1 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
2 use rustc_middle::ty::{self, Ty};
3 use rustc_span::{self, Span};
5 use super::Expectation::*;
8 /// When type-checking an expression, we propagate downward
9 /// whatever type hint we are able in the form of an `Expectation`.
10 #[derive(Copy, Clone, Debug)]
11 pub enum Expectation<'tcx> {
12 /// We know nothing about what type this expression should have.
15 /// This expression should have the type given (or some subtype).
16 ExpectHasType(Ty<'tcx>),
18 /// This expression will be cast to the `Ty`.
19 ExpectCastableToType(Ty<'tcx>),
21 /// This rvalue expression will be wrapped in `&` or `Box` and coerced
22 /// to `&Ty` or `Box<Ty>`, respectively. `Ty` is `[A]` or `Trait`.
23 ExpectRvalueLikeUnsized(Ty<'tcx>),
26 impl<'a, 'tcx> Expectation<'tcx> {
27 // Disregard "castable to" expectations because they
28 // can lead us astray. Consider for example `if cond
29 // {22} else {c} as u8` -- if we propagate the
30 // "castable to u8" constraint to 22, it will pick the
31 // type 22u8, which is overly constrained (c might not
32 // be a u8). In effect, the problem is that the
33 // "castable to" expectation is not the tightest thing
34 // we can say, so we want to drop it in this case.
35 // The tightest thing we can say is "must unify with
36 // else branch". Note that in the case of a "has type"
37 // constraint, this limitation does not hold.
39 // If the expected type is just a type variable, then don't use
40 // an expected type. Otherwise, we might write parts of the type
41 // when checking the 'then' block which are incompatible with the
43 pub(super) fn adjust_for_branches(&self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx> {
45 ExpectHasType(ety) => {
46 let ety = fcx.shallow_resolve(ety);
47 if !ety.is_ty_var() { ExpectHasType(ety) } else { NoExpectation }
49 ExpectRvalueLikeUnsized(ety) => ExpectRvalueLikeUnsized(ety),
54 /// Provides an expectation for an rvalue expression given an *optional*
55 /// hint, which is not required for type safety (the resulting type might
56 /// be checked higher up, as is the case with `&expr` and `box expr`), but
57 /// is useful in determining the concrete type.
59 /// The primary use case is where the expected type is a fat pointer,
60 /// like `&[isize]`. For example, consider the following statement:
62 /// let x: &[isize] = &[1, 2, 3];
64 /// In this case, the expected type for the `&[1, 2, 3]` expression is
65 /// `&[isize]`. If however we were to say that `[1, 2, 3]` has the
66 /// expectation `ExpectHasType([isize])`, that would be too strong --
67 /// `[1, 2, 3]` does not have the type `[isize]` but rather `[isize; 3]`.
68 /// It is only the `&[1, 2, 3]` expression as a whole that can be coerced
69 /// to the type `&[isize]`. Therefore, we propagate this more limited hint,
70 /// which still is useful, because it informs integer literals and the like.
71 /// See the test case `test/ui/coerce-expect-unsized.rs` and #20169
72 /// for examples of where this comes up,.
73 pub(super) fn rvalue_hint(fcx: &FnCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Expectation<'tcx> {
74 match fcx.tcx.struct_tail_without_normalization(ty).kind() {
75 ty::Slice(_) | ty::Str | ty::Dynamic(..) => ExpectRvalueLikeUnsized(ty),
76 _ => ExpectHasType(ty),
80 // Resolves `expected` by a single level if it is a variable. If
81 // there is no expected type or resolution is not possible (e.g.,
82 // no constraints yet present), just returns `None`.
83 fn resolve(self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx> {
85 NoExpectation => NoExpectation,
86 ExpectCastableToType(t) => ExpectCastableToType(fcx.resolve_vars_if_possible(t)),
87 ExpectHasType(t) => ExpectHasType(fcx.resolve_vars_if_possible(t)),
88 ExpectRvalueLikeUnsized(t) => ExpectRvalueLikeUnsized(fcx.resolve_vars_if_possible(t)),
92 pub(super) fn to_option(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>> {
93 match self.resolve(fcx) {
94 NoExpectation => None,
95 ExpectCastableToType(ty) | ExpectHasType(ty) | ExpectRvalueLikeUnsized(ty) => Some(ty),
99 /// It sometimes happens that we want to turn an expectation into
100 /// a **hard constraint** (i.e., something that must be satisfied
101 /// for the program to type-check). `only_has_type` will return
102 /// such a constraint, if it exists.
103 pub(super) fn only_has_type(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>> {
104 match self.resolve(fcx) {
105 ExpectHasType(ty) => Some(ty),
106 NoExpectation | ExpectCastableToType(_) | ExpectRvalueLikeUnsized(_) => None,
110 /// Like `only_has_type`, but instead of returning `None` if no
111 /// hard constraint exists, creates a fresh type variable.
112 pub(super) fn coercion_target_type(self, fcx: &FnCtxt<'a, 'tcx>, span: Span) -> Ty<'tcx> {
113 self.only_has_type(fcx).unwrap_or_else(|| {
114 fcx.next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::MiscVariable, span })