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recursively evaluate the constants in everything that is 'mentioned' This is another attempt at fixing https://github.com/rust-lang/rust/issues/107503. The previous attempt at https://github.com/rust-lang/rust/pull/112879 seems stuck in figuring out where the [perf regression](https://perf.rust-lang.org/compare.html?start=c55d1ee8d4e3162187214692229a63c2cc5e0f31&end=ec8de1ebe0d698b109beeaaac83e60f4ef8bb7d1&stat=instructions:u) comes from. In https://github.com/rust-lang/rust/pull/122258 I learned some things, which informed the approach this PR is taking. Quoting from the new collector docs, which explain the high-level idea: ```rust //! One important role of collection is to evaluate all constants that are used by all the items //! which are being collected. Codegen can then rely on only encountering constants that evaluate //! successfully, and if a constant fails to evaluate, the collector has much better context to be //! able to show where this constant comes up. //! //! However, the exact set of "used" items (collected as described above), and therefore the exact //! set of used constants, can depend on optimizations. Optimizing away dead code may optimize away //! a function call that uses a failing constant, so an unoptimized build may fail where an //! optimized build succeeds. This is undesirable. //! //! To fix this, the collector has the concept of "mentioned" items. Some time during the MIR //! pipeline, before any optimization-level-dependent optimizations, we compute a list of all items //! that syntactically appear in the code. These are considered "mentioned", and even if they are in //! dead code and get optimized away (which makes them no longer "used"), they are still //! "mentioned". For every used item, the collector ensures that all mentioned items, recursively, //! do not use a failing constant. This is reflected via the [`CollectionMode`], which determines //! whether we are visiting a used item or merely a mentioned item. //! //! The collector and "mentioned items" gathering (which lives in `rustc_mir_transform::mentioned_items`) //! need to stay in sync in the following sense: //! //! - For every item that the collector gather that could eventually lead to build failure (most //! likely due to containing a constant that fails to evaluate), a corresponding mentioned item //! must be added. This should use the exact same strategy as the ecollector to make sure they are //! in sync. However, while the collector works on monomorphized types, mentioned items are //! collected on generic MIR -- so any time the collector checks for a particular type (such as //! `ty::FnDef`), we have to just onconditionally add this as a mentioned item. //! - In `visit_mentioned_item`, we then do with that mentioned item exactly what the collector //! would have done during regular MIR visiting. Basically you can think of the collector having //! two stages, a pre-monomorphization stage and a post-monomorphization stage (usually quite //! literally separated by a call to `self.monomorphize`); the pre-monomorphizationn stage is //! duplicated in mentioned items gathering and the post-monomorphization stage is duplicated in //! `visit_mentioned_item`. //! - Finally, as a performance optimization, the collector should fill `used_mentioned_item` during //! its MIR traversal with exactly what mentioned item gathering would have added in the same //! situation. This detects mentioned items that have *not* been optimized away and hence don't //! need a dedicated traversal. enum CollectionMode { /// Collect items that are used, i.e., actually needed for codegen. /// /// Which items are used can depend on optimization levels, as MIR optimizations can remove /// uses. UsedItems, /// Collect items that are mentioned. The goal of this mode is that it is independent of /// optimizations: the set of "mentioned" items is computed before optimizations are run. /// /// The exact contents of this set are *not* a stable guarantee. (For instance, it is currently /// computed after drop-elaboration. If we ever do some optimizations even in debug builds, we /// might decide to run them before computing mentioned items.) The key property of this set is /// that it is optimization-independent. MentionedItems, } ``` And the `mentioned_items` MIR body field docs: ```rust /// Further items that were mentioned in this function and hence *may* become monomorphized, /// depending on optimizations. We use this to avoid optimization-dependent compile errors: the /// collector recursively traverses all "mentioned" items and evaluates all their /// `required_consts`. /// /// This is *not* soundness-critical and the contents of this list are *not* a stable guarantee. /// All that's relevant is that this set is optimization-level-independent, and that it includes /// everything that the collector would consider "used". (For example, we currently compute this /// set after drop elaboration, so some drop calls that can never be reached are not considered /// "mentioned".) See the documentation of `CollectionMode` in /// `compiler/rustc_monomorphize/src/collector.rs` for more context. pub mentioned_items: Vec<Spanned<MentionedItem<'tcx>>>, ``` Fixes #107503 |
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