auto merge of #17378 : Gankro/rust/hashmap-entry, r=aturon

Deprecates the `find_or_*` family of "internal mutation" methods on `HashMap` in
favour of the "external mutation" Entry API as part of RFC 60. Part of #17320,
but this still needs to be done on the rest of the maps. However they don't have
any internal mutation methods defined, so they can be done without deprecating
or breaking anything. Work on `BTree` is part of the complete rewrite in #17334.

The implemented API deviates from the API described in the RFC in two key places:

* `VacantEntry.set` yields a mutable reference to the inserted element to avoid code
duplication where complex logic needs to be done *regardless* of whether the entry
was vacant or not.
* `OccupiedEntry.into_mut` was added so that it is possible to return a reference
into the map beyond the lifetime of the Entry itself, providing functional parity
to `VacantEntry.set`.

This allows the full find_or_insert functionality to be implemented using this API.
A PR will be submitted to the RFC to amend this.

[breaking-change]

1 files changed, 9 insertions, 7 deletions

diff --git a/src/libsyntax/ext/mtwt.rs b/src/libsyntax/ext/mtwt.rs
index 2c94db52967..6fe4f5b324c 100644
--- a/src/libsyntax/ext/mtwt.rs
+++ b/src/libsyntax/ext/mtwt.rs
@@ -20,6 +20,7 @@ use ast::{Ident, Mrk, Name, SyntaxContext};
 use std::cell::RefCell;
 use std::rc::Rc;
 use std::collections::HashMap;
+use std::collections::hashmap::{Occupied, Vacant};
 
 /// The SCTable contains a table of SyntaxContext_'s. It
 /// represents a flattened tree structure, to avoid having
@@ -65,10 +66,10 @@ pub fn apply_mark(m: Mrk, ctxt: SyntaxContext) -> SyntaxContext {
 /// Extend a syntax context with a given mark and sctable (explicit memoization)
 fn apply_mark_internal(m: Mrk, ctxt: SyntaxContext, table: &SCTable) -> SyntaxContext {
     let key = (ctxt, m);
-    let new_ctxt = |_: &(SyntaxContext, Mrk)|
-                   idx_push(&mut *table.table.borrow_mut(), Mark(m, ctxt));
-
-    *table.mark_memo.borrow_mut().find_or_insert_with(key, new_ctxt)
+    * match table.mark_memo.borrow_mut().entry(key) {
+        Vacant(entry) => entry.set(idx_push(&mut *table.table.borrow_mut(), Mark(m, ctxt))),
+        Occupied(entry) => entry.into_mut(),
+    }
 }
 
 /// Extend a syntax context with a given rename
@@ -83,10 +84,11 @@ fn apply_rename_internal(id: Ident,
                        ctxt: SyntaxContext,
                        table: &SCTable) -> SyntaxContext {
     let key = (ctxt, id, to);
-    let new_ctxt = |_: &(SyntaxContext, Ident, Name)|
-                   idx_push(&mut *table.table.borrow_mut(), Rename(id, to, ctxt));
 
-    *table.rename_memo.borrow_mut().find_or_insert_with(key, new_ctxt)
+    * match table.rename_memo.borrow_mut().entry(key) {
+        Vacant(entry) => entry.set(idx_push(&mut *table.table.borrow_mut(), Rename(id, to, ctxt))),
+        Occupied(entry) => entry.into_mut(),
+    }
 }
 
 /// Apply a list of renamings to a context