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Rollup merge of #124904 - RalfJung:reachable, r=tmiasko

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reachable computation: extend explanation of what this does, and why

Follow-up to https://github.com/rust-lang/rust/pull/122769. I had the time to think about this some more, in particular in the context of https://github.com/rust-lang/rust/issues/119214, so I felt it was worth extending these comments some more.

I also gave up on the context of "externally reachable" as it is not called that way anywhere else in the compiler.

Cc `@tmiasko` `@saethlin`
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许杰友 Jieyou Xu (Joe) 2024-05-11 01:15:10 +01:00 committed by GitHub
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compiler/rustc_passes/src/reachable.rs
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@ -1,14 +1,26 @@
//! Finds local items that are externally reachable, which means that other crates need access to
//! their compiled machine code or their MIR.
//! Finds local items that are "reachable", which means that other crates need access to their
//! compiled code or their *runtime* MIR. (Compile-time MIR is always encoded anyway, so we don't
//! worry about that here.)
//!
//! An item is "externally reachable" if it is relevant for other crates. This obviously includes
//! all public items. However, some of these items cannot be compiled to machine code (because they
//! are generic), and for some the machine code is not sufficient (because we want to cross-crate
//! inline them). These items "need cross-crate MIR". When a reachable function `f` needs
//! cross-crate MIR, then all the functions it calls also become reachable, as they will be
//! necessary to use the MIR of `f` from another crate. Furthermore, an item can become "externally
//! reachable" by having a `const`/`const fn` return a pointer to that item, so we also need to
//! recurse into reachable `const`/`const fn`.
//! An item is "reachable" if codegen that happens in downstream crates can end up referencing this
//! item. This obviously includes all public items. However, some of these items cannot be codegen'd
//! (because they are generic), and for some the compiled code is not sufficient (because we want to
//! cross-crate inline them). These items "need cross-crate MIR". When a reachable function `f`
//! needs cross-crate MIR, then its MIR may be codegen'd in a downstream crate, and hence items it
//! mentions need to be considered reachable.
//!
//! Furthermore, if a `const`/`const fn` is reachable, then it can return pointers to other items,
//! making those reachable as well. For instance, consider a `const fn` returning a pointer to an
//! otherwise entirely private function: if a downstream crate calls that `const fn` to compute the
//! initial value of a `static`, then it needs to generate a direct reference to this function --
//! i.e., the function is directly reachable from that downstream crate! Hence we have to recurse
//! into `const` and `const fn`.
//!
//! Conversely, reachability *stops* when it hits a monomorphic non-`const` function that we do not
//! want to cross-crate inline. That function will just be codegen'd in this crate, which means the
//! monomorphization collector will consider it a root and then do another graph traversal to
//! codegen everything called by this function -- but that's a very different graph from what we are
//! considering here as at that point, everything is monomorphic.
use hir::def_id::LocalDefIdSet;
use rustc_data_structures::stack::ensure_sufficient_stack;