compiler/rustc_middle/src/ty/adjustment.rs

   1 use crate::ty::subst::SubstsRef;
   2 use crate::ty::{self, Ty, TyCtxt};
   3 use rustc_hir as hir;
   4 use rustc_hir::def_id::DefId;
   5 use rustc_hir::lang_items::LangItem;
   6 use rustc_macros::HashStable;
   7 use rustc_span::Span;
   8
   9 #[derive(Clone, Copy, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
  10 pub enum PointerCast {
  11     /// Go from a fn-item type to a fn-pointer type.
  12     ReifyFnPointer,
  13
  14     /// Go from a safe fn pointer to an unsafe fn pointer.
  15     UnsafeFnPointer,
  16
  17     /// Go from a non-capturing closure to an fn pointer or an unsafe fn pointer.
  18     /// It cannot convert a closure that requires unsafe.
  19     ClosureFnPointer(hir::Unsafety),
  20
  21     /// Go from a mut raw pointer to a const raw pointer.
  22     MutToConstPointer,
  23
  24     /// Go from `*const [T; N]` to `*const T`
  25     ArrayToPointer,
  26
  27     /// Unsize a pointer/reference value, e.g., `&[T; n]` to
  28     /// `&[T]`. Note that the source could be a thin or fat pointer.
  29     /// This will do things like convert thin pointers to fat
  30     /// pointers, or convert structs containing thin pointers to
  31     /// structs containing fat pointers, or convert between fat
  32     /// pointers. We don't store the details of how the transform is
  33     /// done (in fact, we don't know that, because it might depend on
  34     /// the precise type parameters). We just store the target
  35     /// type. Codegen backends and miri figure out what has to be done
  36     /// based on the precise source/target type at hand.
  37     Unsize,
  38 }
  39
  40 /// Represents coercing a value to a different type of value.
  41 ///
  42 /// We transform values by following a number of `Adjust` steps in order.
  43 /// See the documentation on variants of `Adjust` for more details.
  44 ///
  45 /// Here are some common scenarios:
  46 ///
  47 /// 1. The simplest cases are where a pointer is not adjusted fat vs thin.
  48 ///    Here the pointer will be dereferenced N times (where a dereference can
  49 ///    happen to raw or borrowed pointers or any smart pointer which implements
  50 ///    `Deref`, including `Box<_>`). The types of dereferences is given by
  51 ///    `autoderefs`. It can then be auto-referenced zero or one times, indicated
  52 ///    by `autoref`, to either a raw or borrowed pointer. In these cases unsize is
  53 ///    `false`.
  54 ///
  55 /// 2. A thin-to-fat coercion involves unsizing the underlying data. We start
  56 ///    with a thin pointer, deref a number of times, unsize the underlying data,
  57 ///    then autoref. The 'unsize' phase may change a fixed length array to a
  58 ///    dynamically sized one, a concrete object to a trait object, or statically
  59 ///    sized struct to a dynamically sized one. E.g., `&[i32; 4]` -> `&[i32]` is
  60 ///    represented by:
  61 ///
  62 ///    ```ignore (illustrative)
  63 ///    Deref(None) -> [i32; 4],
  64 ///    Borrow(AutoBorrow::Ref) -> &[i32; 4],
  65 ///    Unsize -> &[i32],
  66 ///    ```
  67 ///
  68 ///    Note that for a struct, the 'deep' unsizing of the struct is not recorded.
  69 ///    E.g., `struct Foo<T> { x: T }` we can coerce `&Foo<[i32; 4]>` to `&Foo<[i32]>`
  70 ///    The autoderef and -ref are the same as in the above example, but the type
  71 ///    stored in `unsize` is `Foo<[i32]>`, we don't store any further detail about
  72 ///    the underlying conversions from `[i32; 4]` to `[i32]`.
  73 ///
  74 /// 3. Coercing a `Box<T>` to `Box<dyn Trait>` is an interesting special case. In
  75 ///    that case, we have the pointer we need coming in, so there are no
  76 ///    autoderefs, and no autoref. Instead we just do the `Unsize` transformation.
  77 ///    At some point, of course, `Box` should move out of the compiler, in which
  78 ///    case this is analogous to transforming a struct. E.g., `Box<[i32; 4]>` ->
  79 ///    `Box<[i32]>` is an `Adjust::Unsize` with the target `Box<[i32]>`.
  80 #[derive(Clone, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable, Lift)]
  81 pub struct Adjustment<'tcx> {
  82     pub kind: Adjust<'tcx>,
  83     pub target: Ty<'tcx>,
  84 }
  85
  86 impl<'tcx> Adjustment<'tcx> {
  87     pub fn is_region_borrow(&self) -> bool {
  88         matches!(self.kind, Adjust::Borrow(AutoBorrow::Ref(..)))
  89     }
  90 }
  91
  92 #[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable, Lift)]
  93 pub enum Adjust<'tcx> {
  94     /// Go from ! to any type.
  95     NeverToAny,
  96
  97     /// Dereference once, producing a place.
  98     Deref(Option<OverloadedDeref<'tcx>>),
  99
 100     /// Take the address and produce either a `&` or `*` pointer.
 101     Borrow(AutoBorrow<'tcx>),
 102
 103     Pointer(PointerCast),
 104 }
 105
 106 /// An overloaded autoderef step, representing a `Deref(Mut)::deref(_mut)`
 107 /// call, with the signature `&'a T -> &'a U` or `&'a mut T -> &'a mut U`.
 108 /// The target type is `U` in both cases, with the region and mutability
 109 /// being those shared by both the receiver and the returned reference.
 110 #[derive(Copy, Clone, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
 111 #[derive(TypeFoldable, TypeVisitable, Lift)]
 112 pub struct OverloadedDeref<'tcx> {
 113     pub region: ty::Region<'tcx>,
 114     pub mutbl: hir::Mutability,
 115     /// The `Span` associated with the field access or method call
 116     /// that triggered this overloaded deref.
 117     pub span: Span,
 118 }
 119
 120 impl<'tcx> OverloadedDeref<'tcx> {
 121     pub fn method_call(&self, tcx: TyCtxt<'tcx>, source: Ty<'tcx>) -> (DefId, SubstsRef<'tcx>) {
 122         let trait_def_id = match self.mutbl {
 123             hir::Mutability::Not => tcx.require_lang_item(LangItem::Deref, None),
 124             hir::Mutability::Mut => tcx.require_lang_item(LangItem::DerefMut, None),
 125         };
 126         let method_def_id = tcx
 127             .associated_items(trait_def_id)
 128             .in_definition_order()
 129             .find(|m| m.kind == ty::AssocKind::Fn)
 130             .unwrap()
 131             .def_id;
 132         (method_def_id, tcx.mk_substs_trait(source, &[]))
 133     }
 134 }
 135
 136 /// At least for initial deployment, we want to limit two-phase borrows to
 137 /// only a few specific cases. Right now, those are mostly "things that desugar"
 138 /// into method calls:
 139 /// - using `x.some_method()` syntax, where some_method takes `&mut self`,
 140 /// - using `Foo::some_method(&mut x, ...)` syntax,
 141 /// - binary assignment operators (`+=`, `-=`, `*=`, etc.).
 142 /// Anything else should be rejected until generalized two-phase borrow support
 143 /// is implemented. Right now, dataflow can't handle the general case where there
 144 /// is more than one use of a mutable borrow, and we don't want to accept too much
 145 /// new code via two-phase borrows, so we try to limit where we create two-phase
 146 /// capable mutable borrows.
 147 /// See #49434 for tracking.
 148 #[derive(Copy, Clone, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
 149 pub enum AllowTwoPhase {
 150     Yes,
 151     No,
 152 }
 153
 154 #[derive(Copy, Clone, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
 155 pub enum AutoBorrowMutability {
 156     Mut { allow_two_phase_borrow: AllowTwoPhase },
 157     Not,
 158 }
 159
 160 impl From<AutoBorrowMutability> for hir::Mutability {
 161     fn from(m: AutoBorrowMutability) -> Self {
 162         match m {
 163             AutoBorrowMutability::Mut { .. } => hir::Mutability::Mut,
 164             AutoBorrowMutability::Not => hir::Mutability::Not,
 165         }
 166     }
 167 }
 168
 169 #[derive(Copy, Clone, PartialEq, Debug, TyEncodable, TyDecodable, HashStable)]
 170 #[derive(TypeFoldable, TypeVisitable, Lift)]
 171 pub enum AutoBorrow<'tcx> {
 172     /// Converts from T to &T.
 173     Ref(ty::Region<'tcx>, AutoBorrowMutability),
 174
 175     /// Converts from T to *T.
 176     RawPtr(hir::Mutability),
 177 }
 178
 179 /// Information for `CoerceUnsized` impls, storing information we
 180 /// have computed about the coercion.
 181 ///
 182 /// This struct can be obtained via the `coerce_impl_info` query.
 183 /// Demanding this struct also has the side-effect of reporting errors
 184 /// for inappropriate impls.
 185 #[derive(Clone, Copy, TyEncodable, TyDecodable, Debug, HashStable)]
 186 pub struct CoerceUnsizedInfo {
 187     /// If this is a "custom coerce" impl, then what kind of custom
 188     /// coercion is it? This applies to impls of `CoerceUnsized` for
 189     /// structs, primarily, where we store a bit of info about which
 190     /// fields need to be coerced.
 191     pub custom_kind: Option<CustomCoerceUnsized>,
 192 }
 193
 194 #[derive(Clone, Copy, TyEncodable, TyDecodable, Debug, HashStable)]
 195 pub enum CustomCoerceUnsized {
 196     /// Records the index of the field being coerced.
 197     Struct(usize),
 198 }