40 lines
1.2 KiB
Rust
40 lines
1.2 KiB
Rust
//! This module implements deferred display helpers.
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//!
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//! These are particularly useful in logging contexts, where the maximum logging level filter might
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//! be enabled, but we don't want the arguments to be evaluated early:
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//!
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//! ```
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//! log::set_max_level(log::LevelFilter::max());
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//! fn expensive_calculation() -> String {
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//! "a string that is very slow to generate".into()
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//! }
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//! log::debug!("{}", expensive_calculation());
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//! ```
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//!
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//! If the associated log implementation filters out log debug entries, the expensive calculation
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//! would have been spurious. In this case, we can wrap the expensive computation within an
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//! `DeferredDisplay`, so that the computation only happens when the actual `fmt` function is
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//! called.
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use core::fmt;
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pub(crate) struct DeferredDisplay<F>(F);
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impl<F: Fn() -> T, T: fmt::Display> DeferredDisplay<F> {
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pub(crate) fn new(f: F) -> Self {
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Self(f)
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}
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}
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impl<F: Fn() -> T, T: fmt::Display> fmt::Display for DeferredDisplay<F> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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self.0().fmt(f)
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}
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}
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impl<F: Fn() -> T, T: fmt::Debug> fmt::Debug for DeferredDisplay<F> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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self.0().fmt(f)
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}
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}
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