Crate delegate_display

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Expand description

Lets you derive Display & Debug traits on structs with 0..=1 fields & enums where each variant has 0..=1 fields - see input/output examples below.

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Examples

Newtype structs 
// Input
#[derive(delegate_display::DelegateDisplay)]
struct Foo(SomeType);

// Output
impl fmt::Display for Foo {
  #[inline]
  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    fmt::Display::fmt(&self.0, f)
  }
}
Structs with one field 
// Input
#[derive(delegate_display::DelegateDebug)]
struct Foo { some_field: SomeType }

// Output
impl fmt::Debug for Foo {
  #[inline]
  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    fmt::Debug::fmt(&self.some_field, f)
  }
}
Enums 
// Input
enum MyEnum {
  Foo,
  Bar(SomeType),
  Qux { baz: SomeType }
}

// Output
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  match self {
    Self::Foo => f.write_str("Foo"),
    Self::Bar(inner) => DebugOrDisplay::fmt(inner, f),
    Self::Qux { baz } => DebugOrDisplay::fmt(baz, f),
  }
}
Empty structs & enums 
// Input
struct Foo;
struct Bar{}
struct Qux();
enum Baz {}

// Output
fn fmt(&self, _: &mut fmt::Formatter<'_>) -> fmt::Result {
  Ok(())
}
Custom generic bounds

The attribute names are ddebug for Debug, ddisplay for Display and dboth for a common config for both. ddebug and ddisplay take precendence over dboth.

  • base_bounds will add whatever trait is being derived as a generic bound to each of the struct/enum’s generic params
  • bounds(...) will let you specify specific bounds
// Input
#[derive(DelegateDisplay, DelegateDebug)]
#[dboth(base_bounds)]
#[ddisplay(bounds(F: Display, B: Clone + Display))]
enum Foo<F, B> {
  Foo(F),
  Bar(B),
}

// Output
impl<F: Display, B: Clone + Display> Display for Foo<F, B> { /* ... */}
impl<F: Debug, B: Debug> Debug for Foo<F, B> { /* ... */ }
Typed delegations

Can be useful for further prettifying the output.

/// Some type that `Deref`s to the type we want to use in our formatting, in this case, `str`.
#[derive(Debug)]
struct Wrapper(&'static str);

#[derive(DelegateDebug)]
#[ddebug(delegate_to(str))] // ignore `Wrapper` and debug the `str` it `Deref`s instead
struct Typed(Wrapper);

#[derive(DelegateDebug)] // Included for comparison
struct Base(Wrapper);

assert_eq!(format!("{:?}", Typed(Wrapper("foo"))), "\"foo\"");
assert_eq!(format!("{:?}", Base(Wrapper("bar"))), "Wrapper(\"bar\")");
Invalid inputs
ⓘ
#[derive(DelegateDisplay, Debug)]
#[dboth(delegate_to(String))] // `delegate_to` is not supported on enums
enum SomeEnum {
  Foo(Arc<String>)
}
ⓘ
#[derive(delegate_display::DelegateDisplay)]
#[ddisplay(base_bounds, bounds(T: Display))] // `base_bounds` and `bounds` are mutually exclusive
struct Generic<T>(T);
ⓘ
#[derive(delegate_display::DelegateDisplay)]
#[ddisplay(base_bounds)]
#[ddisplay(base_bounds)] // `dbodh` and `ddisplay` can be mixed, but the same option can't be used twice
struct Foo<T>(T);
ⓘ
#[derive(delegate_display::DelegateDebug)]
struct TooManyFields1 {
  foo: u8,
  bar: u8, // Only one field permitted
}
ⓘ
#[derive(delegate_display::DelegateDebug)]
struct TooManyFields2(u8, u8); // too many fields
ⓘ
#[derive(delegate_display::DelegateDebug)]
enum SomeEnum {
  A, // this is ok
  B(u8), // this is ok
  C { foo: u8 }, // this is ok
  D(u8, u8), // Only one field permitted
  E { foo: u8, bar: u8 } // Only one field permitted
}
ⓘ
#[derive(delegate_display::DelegateDebug)]
union Foo { bar: u8 } // Unions are not supported

Derive Macros