library/core/src/asserting.rs

   1 // Contains the machinery necessary to print useful `assert!` messages. Not intended for public
   2 // usage, not even nightly use-cases.
   3 //
   4 // Based on https://github.com/dtolnay/case-studies/tree/master/autoref-specialization. When
   5 // 'specialization' is robust enough (5 years? 10 years? Never?), `Capture` can be specialized
   6 // to [Printable].
   7
   8 #![allow(missing_debug_implementations)]
   9 #![doc(hidden)]
  10 #![unstable(feature = "generic_assert_internals", issue = "44838")]
  11
  12 use crate::{
  13     fmt::{Debug, Formatter},
  14     marker::PhantomData,
  15 };
  16
  17 // ***** TryCapture - Generic *****
  18
  19 /// Marker used by [Capture]
  20 #[unstable(feature = "generic_assert_internals", issue = "44838")]
  21 pub struct TryCaptureWithoutDebug;
  22
  23 /// Catches an arbitrary `E` and modifies `to` accordingly
  24 #[unstable(feature = "generic_assert_internals", issue = "44838")]
  25 pub trait TryCaptureGeneric<E, M> {
  26     /// Similar to [TryCapturePrintable] but generic to any `E`.
  27     fn try_capture(&self, to: &mut Capture<E, M>);
  28 }
  29
  30 impl<E> TryCaptureGeneric<E, TryCaptureWithoutDebug> for &Wrapper<&E> {
  31     #[inline]
  32     fn try_capture(&self, _: &mut Capture<E, TryCaptureWithoutDebug>) {}
  33 }
  34
  35 impl<E> Debug for Capture<E, TryCaptureWithoutDebug> {
  36     fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), core::fmt::Error> {
  37         f.write_str("N/A")
  38     }
  39 }
  40
  41 // ***** TryCapture - Printable *****
  42
  43 /// Marker used by [Capture]
  44 #[unstable(feature = "generic_assert_internals", issue = "44838")]
  45 pub struct TryCaptureWithDebug;
  46
  47 /// Catches an arbitrary `E: Printable` and modifies `to` accordingly
  48 #[unstable(feature = "generic_assert_internals", issue = "44838")]
  49 pub trait TryCapturePrintable<E, M> {
  50     /// Similar as [TryCaptureGeneric] but specialized to any `E: Printable`.
  51     fn try_capture(&self, to: &mut Capture<E, M>);
  52 }
  53
  54 impl<E> TryCapturePrintable<E, TryCaptureWithDebug> for Wrapper<&E>
  55 where
  56     E: Printable,
  57 {
  58     #[inline]
  59     fn try_capture(&self, to: &mut Capture<E, TryCaptureWithDebug>) {
  60         to.elem = Some(*self.0);
  61     }
  62 }
  63
  64 impl<E> Debug for Capture<E, TryCaptureWithDebug>
  65 where
  66     E: Printable,
  67 {
  68     fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), core::fmt::Error> {
  69         match self.elem {
  70             None => f.write_str("N/A"),
  71             Some(ref value) => Debug::fmt(value, f),
  72         }
  73     }
  74 }
  75
  76 // ***** Others *****
  77
  78 /// All possible captured `assert!` elements
  79 ///
  80 /// # Types
  81 ///
  82 /// * `E`: **E**lement that is going to be displayed.
  83 /// * `M`: **M**arker used to differentiate [Capture]s in regards to [Debug].
  84 #[unstable(feature = "generic_assert_internals", issue = "44838")]
  85 pub struct Capture<E, M> {
  86     // If None, then `E` does not implements [Printable] or `E` wasn't evaluated (`assert!( ... )`
  87     // short-circuited).
  88     //
  89     // If Some, then `E` implements [Printable] and was evaluated.
  90     pub elem: Option<E>,
  91     phantom: PhantomData<M>,
  92 }
  93
  94 impl<M, T> Capture<M, T> {
  95     #[inline]
  96     pub const fn new() -> Self {
  97         Self { elem: None, phantom: PhantomData }
  98     }
  99 }
 100
 101 /// Necessary for the implementations of `TryCapture*`
 102 #[unstable(feature = "generic_assert_internals", issue = "44838")]
 103 pub struct Wrapper<T>(pub T);
 104
 105 /// Tells which elements can be copied and displayed
 106 #[unstable(feature = "generic_assert_internals", issue = "44838")]
 107 pub trait Printable: Copy + Debug {}
 108
 109 impl<T> Printable for T where T: Copy + Debug {}