Auto merge of rust-lang#12931 - oxalica:feat/la-arena-apis, r=lnicola

Add more constructors and entry-APIs for la-arena

`la-arena` on crates.io is quite helpful when just a thin wrapper for Vec with u32 indices is needed.
But the current API is not ergonomic enough.

This PR
- Adds `ArenaMap::new`. Not sure why only `Arena` has it now.
- Adds `Arena{,Map}::with_capacity` for known-size storage.
- Adds entry-API for `ArenaMap` for easier `.entry(idx).or_default().push(value)` or `.entry(idx).or_insert(...)` operations.

lib/la-arena/src/lib.rs

@@ -12,7 +12,7 @@ use std::{
 };
 
 mod map;
-pub use map::ArenaMap;
+pub use map::{ArenaMap, Entry, OccupiedEntry, VacantEntry};
 
 /// The raw index of a value in an arena.
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
@@ -208,6 +208,16 @@ impl<T> Arena<T> {
         Arena { data: Vec::new() }
     }
 
+    /// Create a new empty arena with specific capacity.
+    ///
+    /// ```
+    /// let arena: la_arena::Arena<i32> = la_arena::Arena::with_capacity(42);
+    /// assert!(arena.is_empty());
+    /// ```
+    pub fn with_capacity(capacity: usize) -> Arena<T> {
+        Arena { data: Vec::with_capacity(capacity) }
+    }
+
     /// Empties the arena, removing all contained values.
     ///
     /// ```

lib/la-arena/src/map.rs

@@ -11,6 +11,16 @@ pub struct ArenaMap<IDX, V> {
 }
 
 impl<T, V> ArenaMap<Idx<T>, V> {
+    /// Creates a new empty map.
+    pub const fn new() -> Self {
+        Self { v: Vec::new(), _ty: PhantomData }
+    }
+
+    /// Create a new empty map with specific capacity.
+    pub fn with_capacity(capacity: usize) -> Self {
+        Self { v: Vec::with_capacity(capacity), _ty: PhantomData }
+    }
+
     /// Inserts a value associated with a given arena index into the map.
     pub fn insert(&mut self, idx: Idx<T>, t: V) {
         let idx = Self::to_idx(idx);
@@ -46,6 +56,16 @@ impl<T, V> ArenaMap<Idx<T>, V> {
         self.v.iter().enumerate().filter_map(|(idx, o)| Some((Self::from_idx(idx), o.as_ref()?)))
     }
 
+    /// Gets the given key's corresponding entry in the map for in-place manipulation.
+    pub fn entry(&mut self, idx: Idx<T>) -> Entry<'_, Idx<T>, V> {
+        let idx = Self::to_idx(idx);
+        self.v.resize_with((idx + 1).max(self.v.len()), || None);
+        match &mut self.v[idx] {
+            slot @ Some(_) => Entry::Occupied(OccupiedEntry { slot, _ty: PhantomData }),
+            slot @ None => Entry::Vacant(VacantEntry { slot, _ty: PhantomData }),
+        }
+    }
+
     fn to_idx(idx: Idx<T>) -> usize {
         u32::from(idx.into_raw()) as usize
     }
@@ -70,6 +90,103 @@ impl<T, V> std::ops::IndexMut<Idx<V>> for ArenaMap<Idx<V>, T> {
 
 impl<T, V> Default for ArenaMap<Idx<V>, T> {
     fn default() -> Self {
-        ArenaMap { v: Vec::new(), _ty: PhantomData }
+        Self::new()
+    }
+}
+
+/// A view into a single entry in a map, which may either be vacant or occupied.
+///
+/// This `enum` is constructed from the [`entry`] method on [`ArenaMap`].
+///
+/// [`entry`]: ArenaMap::entry
+pub enum Entry<'a, IDX, V> {
+    /// A vacant entry.
+    Vacant(VacantEntry<'a, IDX, V>),
+    /// An occupied entry.
+    Occupied(OccupiedEntry<'a, IDX, V>),
+}
+
+impl<'a, IDX, V> Entry<'a, IDX, V> {
+    /// Ensures a value is in the entry by inserting the default if empty, and returns a mutable reference to
+    /// the value in the entry.
+    pub fn or_insert(self, default: V) -> &'a mut V {
+        match self {
+            Self::Vacant(ent) => ent.insert(default),
+            Self::Occupied(ent) => ent.into_mut(),
+        }
+    }
+
+    /// Ensures a value is in the entry by inserting the result of the default function if empty, and returns
+    /// a mutable reference to the value in the entry.
+    pub fn or_insert_with<F: FnOnce() -> V>(self, default: F) -> &'a mut V {
+        match self {
+            Self::Vacant(ent) => ent.insert(default()),
+            Self::Occupied(ent) => ent.into_mut(),
+        }
+    }
+
+    /// Provides in-place mutable access to an occupied entry before any potential inserts into the map.
+    pub fn and_modify<F: FnOnce(&mut V)>(mut self, f: F) -> Self {
+        if let Self::Occupied(ent) = &mut self {
+            f(ent.get_mut());
+        }
+        self
+    }
+}
+
+impl<'a, IDX, V> Entry<'a, IDX, V>
+where
+    V: Default,
+{
+    /// Ensures a value is in the entry by inserting the default value if empty, and returns a mutable reference
+    /// to the value in the entry.
+    pub fn or_default(self) -> &'a mut V {
+        self.or_insert_with(Default::default)
+    }
+}
+
+/// A view into an vacant entry in a [`ArenaMap`]. It is part of the [`Entry`] enum.
+pub struct VacantEntry<'a, IDX, V> {
+    slot: &'a mut Option<V>,
+    _ty: PhantomData<IDX>,
+}
+
+impl<'a, IDX, V> VacantEntry<'a, IDX, V> {
+    /// Sets the value of the entry with the `VacantEntry`’s key, and returns a mutable reference to it.
+    pub fn insert(self, value: V) -> &'a mut V {
+        self.slot.insert(value)
+    }
+}
+
+/// A view into an occupied entry in a [`ArenaMap`]. It is part of the [`Entry`] enum.
+pub struct OccupiedEntry<'a, IDX, V> {
+    slot: &'a mut Option<V>,
+    _ty: PhantomData<IDX>,
+}
+
+impl<'a, IDX, V> OccupiedEntry<'a, IDX, V> {
+    /// Gets a reference to the value in the entry.
+    pub fn get(&self) -> &V {
+        self.slot.as_ref().expect("Occupied")
+    }
+
+    /// Gets a mutable reference to the value in the entry.
+    pub fn get_mut(&mut self) -> &mut V {
+        self.slot.as_mut().expect("Occupied")
+    }
+
+    /// Converts the entry into a mutable reference to its value.
+    pub fn into_mut(self) -> &'a mut V {
+        self.slot.as_mut().expect("Occupied")
+    }
+
+    /// Sets the value of the entry with the `OccupiedEntry`’s key, and returns the entry’s old value.
+    pub fn insert(&mut self, value: V) -> V {
+        self.slot.replace(value).expect("Occupied")
+    }
+
+    /// Takes the value of the entry out of the map, and returns it.
+    pub fn remove(self) -> V {
+        self.slot.take().expect("Occupied")
     }
 }