Decouple Autoderef with FnCtxt and move Autoderef to librustc_trait_selection.

Author: ldm0

src/librustc_trait_selection/autoderef.rs

@@ -0,0 +1,229 @@
+use crate::traits::query::evaluate_obligation::InferCtxtExt;
+use crate::traits::{self, TraitEngine};
+use rustc_errors::struct_span_err;
+use rustc_hir as hir;
+use rustc_infer::infer::InferCtxt;
+use rustc_middle::ty::{self, TraitRef, Ty, TyCtxt, WithConstness};
+use rustc_middle::ty::{ToPredicate, TypeFoldable};
+use rustc_session::DiagnosticMessageId;
+use rustc_span::symbol::Ident;
+use rustc_span::Span;
+
+#[derive(Copy, Clone, Debug)]
+pub enum AutoderefKind {
+    Builtin,
+    Overloaded,
+}
+
+struct AutoderefSnapshot<'tcx> {
+    at_start: bool,
+    reached_recursion_limit: bool,
+    steps: Vec<(Ty<'tcx>, AutoderefKind)>,
+    cur_ty: Ty<'tcx>,
+    obligations: Vec<traits::PredicateObligation<'tcx>>,
+}
+
+pub struct Autoderef<'a, 'tcx> {
+    // Meta infos:
+    infcx: &'a InferCtxt<'a, 'tcx>,
+    span: Span,
+    body_id: hir::HirId,
+    param_env: ty::ParamEnv<'tcx>,
+
+    // Current state:
+    state: AutoderefSnapshot<'tcx>,
+
+    // Configurations:
+    include_raw_pointers: bool,
+    silence_errors: bool,
+}
+
+impl<'a, 'tcx> Iterator for Autoderef<'a, 'tcx> {
+    type Item = (Ty<'tcx>, usize);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let tcx = self.infcx.tcx;
+
+        debug!("autoderef: steps={:?}, cur_ty={:?}", self.state.steps, self.state.cur_ty);
+        if self.state.at_start {
+            self.state.at_start = false;
+            debug!("autoderef stage #0 is {:?}", self.state.cur_ty);
+            return Some((self.state.cur_ty, 0));
+        }
+
+        // If we have reached the recursion limit, error gracefully.
+        if !tcx.sess.recursion_limit().value_within_limit(self.state.steps.len()) {
+            if !self.silence_errors {
+                report_autoderef_recursion_limit_error(tcx, self.span, self.state.cur_ty);
+            }
+            self.state.reached_recursion_limit = true;
+            return None;
+        }
+
+        if self.state.cur_ty.is_ty_var() {
+            return None;
+        }
+
+        // Otherwise, deref if type is derefable:
+        let (kind, new_ty) =
+            if let Some(mt) = self.state.cur_ty.builtin_deref(self.include_raw_pointers) {
+                (AutoderefKind::Builtin, mt.ty)
+            } else if let Some(ty) = self.overloaded_deref_ty(self.state.cur_ty) {
+                (AutoderefKind::Overloaded, ty)
+            } else {
+                return None;
+            };
+
+        if new_ty.references_error() {
+            return None;
+        }
+
+        self.state.steps.push((self.state.cur_ty, kind));
+        debug!(
+            "autoderef stage #{:?} is {:?} from {:?}",
+            self.step_count(),
+            new_ty,
+            (self.state.cur_ty, kind)
+        );
+        self.state.cur_ty = new_ty;
+
+        Some((self.state.cur_ty, self.step_count()))
+    }
+}
+
+impl<'a, 'tcx> Autoderef<'a, 'tcx> {
+    pub fn new(
+        infcx: &'a InferCtxt<'a, 'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        body_id: hir::HirId,
+        span: Span,
+        base_ty: Ty<'tcx>,
+    ) -> Autoderef<'a, 'tcx> {
+        Autoderef {
+            infcx,
+            span,
+            body_id,
+            param_env,
+            state: AutoderefSnapshot {
+                steps: vec![],
+                cur_ty: infcx.resolve_vars_if_possible(&base_ty),
+                obligations: vec![],
+                at_start: true,
+                reached_recursion_limit: false,
+            },
+            include_raw_pointers: false,
+            silence_errors: false,
+        }
+    }
+
+    fn overloaded_deref_ty(&mut self, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
+        debug!("overloaded_deref_ty({:?})", ty);
+
+        let tcx = self.infcx.tcx;
+
+        // <ty as Deref>
+        let trait_ref = TraitRef {
+            def_id: tcx.lang_items().deref_trait()?,
+            substs: tcx.mk_substs_trait(ty, &[]),
+        };
+
+        let cause = traits::ObligationCause::misc(self.span, self.body_id);
+
+        let obligation = traits::Obligation::new(
+            cause.clone(),
+            self.param_env,
+            trait_ref.without_const().to_predicate(tcx),
+        );
+        if !self.infcx.predicate_may_hold(&obligation) {
+            debug!("overloaded_deref_ty: cannot match obligation");
+            return None;
+        }
+
+        let mut fulfillcx = traits::FulfillmentContext::new_in_snapshot();
+        let normalized_ty = fulfillcx.normalize_projection_type(
+            &self.infcx,
+            self.param_env,
+            ty::ProjectionTy::from_ref_and_name(tcx, trait_ref, Ident::from_str("Target")),
+            cause,
+        );
+        if let Err(e) = fulfillcx.select_where_possible(&self.infcx) {
+            // This shouldn't happen, except for evaluate/fulfill mismatches,
+            // but that's not a reason for an ICE (`predicate_may_hold` is conservative
+            // by design).
+            debug!("overloaded_deref_ty: encountered errors {:?} while fulfilling", e);
+            return None;
+        }
+        let obligations = fulfillcx.pending_obligations();
+        debug!("overloaded_deref_ty({:?}) = ({:?}, {:?})", ty, normalized_ty, obligations);
+        self.state.obligations.extend(obligations);
+
+        Some(self.infcx.resolve_vars_if_possible(&normalized_ty))
+    }
+
+    /// Returns the final type we ended up with, which may be an inference
+    /// variable (we will resolve it first, if we want).
+    pub fn final_ty(&self, resolve: bool) -> Ty<'tcx> {
+        if resolve {
+            self.infcx.resolve_vars_if_possible(&self.state.cur_ty)
+        } else {
+            self.state.cur_ty
+        }
+    }
+
+    pub fn step_count(&self) -> usize {
+        self.state.steps.len()
+    }
+
+    pub fn into_obligations(self) -> Vec<traits::PredicateObligation<'tcx>> {
+        self.state.obligations
+    }
+
+    pub fn steps(&self) -> &[(Ty<'tcx>, AutoderefKind)] {
+        &self.state.steps
+    }
+
+    pub fn span(&self) -> Span {
+        self.span.clone()
+    }
+
+    pub fn reached_recursion_limit(&self) -> bool {
+        self.state.reached_recursion_limit
+    }
+
+    /// also dereference through raw pointer types
+    /// e.g., assuming ptr_to_Foo is the type `*const Foo`
+    /// fcx.autoderef(span, ptr_to_Foo)  => [*const Foo]
+    /// fcx.autoderef(span, ptr_to_Foo).include_raw_ptrs() => [*const Foo, Foo]
+    pub fn include_raw_pointers(mut self) -> Self {
+        self.include_raw_pointers = true;
+        self
+    }
+
+    pub fn silence_errors(mut self) -> Self {
+        self.silence_errors = true;
+        self
+    }
+}
+
+pub fn report_autoderef_recursion_limit_error<'tcx>(tcx: TyCtxt<'tcx>, span: Span, ty: Ty<'tcx>) {
+    // We've reached the recursion limit, error gracefully.
+    let suggested_limit = tcx.sess.recursion_limit() * 2;
+    let msg = format!("reached the recursion limit while auto-dereferencing `{:?}`", ty);
+    let error_id = (DiagnosticMessageId::ErrorId(55), Some(span), msg);
+    let fresh = tcx.sess.one_time_diagnostics.borrow_mut().insert(error_id);
+    if fresh {
+        struct_span_err!(
+            tcx.sess,
+            span,
+            E0055,
+            "reached the recursion limit while auto-dereferencing `{:?}`",
+            ty
+        )
+        .span_label(span, "deref recursion limit reached")
+        .help(&format!(
+            "consider adding a `#![recursion_limit=\"{}\"]` attribute to your crate (`{}`)",
+            suggested_limit, tcx.crate_name,
+        ))
+        .emit();
+    }
+}

src/librustc_trait_selection/lib.rs

@@ -28,6 +28,7 @@ extern crate log;
 #[macro_use]
 extern crate rustc_middle;
 
+pub mod autoderef;
 pub mod infer;
 pub mod opaque_types;
 pub mod traits;