aliri_braid

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.

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;

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

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

#
let owned = DatabaseName::new("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("mongo");
borrow_raw_str(owned.as_str());
take_raw_string(owned.into_string());

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::new("mongo");
let borrowed = DatabaseNameRef::from_str("mongo");

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

#
#[braid]
pub struct DatabaseNameBuf;

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

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 = "TempDb")]
pub struct DatabaseNameBuf;

let owned = DatabaseNameBuf::new("mongo");
let borrowed = TempDb::from_str("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;
#

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::new("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.

# use aliri_braid::braid;
#
# #[braid]
# pub struct DatabaseName;
#
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:

1. Pass the username around as a string, validate that it isn't root at known entry points.
2. Create a username type and allow creation from a raw string, then validate it just after creation.
3. 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.

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

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

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

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

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][std::borrow::Cow], 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.

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.

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::Normalizer for HeaderName {
    type Error = InvalidHeaderName;
    fn normalize(s: &str) -> Resultstr>, Self::Error> {
        if !s.is_ascii() || s.is_empty() {
            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("").is_err());
assert_eq!("mixedcase", HeaderName::new("MixedCase").unwrap().as_str());
assert_eq!("lowercase", HeaderName::new("lowercase").unwrap().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());

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.

# use aliri_braid::braid;
#
# #[derive(Debug, PartialEq, Eq)]
# pub struct InvalidUsername;
# // Error implementation elided
# impl std::fmt::Display for InvalidUsername {
#     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
#         f.write_str("invalid username")
#     }
# }
# impl std::error::Error for InvalidUsername {}
#
# #[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(())
#         }
#     }
# }
#
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("");
    NonRootUsernameRef::from_str_unchecked("root");
}

Provided trait impls

By default, the following traits will be automatically implemented.

For the Owned type

• [std::clone::Clone]
• [std::fmt::Debug]
• [std::fmt::Display]
• [std::hash::Hash]
• [std::cmp::Eq]
• [std::cmp::PartialEq]
• [std::cmp::PartialEq]
• [std::cmp::PartialEq<&Borrowed>]
• [std::cmp::PartialEq>]
• [std::convert::AsRef]
• [std::convert::From<&Borrowed>]
• [std::convert::From>]
• [std::convert::From>]
• [std::borrow::Borrow]
• [std::str::FromStr]
• [std::ops::Deref] where Target = Borrowed

Additionally, unvalidated owned types implement

• [std::convert::From]
• [std::convert::From<&str>]

Validated and normalized owned types will instead implement

• [std::convert::TryFrom]
• [std::convert::TryFrom<&str>]

When normalized, the above conversions will normalize values.

For the Borrowed type

• [std::fmt::Debug]
• [std::fmt::Display]
• [std::hash::Hash]
• [std::cmp::Eq]
• [std::cmp::PartialEq]
• [std::cmp::PartialEq]
• [std::cmp::PartialEq<&Borrowed>]
• [std::cmp::PartialEq>]
• [std::borrow::ToOwned] where Owned = Owned

Additionally, unvalidated borrowed types implement

• [std::convert::From<&str>]

Validated and normalize borrowed types will instead implement

• [std::convert::TryFrom<&str>]

For Cow

• [std::convert::From<&Borrowed>]

For Box

• [std::convert::From]

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

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 omit_clone parameter can be used to prevent automatically deriving [Clone][std::clone::Clone].

#
#[braid(omit_clone)]
pub struct Sensitive;

assert_not_impl_any!(Sensitive: Clone);

Custom Display and Debug

By default, the implementations of [Display][std::fmt::Display] and [Debug][std::fmt::Debug] 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_impl and debug_impl parameters. Both of these parameters accept one of auto, owned, or none. By default, the auto derivation mode is used.

The modes have the following effects:

• auto: 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.
• none: No implementations are provided for the owned or borrowed forms. These must be implemented by the consumer if they are desired.

As an example:

use std::fmt;
#
#[braid(omit_clone, display_impl = "owned", debug_impl = "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::new("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::new("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::("\"\"").is_err());
assert!(serde_json::from_str::("\"root\"").is_err());
assert!(serde_json::from_str::("\"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());

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.