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Add Step::sub_usize #60542

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merged 1 commit into from May 29, 2019
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Just for now

@@ -34,6 +34,13 @@ pub trait Step: Clone + PartialOrd + Sized {

/// Adds a `usize`, returning `None` on overflow.
fn add_usize(&self, n: usize) -> Option<Self>;

/// Subtracts a `usize`, returning `None` on underflow.
fn sub_usize(&self, n: usize) -> Option<Self> {
// this default implementation makes the addition of `sub_usize` a non-breaking change
let _ = n;
unimplemented!()
}
}

// These are still macro-generated because the integer literals resolve to different types.
@@ -85,6 +92,15 @@ macro_rules! step_impl_unsigned {
}
}

#[inline]
#[allow(unreachable_patterns)]
fn sub_usize(&self, n: usize) -> Option<Self> {
match <$t>::try_from(n) {
Ok(n_as_t) => self.checked_sub(n_as_t),
Err(_) => None,
}
}

step_identical_methods!();
}
)*)
@@ -125,6 +141,25 @@ macro_rules! step_impl_signed {
}
}

#[inline]
#[allow(unreachable_patterns)]
fn sub_usize(&self, n: usize) -> Option<Self> {
match <$unsigned>::try_from(n) {
Ok(n_as_unsigned) => {
// Wrapping in unsigned space handles cases like
// `80_i8.sub_usize(200) == Some(-120_i8)`,
// even though 200_usize is out of range for i8.
let wrapped = (*self as $unsigned).wrapping_sub(n_as_unsigned) as $t;
if wrapped <= *self {
Some(wrapped)
} else {
None // Subtraction underflowed
}
}
Err(_) => None,
}
}

step_identical_methods!();
}
)*)
@@ -212,6 +212,11 @@ macro_rules! newtype_index {
fn add_usize(&self, u: usize) -> Option<Self> {
Idx::index(*self).checked_add(u).map(Self::new)
}

#[inline]
fn sub_usize(&self, u: usize) -> Option<Self> {
Idx::index(*self).checked_sub(u).map(Self::new)
}
}

impl From<$type> for u32 {
@@ -180,6 +180,10 @@ impl std::iter::Step for NaiveDate {
fn add_usize(&self, _: usize) -> Option<Self> {
unimplemented!()
}

fn sub_usize(&self, _: usize) -> Option<Self> {
unimplemented!()
}
}

#[derive(Copy, Clone, Debug, Eq, Ord, PartialEq, PartialOrd)]
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