated by including serde in the argument list for the macro.

#[braid(serde)]
pub struct Username;

let username = Username::from_static("root");
let json = serde_json::to_string(&username).unwrap();
let new_username: Username = serde_json::from_str(&json).unwrap();

Such automatic implementations will also properly handle string values that require validation. This automatic validation has the benefit of easing use with Serde while still protecting the integrity of the type.

#[derive(Debug, PartialEq, Eq)]
pub struct InvalidUsername;
// Error implementation elided

#[braid(serde, validator)]
pub struct Username;

impl aliri_braid::Validator for Username {
    type Error = InvalidUsername;
    fn validate(s: &str) -> Result<(), Self::Error> {
        if s.is_empty() || s.eq_ignore_ascii_case("root") {
            Err(InvalidUsername)
        } else {
            Ok(())
        }
    }
}

assert!(serde_json::from_str::<Username>("\"\"").is_err());
assert!(serde_json::from_str::<Username>("\"root\"").is_err());
assert!(serde_json::from_str::<Username>("\"nobody\"").is_ok());

assert!(serde_json::from_str::<&UsernameRef>("\"\"").is_err());
assert!(serde_json::from_str::<&UsernameRef>("\"root\"").is_err());
assert!(serde_json::from_str::<&UsernameRef>("\"nobody\"").is_ok());

§Custom string types

The braid macro can be used to define a custom string type that wraps types other than the standard String. This allows defining a braid that is backed by a type that offers small-string optimizations, such as SmartString or CompactString.

Functions that expose the inner wrapped type can be made private by adding the no_expose parameter to avoid leaking the type in the public interface.

use compact_str::CompactString;
use smartstring::{SmartString, LazyCompact};

#[braid(no_expose)]
pub struct UserId(CompactString);

#[braid(no_expose)]
pub struct AltUserId(SmartString<LazyCompact>);

It can also be used to wrap a ByteString, which is a string backed by Bytes, which may be useful if the type is primarily used in contexts where a zero-copy implementation is preferred.

use bytestring::ByteString;

#[braid]
pub struct ZeroCopyIdentifier(ByteString);

§Requirements

In order to be used as a custom string type, the type must implement the following traits:

core::clone::Clone (unless clone is omit)
core::fmt::Debug (unless debug is omit)
core::fmt::Display (unless display is omit)
core::cmp::Eq
core::cmp::PartialEq
core::hash::Hash
core::cmp::Ord (unless ord is omit)
core::cmp::PartialOrd (unless ord is omit)
serde::Serialize (unless serde is omit)
serde::Deserialize (unless serde is omit)
core::convert::From<&str>
core::convert::From<Box<str>>
core::convert::AsRef<str>
core::convert::Into<String>

§`no_std` support

Braids can be implemented in no_std environments with alloc. By adding the no_std parameter to the macro, all impls will reference the core or alloc crates instead of the std crate, as appropriate.

extern crate alloc;

use aliri_braid::braid;
use alloc::string::String;

#[braid(no_std)]
pub struct NoStdLibWrapper;

In environments without an allocator, braid_ref can be used to create a reference-only braid. In order to remove the alloc dependency in aliri_braid, specify default-features = "false" in the Cargo.toml file.

use aliri_braid::braid_ref;

#[braid_ref(no_std)]
pub struct NoStdValue;

§Safety

Braid uses limited unsafe in order to be able to reinterpret string slices (&str) as the borrowed form. Because this functionality is provided as a macro, using the #![forbid(unsafe_code)] lint level on a crate that generates braids will result in compiler errors. Instead, the crate can be annotated with #![deny(unsafe_code)], which allows for overrides as appropriate. The functions that require unsafe to work correctly are annotated with #[allow(unsafe_code)], and all usages of unsafe that the macro generates are annotated with SAFETY code comments.

If strict adherence to forbid unsafe code is required, then the types can be segregated into an accessory crate without the prohibition, and then consumed safely from crates that otherwise forbid unsafe code.

Macros§

from_infallible: Utility macro for easily defining From<Infallible> for a given type.

Traits§

Normalizer: A normalizer that can verify a given input is valid and performs necessary normalization
Validator: A validator that can verify a given input is valid given certain preconditions

Attribute Macros§

braid: Constructs a braid
braid_ref: Constructs a ref-only braid