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Rollup merge of #126154 - RalfJung:storage-live, r=compiler-errors

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StorageLive: refresh storage (instead of UB) when local is already live

Blocked on [this FCP](https://github.com/rust-lang/rust/issues/99160#issuecomment-2155924538), which also contains the motivation.

Fixes https://github.com/rust-lang/rust/issues/99160
Fixes https://github.com/rust-lang/rust/issues/98896 (by declaring it not-a-bug)
Fixes https://github.com/rust-lang/rust/issues/119366
Fixes https://github.com/rust-lang/unsafe-code-guidelines/issues/129
This commit is contained in:
León Orell Valerian Liehr 2024-06-19 13:04:58 +02:00 • committed by GitHub
commit 035285b464
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7 changed files with 71 additions and 11 deletions
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11
compiler/rustc_middle/src/mir/syntax.rs
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@ -361,16 +361,19 @@ pub enum StatementKind<'tcx> {
/// At any point during the execution of a function, each local is either allocated or
/// unallocated. Except as noted below, all locals except function parameters are initially
/// unallocated. `StorageLive` statements cause memory to be allocated for the local while
/// `StorageDead` statements cause the memory to be freed. Using a local in any way (not only
/// reading/writing from it) while it is unallocated is UB.
/// `StorageDead` statements cause the memory to be freed. In other words,
/// `StorageLive`/`StorageDead` act like the heap operations `allocate`/`deallocate`, but for
/// stack-allocated local variables. Using a local in any way (not only reading/writing from it)
/// while it is unallocated is UB.
///
/// Some locals have no `StorageLive` or `StorageDead` statements within the entire MIR body.
/// These locals are implicitly allocated for the full duration of the function. There is a
/// convenience method at `rustc_mir_dataflow::storage::always_storage_live_locals` for
/// computing these locals.
///
/// If the local is already allocated, calling `StorageLive` again is UB. However, for an
/// unallocated local an additional `StorageDead` all is simply a nop.
/// If the local is already allocated, calling `StorageLive` again will implicitly free the
/// local and then allocate fresh uninitilized memory. If a local is already deallocated,
/// calling `StorageDead` again is a NOP.
StorageLive(Local),
/// See `StorageLive` above.