Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 113 insertions, 0 deletions

diff --git a/library/proc_macro/src/bridge/arena.rs b/library/proc_macro/src/bridge/arena.rs
new file mode 100644
index 000000000..fa72d2816
--- /dev/null
+++ b/library/proc_macro/src/bridge/arena.rs
@@ -0,0 +1,113 @@
+//! A minimal arena allocator inspired by `rustc_arena::DroplessArena`.
+//!
+//! This is unfortunately a minimal re-implementation rather than a dependency
+//! as it is difficult to depend on crates from within `proc_macro`, due to it
+//! being built at the same time as `std`.
+
+use std::cell::{Cell, RefCell};
+use std::cmp;
+use std::mem::MaybeUninit;
+use std::ops::Range;
+use std::ptr;
+use std::slice;
+use std::str;
+
+// The arenas start with PAGE-sized chunks, and then each new chunk is twice as
+// big as its predecessor, up until we reach HUGE_PAGE-sized chunks, whereupon
+// we stop growing. This scales well, from arenas that are barely used up to
+// arenas that are used for 100s of MiBs. Note also that the chosen sizes match
+// the usual sizes of pages and huge pages on Linux.
+const PAGE: usize = 4096;
+const HUGE_PAGE: usize = 2 * 1024 * 1024;
+
+/// A minimal arena allocator inspired by `rustc_arena::DroplessArena`.
+///
+/// This is unfortunately a complete re-implementation rather than a dependency
+/// as it is difficult to depend on crates from within `proc_macro`, due to it
+/// being built at the same time as `std`.
+///
+/// This arena doesn't have support for allocating anything other than byte
+/// slices, as that is all that is necessary.
+pub(crate) struct Arena {
+    start: Cell<*mut MaybeUninit<u8>>,
+    end: Cell<*mut MaybeUninit<u8>>,
+    chunks: RefCell<Vec<Box<[MaybeUninit<u8>]>>>,
+}
+
+impl Arena {
+    pub(crate) fn new() -> Self {
+        Arena {
+            start: Cell::new(ptr::null_mut()),
+            end: Cell::new(ptr::null_mut()),
+            chunks: RefCell::new(Vec::new()),
+        }
+    }
+
+    /// Add a new chunk with at least `additional` free bytes.
+    #[inline(never)]
+    #[cold]
+    fn grow(&self, additional: usize) {
+        let mut chunks = self.chunks.borrow_mut();
+        let mut new_cap;
+        if let Some(last_chunk) = chunks.last_mut() {
+            // 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.len().min(HUGE_PAGE / 2);
+            new_cap *= 2;
+        } else {
+            new_cap = PAGE;
+        }
+        // Also ensure that this chunk can fit `additional`.
+        new_cap = cmp::max(additional, new_cap);
+
+        let mut chunk = Box::new_uninit_slice(new_cap);
+        let Range { start, end } = chunk.as_mut_ptr_range();
+        self.start.set(start);
+        self.end.set(end);
+        chunks.push(chunk);
+    }
+
+    /// Allocates a byte slice with specified size from the current memory
+    /// chunk. Returns `None` if there is no free space left to satisfy the
+    /// request.
+    fn alloc_raw_without_grow(&self, bytes: usize) -> Option<&mut [MaybeUninit<u8>]> {
+        let start = self.start.get().addr();
+        let old_end = self.end.get();
+        let end = old_end.addr();
+
+        let new_end = end.checked_sub(bytes)?;
+        if start <= new_end {
+            let new_end = old_end.with_addr(new_end);
+            self.end.set(new_end);
+            // SAFETY: `bytes` bytes starting at `new_end` were just reserved.
+            Some(unsafe { slice::from_raw_parts_mut(new_end, bytes) })
+        } else {
+            None
+        }
+    }
+
+    fn alloc_raw(&self, bytes: usize) -> &mut [MaybeUninit<u8>] {
+        if bytes == 0 {
+            return &mut [];
+        }
+
+        loop {
+            if let Some(a) = self.alloc_raw_without_grow(bytes) {
+                break a;
+            }
+            // No free space left. Allocate a new chunk to satisfy the request.
+            // On failure the grow will panic or abort.
+            self.grow(bytes);
+        }
+    }
+
+    pub(crate) fn alloc_str<'a>(&'a self, string: &str) -> &'a mut str {
+        let alloc = self.alloc_raw(string.len());
+        let bytes = MaybeUninit::write_slice(alloc, string.as_bytes());
+
+        // SAFETY: we convert from `&str` to `&[u8]`, clone it into the arena,
+        // and immediately convert the clone back to `&str`.
+        unsafe { str::from_utf8_unchecked_mut(bytes) }
+    }
+}