Rollup merge of #138528 - dianne:implicit-deref-patterns, r=Nadrieril

deref patterns: implement implicit deref patterns

This implements implicit deref patterns (per https://hackmd.io/4qDDMcvyQ-GDB089IPcHGg#Implicit-deref-patterns) and adds tests and an unstable book chapter.

Best reviewed commit-by-commit. Overall there's a lot of additions, but a lot of that is tests, documentation, and simple(?) refactoring.

Tracking issue: #87121

r? ``@Nadrieril``

Author: matthiaskrgr

compiler/rustc_hir_analysis/src/autoderef.rs

@@ -2,8 +2,8 @@ use rustc_infer::infer::InferCtxt;
 use rustc_infer::traits::PredicateObligations;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
 use rustc_session::Limit;
-use rustc_span::Span;
 use rustc_span::def_id::{LOCAL_CRATE, LocalDefId};
+use rustc_span::{ErrorGuaranteed, Span};
 use rustc_trait_selection::traits::ObligationCtxt;
 use tracing::{debug, instrument};
 
@@ -259,7 +259,11 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
     }
 }
 
-pub fn report_autoderef_recursion_limit_error<'tcx>(tcx: TyCtxt<'tcx>, span: Span, ty: Ty<'tcx>) {
+pub fn report_autoderef_recursion_limit_error<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    span: Span,
+    ty: Ty<'tcx>,
+) -> ErrorGuaranteed {
     // We've reached the recursion limit, error gracefully.
     let suggested_limit = match tcx.recursion_limit() {
         Limit(0) => Limit(2),
@@ -270,5 +274,5 @@ pub fn report_autoderef_recursion_limit_error<'tcx>(tcx: TyCtxt<'tcx>, span: Spa
         ty,
         suggested_limit,
         crate_name: tcx.crate_name(LOCAL_CRATE),
-    });
+    })
 }

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -1000,6 +1000,8 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                     // determines whether to borrow *at the level of the deref pattern* rather than
                     // borrowing the bound place (since that inner place is inside the temporary that
                     // stores the result of calling `deref()`/`deref_mut()` so can't be captured).
+                    // HACK: this could be a fake pattern corresponding to a deref inserted by match
+                    // ergonomics, in which case `pat.hir_id` will be the id of the subpattern.
                     let mutable = self.cx.typeck_results().pat_has_ref_mut_binding(subpattern);
                     let mutability =
                         if mutable { hir::Mutability::Mut } else { hir::Mutability::Not };
@@ -1227,9 +1229,9 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
         // actually this is somewhat "disjoint" from the code below
         // that aims to account for `ref x`.
         if let Some(vec) = self.cx.typeck_results().pat_adjustments().get(pat.hir_id) {
-            if let Some(first_ty) = vec.first() {
-                debug!("pat_ty(pat={:?}) found adjusted ty `{:?}`", pat, first_ty);
-                return Ok(*first_ty);
+            if let Some(first_adjust) = vec.first() {
+                debug!("pat_ty(pat={:?}) found adjustment `{:?}`", pat, first_adjust);
+                return Ok(first_adjust.source);
             }
         } else if let PatKind::Ref(subpat, _) = pat.kind
             && self.cx.typeck_results().skipped_ref_pats().contains(pat.hir_id)
@@ -1680,12 +1682,31 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
         // Then we see that to get the same result, we must start with
         // `deref { deref { place_foo }}` instead of `place_foo` since the pattern is now `Some(x,)`
         // and not `&&Some(x,)`, even though its assigned type is that of `&&Some(x,)`.
-        for _ in
-            0..self.cx.typeck_results().pat_adjustments().get(pat.hir_id).map_or(0, |v| v.len())
-        {
+        let typeck_results = self.cx.typeck_results();
+        let adjustments: &[adjustment::PatAdjustment<'tcx>] =
+            typeck_results.pat_adjustments().get(pat.hir_id).map_or(&[], |v| &**v);
+        let mut adjusts = adjustments.iter().peekable();
+        while let Some(adjust) = adjusts.next() {
             debug!("applying adjustment to place_with_id={:?}", place_with_id);
-            place_with_id = self.cat_deref(pat.hir_id, place_with_id)?;
+            place_with_id = match adjust.kind {
+                adjustment::PatAdjust::BuiltinDeref => self.cat_deref(pat.hir_id, place_with_id)?,
+                adjustment::PatAdjust::OverloadedDeref => {
+                    // This adjustment corresponds to an overloaded deref; it borrows the scrutinee to
+                    // call `Deref::deref` or `DerefMut::deref_mut`. Invoke the callback before setting
+                    // `place_with_id` to the temporary storing the result of the deref.
+                    // HACK(dianne): giving the callback a fake deref pattern makes sure it behaves the
+                    // same as it would if this were an explicit deref pattern.
+                    op(&place_with_id, &hir::Pat { kind: PatKind::Deref(pat), ..*pat })?;
+                    let target_ty = match adjusts.peek() {
+                        Some(&&next_adjust) => next_adjust.source,
+                        // At the end of the deref chain, we get `pat`'s scrutinee.
+                        None => self.pat_ty_unadjusted(pat)?,
+                    };
+                    self.pat_deref_temp(pat.hir_id, pat, target_ty)?
+                }
+            };
         }
+        drop(typeck_results); // explicitly release borrow of typeck results, just in case.
         let place_with_id = place_with_id; // lose mutability
         debug!("applied adjustment derefs to get place_with_id={:?}", place_with_id);
 
@@ -1788,14 +1809,8 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
                 self.cat_pattern(subplace, subpat, op)?;
             }
             PatKind::Deref(subpat) => {
-                let mutable = self.cx.typeck_results().pat_has_ref_mut_binding(subpat);
-                let mutability = if mutable { hir::Mutability::Mut } else { hir::Mutability::Not };
-                let re_erased = self.cx.tcx().lifetimes.re_erased;
                 let ty = self.pat_ty_adjusted(subpat)?;
-                let ty = Ty::new_ref(self.cx.tcx(), re_erased, ty, mutability);
-                // A deref pattern generates a temporary.
-                let base = self.cat_rvalue(pat.hir_id, ty);
-                let place = self.cat_deref(pat.hir_id, base)?;
+                let place = self.pat_deref_temp(pat.hir_id, subpat, ty)?;
                 self.cat_pattern(place, subpat, op)?;
             }
 
@@ -1848,6 +1863,23 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
         Ok(())
     }
 
+    /// Represents the place of the temp that stores the scrutinee of a deref pattern's interior.
+    fn pat_deref_temp(
+        &self,
+        hir_id: HirId,
+        inner: &hir::Pat<'_>,
+        target_ty: Ty<'tcx>,
+    ) -> Result<PlaceWithHirId<'tcx>, Cx::Error> {
+        let mutable = self.cx.typeck_results().pat_has_ref_mut_binding(inner);
+        let mutability = if mutable { hir::Mutability::Mut } else { hir::Mutability::Not };
+        let re_erased = self.cx.tcx().lifetimes.re_erased;
+        let ty = Ty::new_ref(self.cx.tcx(), re_erased, target_ty, mutability);
+        // A deref pattern stores the result of `Deref::deref` or `DerefMut::deref_mut` ...
+        let base = self.cat_rvalue(hir_id, ty);
+        // ... and the inner pattern matches on the place behind that reference.
+        self.cat_deref(hir_id, base)
+    }
+
     fn is_multivariant_adt(&self, ty: Ty<'tcx>, span: Span) -> bool {
         if let ty::Adt(def, _) = self.cx.try_structurally_resolve_type(span, ty).kind() {
             // Note that if a non-exhaustive SingleVariant is defined in another crate, we need