renamed to as_mut_bytes #5
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dorryspears
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Previously, our test harness for `TryFromBytes` types assumed that those types also implemented `NoCell`. This commit introduces machinery that allows us to conditionally execute certain steps of our tests depending on whether or not the type under test implements `NoCell`. This commit also lifts the `T: NoCell` bound in `TryFromBytes for MaybeUninit<T>` in order to test this machinery. Makes progress on #5 Co-authored-by: Jack Wrenn <jswrenn@amazon.com>
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This allows the callback passed to `with_passing_test_cases` to access its argument mutably. We don't make use of this ability in this commit, but it will become important later when we implement `TryFromBytes` for `UnsafeCell`. Makes progress on #5
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Credit to jswrenn@amazon.com for figuring out how to make this design more misuse-proof. Implementing `TryFromBytes` for `UnsafeCell` isn't possible without changes to `TryFromBytes`. Previously, we passed `is_bit_valid` a `Ptr` with shared aliasing. There is no way to soundly implement `is_bit_valid` for `UnsafeCell` with a shared-aliased pointer, as there's no way to soundly inspect the bytes of an `UnsafeCell` when other references may exist to the same memory. The only way to inspect the bytes of an `UnsafeCell` in a generic context is given a mutable reference to one. This is in principle possible from `TryFromBytes`. Notably, `try_read_from` reads a `MaybeUninit<Self>` into its local stack frame, and that has `UnsafeCell`s at the same byte ranges as `Self`, so it is sound to construct a mutably-aliased `Ptr` to that local variable. Unfortunately, `is_bit_valid` previously had no way of reasoning about a mutably-aliased pointer. In order to lift this restriction, we modify `is_bit_valid` to be generic over aliasing using `A: invariant::at_least::Shared`. When `A` is `invariant::Exclusive`, we can implement `TryFromBytes::is_bit_valid` for `UnsafeCell`. However, this isn't the whole story. We need to implement `TryFromBytes` for `UnsafeCell` in the general case, even when a shared-aliased `Ptr` is passed to `is_bit_valid`. Our first design was to define calling `UnsafeCell::is_bit_valid` with a shared-aliased to be a panic condition of `is_bit_valid`. Since we only call `is_bit_valid` from our own code (`is_bit_valid` is `#[doc(hidden)]`), we can in theory simply avoid calling `UnsafeCell::is_bit_valid` with a shared-aliased pointer. However, that's very error-prone. jswrenn had the idea to instead use a post-monomorphization error, which is implemented in `Ptr::into_exclusive_or_post_monomorphization_error`. This allows us to test *at compile time* to see whether the argument to `UnsafeCell::is_bit_valid` is a shared or exclusive pointer, and fail compilation in the former case. Some readers might wonder why we couldn't use a `NoCell` bound to enforce that `is_bit_valid` could only be called with a shared-aliased pointer when `Self: NoCell`. We experimented with this, but it turns out that there is some existing code that cannot be made to work with this pattern - the type system isn't smart enough to infer bounds in some cases. Makes progress on #5
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Makes progress towards #5.
dorryspears
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Makes progress on #5 Closes google#962 Co-authored-by: Jack Wrenn <jswrenn@amazon.com>
joshlf
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…le#1119) For example, `try_from_ref` is now `try_ref_from`. See: google#1095 Makes progress on #5
dorryspears
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dorryspears
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Closes google#1013. Makes progress towards #5.
dorryspears
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We do support deriving `TryFromBytes` on unions, and our conservative assumptions of their validity match intuition. Makes progress towards #5.
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