Introduce a Slot type (#7997)

* Introduce a `Slot` type

Instead of having some type definition that only was used in half of the
code or directly using `u64`, this adds a new unit type wrapper `Slot`.
This makes it especially easy for the outside api to know what type is
expected/returned.

* Change epoch duratioC

* rename all instances of slot number to slot

* Make the constructor private

Co-authored-by: André Silva <andrerfosilva@gmail.com>

substrate/primitives/consensus/slots/src/lib.rs

@@ -21,8 +21,76 @@
 
 use codec::{Decode, Encode};
 
-/// A slot number.
-pub type SlotNumber = u64;
+/// Unit type wrapper that represents a slot.
+#[derive(Debug, Encode, Decode, Eq, Clone, Copy, Default, Ord)]
+pub struct Slot(u64);
+
+impl core::ops::Deref for Slot {
+	type Target = u64;
+
+	fn deref(&self) -> &u64 {
+		&self.0
+	}
+}
+
+impl core::ops::Add for Slot {
+	type Output = Self;
+
+	fn add(self, other: Self) -> Self {
+		Self(self.0 + other.0)
+	}
+}
+
+impl core::ops::Add<u64> for Slot {
+	type Output = Self;
+
+	fn add(self, other: u64) -> Self {
+		Self(self.0 + other)
+	}
+}
+
+impl<T: Into<u64> + Copy> core::cmp::PartialEq<T> for Slot {
+	fn eq(&self, eq: &T) -> bool {
+		self.0 == (*eq).into()
+	}
+}
+
+impl<T: Into<u64> + Copy> core::cmp::PartialOrd<T> for Slot {
+	fn partial_cmp(&self, other: &T) -> Option<core::cmp::Ordering> {
+		self.0.partial_cmp(&(*other).into())
+	}
+}
+
+impl Slot {
+	/// Saturating addition.
+	pub fn saturating_add<T: Into<u64>>(self, rhs: T) -> Self {
+		Self(self.0.saturating_add(rhs.into()))
+	}
+
+	/// Saturating subtraction.
+	pub fn saturating_sub<T: Into<u64>>(self, rhs: T) -> Self {
+		Self(self.0.saturating_sub(rhs.into()))
+	}
+}
+
+#[cfg(feature = "std")]
+impl std::fmt::Display for Slot {
+	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+		write!(f, "{}", self.0)
+	}
+}
+
+impl From<u64> for Slot {
+	fn from(slot: u64) -> Slot {
+		Slot(slot)
+	}
+}
+
+impl From<Slot> for u64 {
+	fn from(slot: Slot) -> u64 {
+		slot.0
+	}
+}
 
 /// Represents an equivocation proof. An equivocation happens when a validator
 /// produces more than one block on the same slot. The proof of equivocation
@@ -32,8 +100,8 @@ pub type SlotNumber = u64;
 pub struct EquivocationProof<Header, Id> {
 	/// Returns the authority id of the equivocator.
 	pub offender: Id,
-	/// The slot number at which the equivocation happened.
-	pub slot_number: SlotNumber,
+	/// The slot at which the equivocation happened.
+	pub slot: Slot,
 	/// The first header involved in the equivocation.
 	pub first_header: Header,
 	/// The second header involved in the equivocation.