{BTreeMap,BTreeSet}::append behaves differently to ::insert and ::extend #145614
Description
I tried this code:
#[derive(Debug, Clone)] struct Key { id: u32, name: String, } use std::cmp::Ordering; impl PartialEq for Key { fn eq(&self, other: &Self) -> bool { self.id == other.id } } impl Eq for Key {} impl PartialOrd for Key { fn partial_cmp(&self, other: &Self) -> Option<Ordering> { Some(self.cmp(other)) } } impl Ord for Key { fn cmp(&self, other: &Self) -> Ordering { self.id.cmp(&other.id) } } use std::collections::BTreeMap; fn append(mut lhs: BTreeMap<Key, String>, mut rhs: BTreeMap<Key, String>) { lhs.append(&mut rhs); println!("append: {lhs:?}"); } fn insert(mut lhs: BTreeMap<Key, String>, rhs: BTreeMap<Key, String>) { for (k, v) in rhs.into_iter() { lhs.insert(k, v); } println!("insert: {lhs:?}"); } fn main() { let mut lhs = BTreeMap::new(); let mut rhs = BTreeMap::new(); lhs.insert(Key { id: 0, name: "lhs_k".to_string(), }, "lhs_v".to_string()); rhs.insert(Key { id: 0, name: "rhs_k".to_string(), }, "rhs_v".to_string()); append(lhs.clone(), rhs.clone()); insert(lhs.clone(), rhs.clone()); }The documentation for BTreeMap::append says If a key from other is already present in self, the respective value from self will be overwritten with the respective value from other.. There is no mention of the overwriting of keys, which aligns with the documentation and behaviour of BTreeMap::insert. (If the map did have this key present, the value is updated, and the old value is returned. The key is not updated, though; this matters for types that can be == without being identical.)
Therefore, I expected to see this happen:
append: {Key { id: 0, name: "lhs_k" }: "rhs_v"} insert: {Key { id: 0, name: "lhs_k" }: "rhs_v"} Instead, this happened:
append: {Key { id: 0, name: "rhs_k" }: "rhs_v"} insert: {Key { id: 0, name: "lhs_k" }: "rhs_v"} Meta
Tested on Rust Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=fa5d69959152003932b622e9f57e9b5e