Safe Transmute: Compute transmutability from rustc_target::abi::Layout
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jswrenn
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UnsafeCell was, previously, unsupported by virtue of us not implementing support for anything besides repr(C) structs. We didn't have to worry about the presence/absence of UnsafeCell in references, because we simply bailed at the first sight of any UnsafeCell. Now that we support analyzing types with UnsafeCell, we also need to be careful not permit transmutations that would lead to data races. We do so by emitting Freeze bounds when appropriate (made possible by #121840; thanks @oli-obk!).
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N.B.: This is what the UX of analyzing an invalid UnsafeCell-containing transmute looks like.
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The meat of the PR is here.
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Apr 8, 2024
| @@ -8,7 +8,7 @@ error[E0277]: `Src` cannot be safely transmuted into `Dst` | |||
| --> $DIR/unknown_src_field.rs:19:36 | |||
| | | |||
| LL | assert::is_transmutable::<Src, Dst>(); | |||
| | ^^^ `Dst` has an unknown layout | |||
| | ^^^ analyzing the transmutability of `Dst` is not yet supported. | |||
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Remove the . -- it's inconsistent with diagnostics
| let align = layout.align(); | ||
| let size = layout.size(); | ||
| let ty_and_layout = cx.layout_of(*ty)?; | ||
| let align = ty_and_layout.align.abi.bytes() as _; |
In its first step of computing transmutability, `rustc_transmutability` constructs a byte-level representation of type layout (`Tree`). Previously, this representation was computed for ADTs by inspecting the ADT definition and performing our own layout computations. This process was error-prone, verbose, and limited our ability to analyze many types (particularly default-repr types). In this PR, we instead construct `Tree`s from `rustc_target::abi::Layout`s. This helps ensure that layout optimizations are reflected our analyses, and increases the kinds of types we can now analyze, including: - default repr ADTs - transparent unions - `UnsafeCell`-containing types Overall, this PR expands the expressvity of `rustc_transmutability` to be much closer to the transmutability analysis performed by miri. Future PRs will work to close the remaining gaps (e.g., support for `Box`, raw pointers, `NonZero*`, coroutines, etc.).
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@bors r+ |
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…iaskrgr Rollup of 9 pull requests Successful merges: - rust-lang#122781 (Fix argument ABI for overaligned structs on ppc64le) - rust-lang#123367 (Safe Transmute: Compute transmutability from `rustc_target::abi::Layout`) - rust-lang#123518 (Fix `ByMove` coroutine-closure shim (for 2021 precise closure capturing behavior)) - rust-lang#123547 (bootstrap: remove unused pub fns) - rust-lang#123564 (Don't emit divide-by-zero panic paths in `StepBy::len`) - rust-lang#123578 (Restore `pred_known_to_hold_modulo_regions`) - rust-lang#123591 (Remove unnecessary cast from `LLVMRustGetInstrProfIncrementIntrinsic`) - rust-lang#123632 (parser: reduce visibility of unnecessary public `UnmatchedDelim`) - rust-lang#123635 (CFI: Fix ICE in KCFI non-associated function pointers) r? `@ghost` `@rustbot` modify labels: rollup
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Rollup merge of rust-lang#123367 - jswrenn:layoutify, r=compiler-errors Safe Transmute: Compute transmutability from `rustc_target::abi::Layout` In its first step of computing transmutability, `rustc_transmutability` constructs a byte-level representation of type layout (`Tree`). Previously, this representation was computed for ADTs by inspecting the ADT definition and performing our own layout computations. This process was error-prone, verbose, and limited our ability to analyze many types (particularly default-repr types). In this PR, we instead construct `Tree`s from `rustc_target::abi::Layout`s. This helps ensure that layout optimizations are reflected our analyses, and increases the kinds of types we can now analyze, including: - default repr ADTs - transparent unions - `UnsafeCell`-containing types Overall, this PR expands the expressvity of `rustc_transmutability` to be much closer to the transmutability analysis performed by miri. Future PRs will work to close the remaining gaps (e.g., support for `Box`, raw pointers, `NonZero*`, coroutines, etc.). r? `@compiler-errors`
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In its first step of computing transmutability,
rustc_transmutabilityconstructs a byte-level representation of type layout (Tree). Previously, this representation was computed for ADTs by inspecting the ADT definition and performing our own layout computations. This process was error-prone, verbose, and limited our ability to analyze many types (particularly default-repr types).In this PR, we instead construct
Trees fromrustc_target::abi::Layouts. This helps ensure that layout optimizations are reflected our analyses, and increases the kinds of types we can now analyze, including:UnsafeCell-containing typesOverall, this PR expands the expressvity of
rustc_transmutabilityto be much closer to the transmutability analysis performed by miri. Future PRs will work to close the remaining gaps (e.g., support forBox, raw pointers,NonZero*, coroutines, etc.).r? @compiler-errors