miri: don't mutate ecx in read_discriminant_value #51471
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Do a basic sanity check for all constant values ## Motivation and high level overview There has been some back and forth in this PR between @RalfJung and me in here about the motivation for this change and the stance it takes on unsafe coding guidelines. The initial implementation ran its checks on every value read (so `*x`, `y = x`, ...). In unsafe code that isn't reasonable, because we might be invalidating invariants for a short time in order to build up a proper value. The current implementation is a lint that runs its checks statics and constants. There is no need to check array lengths and enum variants, because it's a hard error to end up with anything but a number, and that one even has to have the required bits to be defined. ## What checks are done? * Some type related checks * `char` needs to be a correct unicode character as defined by `char::from_u32` * A reference to a ZST must have the correct alignment (and be nonzero) * A reference to anything is dereferenced and its value is checked * Layout checks use the information from `ty::Layout` to check * all fields of structs * all elements of arrays * enum discriminants * the fields of an enum variant (the variant is decided by the discriminant) * whether any union field succeeds in being checked (if none match the memory pattern, the check fails) * if the value is in the range described by the layout (e.g. for `NonZero*` types) Changing the layout of a type will thus automatically cause the checks to check for the new layout. fixes #51330 fixes #51471 cc @RalfJung r? @eddyb
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Jul 27, 2018
…matsakis Do a basic sanity check for all constant values ## Motivation and high level overview There has been some back and forth in this PR between @RalfJung and me in here about the motivation for this change and the stance it takes on unsafe coding guidelines. The initial implementation ran its checks on every value read (so `*x`, `y = x`, ...). In unsafe code that isn't reasonable, because we might be invalidating invariants for a short time in order to build up a proper value. The current implementation is a lint that runs its checks statics and constants. There is no need to check array lengths and enum variants, because it's a hard error to end up with anything but a number, and that one even has to have the required bits to be defined. ## What checks are done? * Some type related checks * `char` needs to be a correct unicode character as defined by `char::from_u32` * A reference to a ZST must have the correct alignment (and be nonzero) * A reference to anything is dereferenced and its value is checked * Layout checks use the information from `ty::Layout` to check * all fields of structs * all elements of arrays * enum discriminants * the fields of an enum variant (the variant is decided by the discriminant) * whether any union field succeeds in being checked (if none match the memory pattern, the check fails) * if the value is in the range described by the layout (e.g. for `NonZero*` types) Changing the layout of a type will thus automatically cause the checks to check for the new layout. fixes rust-lang#51330 fixes rust-lang#51471 cc @RalfJung r? @eddyb
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Do a basic sanity check for all constant values ## Motivation and high level overview There has been some back and forth in this PR between @RalfJung and me in here about the motivation for this change and the stance it takes on unsafe coding guidelines. The initial implementation ran its checks on every value read (so `*x`, `y = x`, ...). In unsafe code that isn't reasonable, because we might be invalidating invariants for a short time in order to build up a proper value. The current implementation is a lint that runs its checks statics and constants. There is no need to check array lengths and enum variants, because it's a hard error to end up with anything but a number, and that one even has to have the required bits to be defined. ## What checks are done? * Some type related checks * `char` needs to be a correct unicode character as defined by `char::from_u32` * A reference to a ZST must have the correct alignment (and be nonzero) * A reference to anything is dereferenced and its value is checked * Layout checks use the information from `ty::Layout` to check * all fields of structs * all elements of arrays * enum discriminants * the fields of an enum variant (the variant is decided by the discriminant) * whether any union field succeeds in being checked (if none match the memory pattern, the check fails) * if the value is in the range described by the layout (e.g. for `NonZero*` types) Changing the layout of a type will thus automatically cause the checks to check for the new layout. fixes #51330 fixes #51471 cc @RalfJung r? @eddyb
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This causes a borrowck error in oli-obk/priroda#8 when trying to read the discriminant of a value while the stack is borrowed immutably.
https://github.com/oli-obk/priroda/pull/8/files#diff-1336350dd7c33bda9f60eab418d60f2bR132
cc @oli-obk
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