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Safe Transmute: Compute transmutability from rustc_target::abi::Layout
#123367
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(Left some comments as review waypoints.)
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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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Minor nits
@@ -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
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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 _; |
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as usize
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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.).
@bors r+ |
…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
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`
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 fromrustc_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:UnsafeCell
-containing typesOverall, 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 forBox
, raw pointers,NonZero*
, coroutines, etc.).r? @compiler-errors