Auto merge of #76906 - Nicholas-Baron:shorten_typeck_check, r=oli-obk

Split rustc_typeck::check into separate files

Contributing to #60302.

Author: bors

compiler/rustc_typeck/src/check/check.rs

@@ -0,0 +1,78 @@
+use rustc_span::source_map::DUMMY_SP;
+use rustc_span::{self, Span};
+use std::{cmp, ops};
+
+/// Tracks whether executing a node may exit normally (versus
+/// return/break/panic, which "diverge", leaving dead code in their
+/// wake). Tracked semi-automatically (through type variables marked
+/// as diverging), with some manual adjustments for control-flow
+/// primitives (approximating a CFG).
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub enum Diverges {
+    /// Potentially unknown, some cases converge,
+    /// others require a CFG to determine them.
+    Maybe,
+
+    /// Definitely known to diverge and therefore
+    /// not reach the next sibling or its parent.
+    Always {
+        /// The `Span` points to the expression
+        /// that caused us to diverge
+        /// (e.g. `return`, `break`, etc).
+        span: Span,
+        /// In some cases (e.g. a `match` expression
+        /// where all arms diverge), we may be
+        /// able to provide a more informative
+        /// message to the user.
+        /// If this is `None`, a default message
+        /// will be generated, which is suitable
+        /// for most cases.
+        custom_note: Option<&'static str>,
+    },
+
+    /// Same as `Always` but with a reachability
+    /// warning already emitted.
+    WarnedAlways,
+}
+
+// Convenience impls for combining `Diverges`.
+
+impl ops::BitAnd for Diverges {
+    type Output = Self;
+    fn bitand(self, other: Self) -> Self {
+        cmp::min(self, other)
+    }
+}
+
+impl ops::BitOr for Diverges {
+    type Output = Self;
+    fn bitor(self, other: Self) -> Self {
+        cmp::max(self, other)
+    }
+}
+
+impl ops::BitAndAssign for Diverges {
+    fn bitand_assign(&mut self, other: Self) {
+        *self = *self & other;
+    }
+}
+
+impl ops::BitOrAssign for Diverges {
+    fn bitor_assign(&mut self, other: Self) {
+        *self = *self | other;
+    }
+}
+
+impl Diverges {
+    /// Creates a `Diverges::Always` with the provided `span` and the default note message.
+    pub(super) fn always(span: Span) -> Diverges {
+        Diverges::Always { span, custom_note: None }
+    }
+
+    pub(super) fn is_always(self) -> bool {
+        // Enum comparison ignores the
+        // contents of fields, so we just
+        // fill them in with garbage here.
+        self >= Diverges::Always { span: DUMMY_SP, custom_note: None }
+    }
+}

compiler/rustc_typeck/src/check/diverges.rs

@@ -0,0 +1,117 @@
+use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
+use rustc_middle::ty::{self, Ty};
+use rustc_span::{self, 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 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 fat 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> {
+        match fcx.tcx.struct_tail_without_normalization(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 `None`.
+    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.resolve(fcx) {
+            ExpectHasType(ty) => Some(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(TypeVariableOrigin { kind: TypeVariableOriginKind::MiscVariable, span })
+        })
+    }
+}