1 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
2 use rustc_middle::ty::{self, Ty};
3 use rustc_span::DUMMY_SP;
4 use rustc_span::{self, Span};
6 use super::Expectation::*;
9 /// When type-checking an expression, we propagate downward
10 /// whatever type hint we are able in the form of an `Expectation`.
11 #[derive(Copy, Clone, Debug)]
12 pub enum Expectation<'tcx> {
13 /// We know nothing about what type this expression should have.
16 /// This expression should have the type given (or some subtype).
17 ExpectHasType(Ty<'tcx>),
19 /// This expression will be cast to the `Ty`.
20 ExpectCastableToType(Ty<'tcx>),
22 /// This rvalue expression will be wrapped in `&` or `Box` and coerced
23 /// to `&Ty` or `Box<Ty>`, respectively. `Ty` is `[A]` or `Trait`.
24 ExpectRvalueLikeUnsized(Ty<'tcx>),
29 impl<'a, 'tcx> Expectation<'tcx> {
30 // Disregard "castable to" expectations because they
31 // can lead us astray. Consider for example `if cond
32 // {22} else {c} as u8` -- if we propagate the
33 // "castable to u8" constraint to 22, it will pick the
34 // type 22u8, which is overly constrained (c might not
35 // be a u8). In effect, the problem is that the
36 // "castable to" expectation is not the tightest thing
37 // we can say, so we want to drop it in this case.
38 // The tightest thing we can say is "must unify with
39 // else branch". Note that in the case of a "has type"
40 // constraint, this limitation does not hold.
42 // If the expected type is just a type variable, then don't use
43 // an expected type. Otherwise, we might write parts of the type
44 // when checking the 'then' block which are incompatible with the
46 pub(super) fn adjust_for_branches(&self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx> {
47 match self.strip_opaque(fcx) {
48 ExpectHasType(ety) => {
49 let ety = fcx.shallow_resolve(ety);
50 if !ety.is_ty_var() { ExpectHasType(ety) } else { NoExpectation }
52 ExpectRvalueLikeUnsized(ety) => ExpectRvalueLikeUnsized(ety),
57 /// Provides an expectation for an rvalue expression given an *optional*
58 /// hint, which is not required for type safety (the resulting type might
59 /// be checked higher up, as is the case with `&expr` and `box expr`), but
60 /// is useful in determining the concrete type.
62 /// The primary use case is where the expected type is a fat pointer,
63 /// like `&[isize]`. For example, consider the following statement:
65 /// let x: &[isize] = &[1, 2, 3];
67 /// In this case, the expected type for the `&[1, 2, 3]` expression is
68 /// `&[isize]`. If however we were to say that `[1, 2, 3]` has the
69 /// expectation `ExpectHasType([isize])`, that would be too strong --
70 /// `[1, 2, 3]` does not have the type `[isize]` but rather `[isize; 3]`.
71 /// It is only the `&[1, 2, 3]` expression as a whole that can be coerced
72 /// to the type `&[isize]`. Therefore, we propagate this more limited hint,
73 /// which still is useful, because it informs integer literals and the like.
74 /// See the test case `test/ui/coerce-expect-unsized.rs` and #20169
75 /// for examples of where this comes up,.
76 pub(super) fn rvalue_hint(fcx: &FnCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Expectation<'tcx> {
77 match fcx.tcx.struct_tail_without_normalization(ty).kind() {
78 ty::Slice(_) | ty::Str | ty::Dynamic(..) => ExpectRvalueLikeUnsized(ty),
79 _ => ExpectHasType(ty),
83 // Resolves `expected` by a single level if it is a variable. If
84 // there is no expected type or resolution is not possible (e.g.,
85 // no constraints yet present), just returns `self`.
86 fn resolve(self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx> {
88 NoExpectation => NoExpectation,
89 ExpectCastableToType(t) => ExpectCastableToType(fcx.resolve_vars_if_possible(t)),
90 ExpectHasType(t) => ExpectHasType(fcx.resolve_vars_if_possible(t)),
91 ExpectRvalueLikeUnsized(t) => ExpectRvalueLikeUnsized(fcx.resolve_vars_if_possible(t)),
92 IsLast(sp) => IsLast(sp),
96 pub(super) fn to_option(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>> {
97 match self.resolve(fcx) {
98 NoExpectation | IsLast(_) => None,
99 ExpectCastableToType(ty) | ExpectHasType(ty) | ExpectRvalueLikeUnsized(ty) => Some(ty),
103 /// It sometimes happens that we want to turn an expectation into
104 /// a **hard constraint** (i.e., something that must be satisfied
105 /// for the program to type-check). `only_has_type` will return
106 /// such a constraint, if it exists.
107 pub(super) fn only_has_type(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>> {
108 match self.strip_opaque(fcx) {
109 ExpectHasType(ty) => Some(ty),
110 NoExpectation | ExpectCastableToType(_) | ExpectRvalueLikeUnsized(_) | IsLast(_) => {
116 /// We must not treat opaque types as expected types in their defining scope, as that
117 /// will break `fn foo() -> impl Trait { if cond { a } else { b } }` if `a` and `b` are
118 /// only "equal" if they coerce to a common target, like two different function items
119 /// coercing to a function pointer if they have the same signature.
120 fn strip_opaque(self, fcx: &FnCtxt<'a, 'tcx>) -> Self {
122 ExpectHasType(ty) => {
123 let ty = fcx.resolve_vars_if_possible(ty);
125 ty::Opaque(def_id, _)
126 if fcx.infcx.opaque_type_origin(def_id, DUMMY_SP).is_some() =>
137 /// Like `only_has_type`, but instead of returning `None` if no
138 /// hard constraint exists, creates a fresh type variable.
139 pub(super) fn coercion_target_type(self, fcx: &FnCtxt<'a, 'tcx>, span: Span) -> Ty<'tcx> {
140 self.only_has_type(fcx).unwrap_or_else(|| {
141 fcx.next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::MiscVariable, span })