Add #[used] to always override

At least when the final linker invocation is driven by `rustc`.

Author: madsmtm

libmimalloc-sys/src/lib.rs

@@ -67,6 +67,47 @@ extern "C" {
     pub fn mi_free(p: *mut c_void);
 }
 
+/// When using the `"override"` feature flag, the user wants us to globally
+/// override the system allocator.
+///
+/// However, since we build and link `mimalloc` as a static library/archive,
+/// the linker may decide to not care about our overrides if it can't directly
+/// see references to the symbols, see the following link for details:
+/// <https://maskray.me/blog/2021-06-20-symbol-processing#archive-processing>
+///
+/// This is problematic if `mimalloc` is used from a library that by itself
+/// doesn't allocate, yet invokes other shared libraries that do, since then
+/// the linker wouldn't see any references to `malloc`/`free`, and the symbols
+/// would not be overridden.
+///
+/// To avoid this problem, we make sure that the allocator functions are
+/// visible to the linker.
+///
+/// To avoid this problem, we reference `mi_malloc` in a `#[used]` static.
+/// This makes it known to `rustc`, which will create a reference to it in a
+/// `symbols.o` stub file that is later passed directly to the linker (instead
+/// of being in an archive). See the following link for details on how this
+/// works: <https://github.com/rust-lang/rust/pull/95604>
+///
+/// NOTE: This works because `mimalloc` is compiled into a single object file
+/// in `static.c`. If it was split across multiple files, we'd need to
+/// reference each symbol. See also the comment at the top of `static.c`.
+///
+/// NOTE: On macOS, mimalloc doesn't just override malloc/free, but also
+/// registers itself with the allocator's zone APIs in a ctor
+/// (`_mi_macos_override_malloc`, marked with `__attribute__((constructor))`).
+/// Similarly to above, for the Mach-O linker to actually consider ctors as
+/// "used" when defined in an archive member in a static library, so we need
+/// to explicitly reference something in the object file. The constructor
+/// symbol itself is static, so we can't get a reference to that, so instead
+/// we reference `mi_malloc` here too).
+#[cfg(feature = "override")]
+mod set_up_statics {
+    use super::*;
+    #[used] // Could be `#[used(linker)]` once stable
+    static USED: unsafe extern "C" fn(usize) -> *mut c_void = mi_malloc;
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;