From 260ec9347838714ad9fa4f7e083bdc0448f4f9be Mon Sep 17 00:00:00 2001 From: Jake Goulding Date: Thu, 21 Jul 2022 21:39:20 -0400 Subject: [PATCH] Add `Provider::{would_be_satisfied_by_value_of,would_be_satisfied_by_ref_of}` While the `provide_*` methods already short-circuit when a value has been provided, there are times where an expensive computation is needed to determine if the `provide_*` method can even be called. --- library/core/src/any.rs | 165 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 165 insertions(+) diff --git a/library/core/src/any.rs b/library/core/src/any.rs index 3bfb675531e64..1a379ecc11c01 100644 --- a/library/core/src/any.rs +++ b/library/core/src/any.rs @@ -1008,6 +1008,156 @@ impl<'a> Demand<'a> { } self } + + /// Check if the `Demand` would be satisfied if provided with a + /// value of the specified type. If the type does not match or has + /// already been provided, returns false. + /// + /// # Examples + /// + /// Check if an `u8` still needs to be provided and then provides + /// it. + /// + /// ```rust + /// #![feature(provide_any)] + /// + /// use std::any::{Provider, Demand}; + /// + /// struct Parent(Option); + /// + /// impl Provider for Parent { + /// fn provide<'a>(&'a self, demand: &mut Demand<'a>) { + /// if let Some(v) = self.0 { + /// demand.provide_value::(v); + /// } + /// } + /// } + /// + /// struct Child { + /// parent: Parent, + /// } + /// + /// impl Child { + /// // Pretend that this takes a lot of resources to evaluate. + /// fn an_expensive_computation(&self) -> Option { + /// Some(99) + /// } + /// } + /// + /// impl Provider for Child { + /// fn provide<'a>(&'a self, demand: &mut Demand<'a>) { + /// // In general, we don't know if this call will provide + /// // an `u8` value or not... + /// self.parent.provide(demand); + /// + /// // ...so we check to see if the `u8` is needed before + /// // we run our expensive computation. + /// if demand.would_be_satisfied_by_value_of::() { + /// if let Some(v) = self.an_expensive_computation() { + /// demand.provide_value::(v); + /// } + /// } + /// + /// // The demand will be satisfied now, regardless of if + /// // the parent provided the value or we did. + /// assert!(!demand.would_be_satisfied_by_value_of::()); + /// } + /// } + /// + /// let parent = Parent(Some(42)); + /// let child = Child { parent }; + /// assert_eq!(Some(42), std::any::request_value::(&child)); + /// + /// let parent = Parent(None); + /// let child = Child { parent }; + /// assert_eq!(Some(99), std::any::request_value::(&child)); + /// ``` + #[unstable(feature = "provide_any", issue = "96024")] + pub fn would_be_satisfied_by_value_of(&self) -> bool + where + T: 'static, + { + self.would_be_satisfied_by::>() + } + + /// Check if the `Demand` would be satisfied if provided with a + /// reference to a value of the specified type. If the type does + /// not match or has already been provided, returns false. + /// + /// # Examples + /// + /// Check if a `&str` still needs to be provided and then provides + /// it. + /// + /// ```rust + /// #![feature(provide_any)] + /// + /// use std::any::{Provider, Demand}; + /// + /// struct Parent(Option); + /// + /// impl Provider for Parent { + /// fn provide<'a>(&'a self, demand: &mut Demand<'a>) { + /// if let Some(v) = &self.0 { + /// demand.provide_ref::(v); + /// } + /// } + /// } + /// + /// struct Child { + /// parent: Parent, + /// name: String, + /// } + /// + /// impl Child { + /// // Pretend that this takes a lot of resources to evaluate. + /// fn an_expensive_computation(&self) -> Option<&str> { + /// Some(&self.name) + /// } + /// } + /// + /// impl Provider for Child { + /// fn provide<'a>(&'a self, demand: &mut Demand<'a>) { + /// // In general, we don't know if this call will provide + /// // a `str` reference or not... + /// self.parent.provide(demand); + /// + /// // ...so we check to see if the `&str` is needed before + /// // we run our expensive computation. + /// if demand.would_be_satisfied_by_ref_of::() { + /// if let Some(v) = self.an_expensive_computation() { + /// demand.provide_ref::(v); + /// } + /// } + /// + /// // The demand will be satisfied now, regardless of if + /// // the parent provided the reference or we did. + /// assert!(!demand.would_be_satisfied_by_ref_of::()); + /// } + /// } + /// + /// let parent = Parent(Some("parent".into())); + /// let child = Child { parent, name: "child".into() }; + /// assert_eq!(Some("parent"), std::any::request_ref::(&child)); + /// + /// let parent = Parent(None); + /// let child = Child { parent, name: "child".into() }; + /// assert_eq!(Some("child"), std::any::request_ref::(&child)); + /// ``` + #[unstable(feature = "provide_any", issue = "96024")] + pub fn would_be_satisfied_by_ref_of(&self) -> bool + where + T: ?Sized + 'static, + { + self.would_be_satisfied_by::>>() + } + + fn would_be_satisfied_by(&self) -> bool + where + I: tags::Type<'a>, + { + matches!(self.0.downcast::(), Some(TaggedOption(None))) + } } #[unstable(feature = "provide_any", issue = "96024")] @@ -1112,6 +1262,21 @@ impl<'a> dyn Erased<'a> + 'a { /// Returns some reference to the dynamic value if it is tagged with `I`, /// or `None` otherwise. #[inline] + fn downcast(&self) -> Option<&TaggedOption<'a, I>> + where + I: tags::Type<'a>, + { + if self.tag_id() == TypeId::of::() { + // SAFETY: Just checked whether we're pointing to an I. + Some(unsafe { &*(self as *const Self).cast::>() }) + } else { + None + } + } + + /// Returns some mutable reference to the dynamic value if it is tagged with `I`, + /// or `None` otherwise. + #[inline] fn downcast_mut(&mut self) -> Option<&mut TaggedOption<'a, I>> where I: tags::Type<'a>,