Arguments in core::fmt - Rust
Description: This structure represents a safely precompiled version of a format string and its arguments. This cannot be generated at runtime because it cannot safely be done, so no constructors are given and the fields are private to prevent modification.

# Struct Arguments

1.6.0 · Source

    pub struct Arguments<'a> { /* private fields */ }

Expand description

This structure represents a safely precompiled version of a format string and its arguments. This cannot be generated at runtime because it cannot safely be done, so no constructors are given and the fields are private to prevent modification.

The `format_args!` macro will safely create an instance of this structure. The macro validates the format string at compile-time so usage of the `write()` and `format()` functions can be safely performed.

You can use the `Arguments<'a>` that `format_args!` returns in `Debug` and `Display` contexts as seen below. The example also shows that `Debug` and `Display` format to the same thing: the interpolated format string in `format_args!`.

    let debug = format!("{:?}", format_args!("{} foo {:?}", 1, 2));
    let display = format!("{}", format_args!("{} foo {:?}", 1, 2));
    assert_eq!("1 foo 2", display);
    assert_eq!(display, debug);

## Implementations§

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### impl<'a> Arguments<'a>

1.52.0 (const: 1.84.0) · Source

#### pub const fn as_str(&self) -> Option<&'static str>

Gets the formatted string, if it has no arguments to be formatted at runtime.

This can be used to avoid allocations in some cases.

##### §Guarantees

For `format_args!("just a literal")`, this function is guaranteed to return `Some("just a literal")`.

For most cases with placeholders, this function will return `None`.

However, the compiler may perform optimizations that can cause this function to return `Some(_)` even if the format string contains placeholders. For example, `format_args!("Hello, {}!", "world")` may be optimized to `format_args!("Hello, world!")`, such that `as_str()` returns `Some("Hello, world!")`.

The behavior for anything but the trivial case (without placeholders) is not guaranteed, and should not be relied upon for anything other than optimization.

##### §Examples

    use std::fmt::Arguments;

    fn write_str(_: &str) { /* ... */ }

    fn write_fmt(args: &Arguments<'_>) {
        if let Some(s) = args.as_str() {
            write_str(s)
        } else {
            write_str(&args.to_string());
        }
    }

    assert_eq!(format_args!("hello").as_str(), Some("hello"));
    assert_eq!(format_args!("").as_str(), Some(""));
    assert_eq!(format_args!("{:?}", std::env::current_dir()).as_str(), None);

## Trait Implementations§

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### impl<'a> Clone for Arguments<'a>

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#### fn clone(&self) -> Arguments<'a>

Returns a copy of the value. Read more
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#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl Debug for Arguments<'_>

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#### fn fmt(&self, fmt: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
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### impl Display for Arguments<'_>

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#### fn fmt(&self, fmt: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
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### impl<'a> Copy for Arguments<'a>

1.0.0 · Source§

### impl !Send for Arguments<'_>

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### impl !Sync for Arguments<'_>

## Auto Trait Implementations§

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### impl<'a> Freeze for Arguments<'a>

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### impl<'a> RefUnwindSafe for Arguments<'a>

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### impl<'a> Unpin for Arguments<'a>

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### impl<'a> UnwindSafe for Arguments<'a>

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit` #126799)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.