src/librustc/ty/adjustment.rs

   1 use crate::hir;
   2 use crate::hir::def_id::DefId;
   3 use crate::ty::{self, Ty, TyCtxt};
   4 use crate::ty::subst::SubstsRef;
   5 use rustc_macros::HashStable;
   6
   7
   8 #[derive(Clone, Copy, Debug, PartialEq, Eq, RustcEncodable, RustcDecodable, HashStable)]
   9 pub enum PointerCast {
  10     /// Go from a fn-item type to a fn-pointer type.
  11     ReifyFnPointer,
  12
  13     /// Go from a safe fn pointer to an unsafe fn pointer.
  14     UnsafeFnPointer,
  15
  16     /// Go from a non-capturing closure to an fn pointer or an unsafe fn pointer.
  17     /// It cannot convert a closure that requires unsafe.
  18     ClosureFnPointer(hir::Unsafety),
  19
  20     /// Go from a mut raw pointer to a const raw pointer.
  21     MutToConstPointer,
  22
  23     /// Go from `*const [T; N]` to `*const T`
  24     ArrayToPointer,
  25
  26     /// Unsize a pointer/reference value, e.g., `&[T; n]` to
  27     /// `&[T]`. Note that the source could be a thin or fat pointer.
  28     /// This will do things like convert thin pointers to fat
  29     /// pointers, or convert structs containing thin pointers to
  30     /// structs containing fat pointers, or convert between fat
  31     /// pointers. We don't store the details of how the transform is
  32     /// done (in fact, we don't know that, because it might depend on
  33     /// the precise type parameters). We just store the target
  34     /// type. Codegen backends and miri figure out what has to be done
  35     /// based on the precise source/target type at hand.
  36     Unsize,
  37 }
  38
  39 /// Represents coercing a value to a different type of value.
  40 ///
  41 /// We transform values by following a number of `Adjust` steps in order.
  42 /// See the documentation on variants of `Adjust` for more details.
  43 ///
  44 /// Here are some common scenarios:
  45 ///
  46 /// 1. The simplest cases are where a pointer is not adjusted fat vs thin.
  47 ///    Here the pointer will be dereferenced N times (where a dereference can
  48 ///    happen to raw or borrowed pointers or any smart pointer which implements
  49 ///    Deref, including Box<_>). The types of dereferences is given by
  50 ///    `autoderefs`. It can then be auto-referenced zero or one times, indicated
  51 ///    by `autoref`, to either a raw or borrowed pointer. In these cases unsize is
  52 ///    `false`.
  53 ///
  54 /// 2. A thin-to-fat coercion involves unsizing the underlying data. We start
  55 ///    with a thin pointer, deref a number of times, unsize the underlying data,
  56 ///    then autoref. The 'unsize' phase may change a fixed length array to a
  57 ///    dynamically sized one, a concrete object to a trait object, or statically
  58 ///    sized struct to a dynamically sized one. E.g., &[i32; 4] -> &[i32] is
  59 ///    represented by:
  60 ///
  61 ///    ```
  62 ///    Deref(None) -> [i32; 4],
  63 ///    Borrow(AutoBorrow::Ref) -> &[i32; 4],
  64 ///    Unsize -> &[i32],
  65 ///    ```
  66 ///
  67 ///    Note that for a struct, the 'deep' unsizing of the struct is not recorded.
  68 ///    E.g., `struct Foo<T> { x: T }` we can coerce &Foo<[i32; 4]> to &Foo<[i32]>
  69 ///    The autoderef and -ref are the same as in the above example, but the type
  70 ///    stored in `unsize` is `Foo<[i32]>`, we don't store any further detail about
  71 ///    the underlying conversions from `[i32; 4]` to `[i32]`.
  72 ///
  73 /// 3. Coercing a `Box<T>` to `Box<dyn Trait>` is an interesting special case. In
  74 ///    that case, we have the pointer we need coming in, so there are no
  75 ///    autoderefs, and no autoref. Instead we just do the `Unsize` transformation.
  76 ///    At some point, of course, `Box` should move out of the compiler, in which
  77 ///    case this is analogous to transforming a struct. E.g., Box<[i32; 4]> ->
  78 ///    Box<[i32]> is an `Adjust::Unsize` with the target `Box<[i32]>`.
  79 #[derive(Clone, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
  80 pub struct Adjustment<'tcx> {
  81     pub kind: Adjust<'tcx>,
  82     pub target: Ty<'tcx>,
  83 }
  84
  85 #[derive(Clone, Debug, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
  86 pub enum Adjust<'tcx> {
  87     /// Go from ! to any type.
  88     NeverToAny,
  89
  90     /// Dereference once, producing a place.
  91     Deref(Option<OverloadedDeref<'tcx>>),
  92
  93     /// Take the address and produce either a `&` or `*` pointer.
  94     Borrow(AutoBorrow<'tcx>),
  95
  96     Pointer(PointerCast),
  97 }
  98
  99 /// An overloaded autoderef step, representing a `Deref(Mut)::deref(_mut)`
 100 /// call, with the signature `&'a T -> &'a U` or `&'a mut T -> &'a mut U`.
 101 /// The target type is `U` in both cases, with the region and mutability
 102 /// being those shared by both the receiver and the returned reference.
 103 #[derive(Copy, Clone, PartialEq, Debug, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 104 pub struct OverloadedDeref<'tcx> {
 105     pub region: ty::Region<'tcx>,
 106     pub mutbl: hir::Mutability,
 107 }
 108
 109 impl<'tcx> OverloadedDeref<'tcx> {
 110     pub fn method_call(&self, tcx: TyCtxt<'tcx>, source: Ty<'tcx>) -> (DefId, SubstsRef<'tcx>) {
 111         let trait_def_id = match self.mutbl {
 112             hir::Mutability::Immutable => tcx.lang_items().deref_trait(),
 113             hir::Mutability::Mutable => tcx.lang_items().deref_mut_trait()
 114         };
 115         let method_def_id = tcx.associated_items(trait_def_id.unwrap())
 116             .find(|m| m.kind == ty::AssocKind::Method).unwrap().def_id;
 117         (method_def_id, tcx.mk_substs_trait(source, &[]))
 118     }
 119 }
 120
 121 /// At least for initial deployment, we want to limit two-phase borrows to
 122 /// only a few specific cases. Right now, those are mostly "things that desugar"
 123 /// into method calls:
 124 /// - using `x.some_method()` syntax, where some_method takes `&mut self`,
 125 /// - using `Foo::some_method(&mut x, ...)` syntax,
 126 /// - binary assignment operators (`+=`, `-=`, `*=`, etc.).
 127 /// Anything else should be rejected until generalized two-phase borrow support
 128 /// is implemented. Right now, dataflow can't handle the general case where there
 129 /// is more than one use of a mutable borrow, and we don't want to accept too much
 130 /// new code via two-phase borrows, so we try to limit where we create two-phase
 131 /// capable mutable borrows.
 132 /// See #49434 for tracking.
 133 #[derive(Copy, Clone, PartialEq, Debug, RustcEncodable, RustcDecodable, HashStable)]
 134 pub enum AllowTwoPhase {
 135     Yes,
 136     No
 137 }
 138
 139 #[derive(Copy, Clone, PartialEq, Debug, RustcEncodable, RustcDecodable, HashStable)]
 140 pub enum AutoBorrowMutability {
 141     Mutable { allow_two_phase_borrow: AllowTwoPhase },
 142     Immutable,
 143 }
 144
 145 impl From<AutoBorrowMutability> for hir::Mutability {
 146     fn from(m: AutoBorrowMutability) -> Self {
 147         match m {
 148             AutoBorrowMutability::Mutable { .. } => hir::Mutability::Mutable,
 149             AutoBorrowMutability::Immutable => hir::Mutability::Immutable,
 150         }
 151     }
 152 }
 153
 154 #[derive(Copy, Clone, PartialEq, Debug, RustcEncodable, RustcDecodable, HashStable, TypeFoldable)]
 155 pub enum AutoBorrow<'tcx> {
 156     /// Converts from T to &T.
 157     Ref(ty::Region<'tcx>, AutoBorrowMutability),
 158
 159     /// Converts from T to *T.
 160     RawPtr(hir::Mutability),
 161 }
 162
 163 /// Information for `CoerceUnsized` impls, storing information we
 164 /// have computed about the coercion.
 165 ///
 166 /// This struct can be obtained via the `coerce_impl_info` query.
 167 /// Demanding this struct also has the side-effect of reporting errors
 168 /// for inappropriate impls.
 169 #[derive(Clone, Copy, RustcEncodable, RustcDecodable, Debug, HashStable)]
 170 pub struct CoerceUnsizedInfo {
 171     /// If this is a "custom coerce" impl, then what kind of custom
 172     /// coercion is it? This applies to impls of `CoerceUnsized` for
 173     /// structs, primarily, where we store a bit of info about which
 174     /// fields need to be coerced.
 175     pub custom_kind: Option<CustomCoerceUnsized>
 176 }
 177
 178 #[derive(Clone, Copy, RustcEncodable, RustcDecodable, Debug, HashStable)]
 179 pub enum CustomCoerceUnsized {
 180     /// Records the index of the field being coerced.
 181     Struct(usize)
 182 }