Sync from rust 2026/09/18 - #980
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Session creation is currently awkward: we build a mostly-initialized session, then use it to initialize a codegen backend, and then use the codegen backend to finish initializing the session. And it's not just awkward: within the Cranelift backend's `init` method `sess.lto()` is called, which consults `sess.thin_lto_supported`, *before* that field has been properly set! In practice it had no effect but it's worth fixing. This commit cleans up this mess. It introduces `EarlySession`, which contains just four `Session` fields, the ones that are needed for codegen backend initialization. It is now a field within `Session`, and `Session` derefs to `EarlySession` to avoid changing a zillion `sess.target`/`sess.opts`/etc. occurrences. `EarlySession` is passed to `init`, which returns a `CodegenBackendInit` that contains the backend-specific information needed to build a `Session`. (It replaces the `replaced_intrinsics`, `fallback_intrinsics`, and `thin_lto_supported` methods.) The `Session` can then be built in a single step. No more `Session`/`CodegenBackend` initialization intermingling. A few functions that previously took a `Session` now take something else, e.g. a `Target`. Some `Session` methods are now `EarlySession` methods. And a new `early_lto` method is used for Cranelift's LTO check.
It currently takes `&self`, which is a bit strange for an `init` method. As a result, the Cranelift and GCC backends have to use types with interior mutability. This commit changes it to `&mut self`. Benefits: - The Cranelift backend can use `Option` instead of `OnceCell` to indicate uninit vs. init. - The GCC backend can avoid `Mutex`, and use `bool` instead of `AtomicBool`, which makes things much simpler. The commit also restructures `GccCodegenBackend` to mirror `CraneliftCodegenBackend`: just contain an `Option<BackendConfig>`, which makes the uninit vs. init distinction foolproof. (E.g. no need to set `lto_supported` to false and then later overwrite it with the real value.) As part of this the `LockedTargetInfo` type is renamed `SharedTargetInfo` because that better matches its new internals. (All this compiles both with and without the "master" feature set.)
It's now possible to get the backend features (a `Vec<String>`) when the codegen backend is started, pass it back through `CodegenBackendInit`, and just store it in the `Session`. This removes the need for the query. Also: - `WriteBackendMethods::target_machine_factory` no longer needs the `target_features` parameter, because it's now available through the `sess` parameter. - `CodegenContext` no longer needs the `backend_features` field because we can use `sess.global_backend_features` instead. - `CodegenBackend::provide` is now a no-op for all the in-tree backends. I haven't removed it because out-of-tree backends still rely on it.
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