Auto merge of rust-lang#78058 - bugadani:arena2, r=lcnr

Make sure arenas don't allocate bigger than HUGE_PAGE

Right now, arenas allocate based on the size of the last chunk. It is possible for a `grow` call to allocate a chunk that is not a multiple of `PAGE`, and this size is doubled for each subsequent allocation. This means, instead of `HUGE_PAGE`, the biggest page possible is actually unknown.

This change fixes this, and also removes an unnecessary checked multiplication. It is still possible to allocate bigger than `HUGE_PAGE` pages, but this will only happen as many times as absolutely necessary.

compiler/rustc_arena/src/lib.rs

@@ -223,10 +223,8 @@ impl<T> TypedArena<T> {
                 // If the previous chunk's len is less than HUGE_PAGE
                 // bytes, then this chunk will be least double the previous
                 // chunk's size.
-                new_cap = last_chunk.storage.len();
-                if new_cap < HUGE_PAGE / elem_size {
-                    new_cap = new_cap.checked_mul(2).unwrap();
-                }
+                new_cap = last_chunk.storage.len().min(HUGE_PAGE / elem_size / 2);
+                new_cap = new_cap * 2;
             } else {
                 new_cap = PAGE / elem_size;
             }
@@ -343,10 +341,8 @@ impl DroplessArena {
                 // If the previous chunk's len is less than HUGE_PAGE
                 // bytes, then this chunk will be least double the previous
                 // chunk's size.
-                new_cap = last_chunk.storage.len();
-                if new_cap < HUGE_PAGE {
-                    new_cap = new_cap.checked_mul(2).unwrap();
-                }
+                new_cap = last_chunk.storage.len().min(HUGE_PAGE / 2);
+                new_cap = new_cap * 2;
             } else {
                 new_cap = PAGE;
             }