Struct frame_support::dispatch::fmt::Arguments
1.0.0 · 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§
source§impl<'a> Arguments<'a>
impl<'a> Arguments<'a>
1.52.0 (const: unstable) · sourcepub fn as_str(&self) -> Option<&'static str>
pub fn as_str(&self) -> Option<&'static str>
Get 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§
source§impl<'a> Serialize for Arguments<'a>
impl<'a> Serialize for Arguments<'a>
source§fn serialize<S>(
&self,
serializer: S
) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>where
S: Serializer,
fn serialize<S>( &self, serializer: S ) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>where S: Serializer,
impl<'a> Copy for Arguments<'a>
Auto Trait Implementations§
impl<'a> !RefUnwindSafe for Arguments<'a>
impl<'a> !Send for Arguments<'a>
impl<'a> !Sync for Arguments<'a>
impl<'a> Unpin for Arguments<'a>
impl<'a> !UnwindSafe for Arguments<'a>
Blanket Implementations§
source§impl<T> CheckedConversion for T
impl<T> CheckedConversion for T
source§impl<T> Instrument for T
impl<T> Instrument for T
source§fn instrument(self, span: Span) -> Instrumented<Self>
fn instrument(self, span: Span) -> Instrumented<Self>
source§fn in_current_span(self) -> Instrumented<Self>
fn in_current_span(self) -> Instrumented<Self>
source§impl<T, Outer> IsWrappedBy<Outer> for Twhere
Outer: AsRef<T> + AsMut<T> + From<T>,
T: From<Outer>,
impl<T, Outer> IsWrappedBy<Outer> for Twhere Outer: AsRef<T> + AsMut<T> + From<T>, T: From<Outer>,
§impl<T> Pointable for T
impl<T> Pointable for T
source§impl<T> SaturatedConversion for T
impl<T> SaturatedConversion for T
source§fn saturated_from<T>(t: T) -> Selfwhere
Self: UniqueSaturatedFrom<T>,
fn saturated_from<T>(t: T) -> Selfwhere Self: UniqueSaturatedFrom<T>,
source§fn saturated_into<T>(self) -> Twhere
Self: UniqueSaturatedInto<T>,
fn saturated_into<T>(self) -> Twhere Self: UniqueSaturatedInto<T>,
T
. Read moresource§impl<S, T> UncheckedInto<T> for Swhere
T: UncheckedFrom<S>,
impl<S, T> UncheckedInto<T> for Swhere T: UncheckedFrom<S>,
source§fn unchecked_into(self) -> T
fn unchecked_into(self) -> T
unchecked_from
.source§impl<T, S> UniqueSaturatedInto<T> for Swhere
T: Bounded,
S: TryInto<T>,
impl<T, S> UniqueSaturatedInto<T> for Swhere T: Bounded, S: TryInto<T>,
source§fn unique_saturated_into(self) -> T
fn unique_saturated_into(self) -> T
T
.