rustc_hir_typeck/
expectation.rs

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use rustc_middle::ty::{self, Ty};
use rustc_span::Span;

use super::Expectation::*;
use super::FnCtxt;

/// When type-checking an expression, we propagate downward
/// whatever type hint we are able in the form of an `Expectation`.
#[derive(Copy, Clone, Debug)]
pub(crate) enum Expectation<'tcx> {
    /// We know nothing about what type this expression should have.
    NoExpectation,

    /// This expression should have the type given (or some subtype).
    ExpectHasType(Ty<'tcx>),

    /// This expression will be cast to the `Ty`.
    ExpectCastableToType(Ty<'tcx>),

    /// This rvalue expression will be wrapped in `&` or `Box` and coerced
    /// to `&Ty` or `Box<Ty>`, respectively. `Ty` is `[A]` or `Trait`.
    ExpectRvalueLikeUnsized(Ty<'tcx>),
}

impl<'a, 'tcx> Expectation<'tcx> {
    // Disregard "castable to" expectations because they
    // can lead us astray. Consider for example `if cond
    // {22} else {c} as u8` -- if we propagate the
    // "castable to u8" constraint to 22, it will pick the
    // type 22u8, which is overly constrained (c might not
    // be a u8). In effect, the problem is that the
    // "castable to" expectation is not the tightest thing
    // we can say, so we want to drop it in this case.
    // The tightest thing we can say is "must unify with
    // else branch". Note that in the case of a "has type"
    // constraint, this limitation does not hold.

    // If the expected type is just a type variable, then don't use
    // an expected type. Otherwise, we might write parts of the type
    // when checking the 'then' block which are incompatible with the
    // 'else' branch.
    pub(super) fn adjust_for_branches(&self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx> {
        match *self {
            ExpectHasType(ety) => {
                let ety = fcx.shallow_resolve(ety);
                if !ety.is_ty_var() { ExpectHasType(ety) } else { NoExpectation }
            }
            ExpectRvalueLikeUnsized(ety) => ExpectRvalueLikeUnsized(ety),
            _ => NoExpectation,
        }
    }

    /// Provides an expectation for an rvalue expression given an *optional*
    /// hint, which is not required for type safety (the resulting type might
    /// be checked higher up, as is the case with `&expr` and `box expr`), but
    /// is useful in determining the concrete type.
    ///
    /// The primary use case is where the expected type is a wide pointer,
    /// like `&[isize]`. For example, consider the following statement:
    ///
    ///    let x: &[isize] = &[1, 2, 3];
    ///
    /// In this case, the expected type for the `&[1, 2, 3]` expression is
    /// `&[isize]`. If however we were to say that `[1, 2, 3]` has the
    /// expectation `ExpectHasType([isize])`, that would be too strong --
    /// `[1, 2, 3]` does not have the type `[isize]` but rather `[isize; 3]`.
    /// It is only the `&[1, 2, 3]` expression as a whole that can be coerced
    /// to the type `&[isize]`. Therefore, we propagate this more limited hint,
    /// which still is useful, because it informs integer literals and the like.
    /// See the test case `test/ui/coerce-expect-unsized.rs` and #20169
    /// for examples of where this comes up,.
    pub(super) fn rvalue_hint(fcx: &FnCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Expectation<'tcx> {
        // FIXME: This is not right, even in the old solver...
        match fcx.tcx.struct_tail_raw(ty, |ty| ty, || {}).kind() {
            ty::Slice(_) | ty::Str | ty::Dynamic(..) => ExpectRvalueLikeUnsized(ty),
            _ => ExpectHasType(ty),
        }
    }

    /// Resolves `expected` by a single level if it is a variable. If
    /// there is no expected type or resolution is not possible (e.g.,
    /// no constraints yet present), just returns `self`.
    fn resolve(self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx> {
        match self {
            NoExpectation => NoExpectation,
            ExpectCastableToType(t) => ExpectCastableToType(fcx.resolve_vars_if_possible(t)),
            ExpectHasType(t) => ExpectHasType(fcx.resolve_vars_if_possible(t)),
            ExpectRvalueLikeUnsized(t) => ExpectRvalueLikeUnsized(fcx.resolve_vars_if_possible(t)),
        }
    }

    pub(super) fn to_option(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>> {
        match self.resolve(fcx) {
            NoExpectation => None,
            ExpectCastableToType(ty) | ExpectHasType(ty) | ExpectRvalueLikeUnsized(ty) => Some(ty),
        }
    }

    /// It sometimes happens that we want to turn an expectation into
    /// a **hard constraint** (i.e., something that must be satisfied
    /// for the program to type-check). `only_has_type` will return
    /// such a constraint, if it exists.
    pub(super) fn only_has_type(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>> {
        match self {
            ExpectHasType(ty) => Some(fcx.resolve_vars_if_possible(ty)),
            NoExpectation | ExpectCastableToType(_) | ExpectRvalueLikeUnsized(_) => None,
        }
    }

    /// Like `only_has_type`, but instead of returning `None` if no
    /// hard constraint exists, creates a fresh type variable.
    pub(super) fn coercion_target_type(self, fcx: &FnCtxt<'a, 'tcx>, span: Span) -> Ty<'tcx> {
        self.only_has_type(fcx).unwrap_or_else(|| fcx.next_ty_var(span))
    }
}