Point out implicit deref coercions in borrow

compiler/rustc_mir/src/borrow_check/diagnostics/conflict_errors.rs

@@ -8,11 +8,10 @@ use rustc_index::vec::Idx;
 use rustc_middle::mir::{
     self, AggregateKind, BindingForm, BorrowKind, ClearCrossCrate, ConstraintCategory,
     FakeReadCause, Local, LocalDecl, LocalInfo, LocalKind, Location, Operand, Place, PlaceRef,
-    ProjectionElem, Rvalue, Statement, StatementKind, TerminatorKind, VarBindingForm,
+    ProjectionElem, Rvalue, Statement, StatementKind, Terminator, TerminatorKind, VarBindingForm,
 };
-use rustc_middle::ty::{self, suggest_constraining_type_param, Ty};
-use rustc_span::source_map::DesugaringKind;
-use rustc_span::Span;
+use rustc_middle::ty::{self, suggest_constraining_type_param, Instance, Ty};
+use rustc_span::{source_map::DesugaringKind, symbol::sym, Span};
 
 use crate::dataflow::drop_flag_effects;
 use crate::dataflow::indexes::{MoveOutIndex, MovePathIndex};
@@ -1543,9 +1542,43 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             None,
         );
 
+        self.explain_deref_coercion(loan, &mut err);
+
         err.buffer(&mut self.errors_buffer);
     }
 
+    fn explain_deref_coercion(&mut self, loan: &BorrowData<'tcx>, err: &mut DiagnosticBuilder<'_>) {
+        let tcx = self.infcx.tcx;
+        if let (
+            Some(Terminator { kind: TerminatorKind::Call { from_hir_call: false, .. }, .. }),
+            Some((method_did, method_substs)),
+        ) = (
+            &self.body[loan.reserve_location.block].terminator,
+            crate::util::find_self_call(
+                tcx,
+                self.body,
+                loan.assigned_place.local,
+                loan.reserve_location.block,
+            ),
+        ) {
+            if tcx.is_diagnostic_item(sym::deref_method, method_did) {
+                let deref_target =
+                    tcx.get_diagnostic_item(sym::deref_target).and_then(|deref_target| {
+                        Instance::resolve(tcx, self.param_env, deref_target, method_substs)
+                            .transpose()
+                    });
+                if let Some(Ok(instance)) = deref_target {
+                    let deref_target_ty = instance.ty(tcx, self.param_env);
+                    err.note(&format!(
+                        "borrow occurs due to deref coercion to `{}`",
+                        deref_target_ty
+                    ));
+                    err.span_note(tcx.def_span(instance.def_id()), "deref defined here");
+                }
+            }
+        }
+    }
+
     /// Reports an illegal reassignment; for example, an assignment to
     /// (part of) a non-`mut` local that occurs potentially after that
     /// local has already been initialized. `place` is the path being

src/test/ui/borrowck/issue-81365-1.rs

@@ -0,0 +1,26 @@
+use std::ops::Deref;
+
+struct DerefTarget {
+    target_field: bool,
+}
+struct Container {
+    target: DerefTarget,
+    container_field: bool,
+}
+
+impl Deref for Container {
+    type Target = DerefTarget;
+    fn deref(&self) -> &Self::Target {
+        &self.target
+    }
+}
+
+impl Container {
+    fn bad_borrow(&mut self) {
+        let first = &self.target_field;
+        self.container_field = true; //~ ERROR E0506
+        first;
+    }
+}
+
+fn main() {}

src/test/ui/borrowck/issue-81365-1.stderr

@@ -0,0 +1,20 @@
+error[E0506]: cannot assign to `self.container_field` because it is borrowed
+  --> $DIR/issue-81365-1.rs:21:9
+   |
+LL |         let first = &self.target_field;
+   |                      ---- borrow of `self.container_field` occurs here
+LL |         self.container_field = true;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ assignment to borrowed `self.container_field` occurs here
+LL |         first;
+   |         ----- borrow later used here
+   |
+   = note: borrow occurs due to deref coercion to `DerefTarget`
+note: deref defined here
+  --> $DIR/issue-81365-1.rs:12:5
+   |
+LL |     type Target = DerefTarget;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0506`.

src/test/ui/borrowck/issue-81365-10.rs

@@ -0,0 +1,26 @@
+use std::ops::Deref;
+
+struct DerefTarget {
+    target_field: bool,
+}
+struct Container {
+    target: DerefTarget,
+    container_field: bool,
+}
+
+impl Deref for Container {
+    type Target = DerefTarget;
+    fn deref(&self) -> &Self::Target {
+        &self.target
+    }
+}
+
+impl Container {
+    fn bad_borrow(&mut self) {
+        let first = &self.deref().target_field;
+        self.container_field = true; //~ ERROR E0506
+        first;
+    }
+}
+
+fn main() {}

src/test/ui/borrowck/issue-81365-10.stderr

@@ -0,0 +1,13 @@
+error[E0506]: cannot assign to `self.container_field` because it is borrowed
+  --> $DIR/issue-81365-10.rs:21:9
+   |
+LL |         let first = &self.deref().target_field;
+   |                      ---- borrow of `self.container_field` occurs here
+LL |         self.container_field = true;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ assignment to borrowed `self.container_field` occurs here
+LL |         first;
+   |         ----- borrow later used here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0506`.

src/test/ui/borrowck/issue-81365-11.rs

@@ -0,0 +1,32 @@
+use std::ops::{Deref, DerefMut};
+
+struct DerefTarget {
+    target_field: bool,
+}
+struct Container {
+    target: DerefTarget,
+    container_field: bool,
+}
+
+impl Deref for Container {
+    type Target = DerefTarget;
+    fn deref(&self) -> &Self::Target {
+        &self.target
+    }
+}
+
+impl DerefMut for Container {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.target
+    }
+}
+
+impl Container {
+    fn bad_borrow(&mut self) {
+        let first = &mut self.target_field;
+        self.container_field = true; //~ ERROR E0506
+        first;
+    }
+}
+
+fn main() {}

src/test/ui/borrowck/issue-81365-11.stderr

@@ -0,0 +1,13 @@
+error[E0506]: cannot assign to `self.container_field` because it is borrowed
+  --> $DIR/issue-81365-11.rs:27:9
+   |
+LL |         let first = &mut self.target_field;
+   |                          ---- borrow of `self.container_field` occurs here
+LL |         self.container_field = true;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ assignment to borrowed `self.container_field` occurs here
+LL |         first;
+   |         ----- borrow later used here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0506`.

src/test/ui/borrowck/issue-81365-2.rs

@@ -0,0 +1,30 @@
+use std::ops::Deref;
+
+struct DerefTarget {
+    target_field: bool,
+}
+struct Container {
+    target: DerefTarget,
+    container_field: bool,
+}
+
+impl Deref for Container {
+    type Target = DerefTarget;
+    fn deref(&self) -> &Self::Target {
+        &self.target
+    }
+}
+
+struct Outer {
+    container: Container,
+}
+
+impl Outer {
+    fn bad_borrow(&mut self) {
+        let first = &self.container.target_field;
+        self.container.container_field = true; //~ ERROR E0506
+        first;
+    }
+}
+
+fn main() {}

src/test/ui/borrowck/issue-81365-2.stderr

@@ -0,0 +1,20 @@
+error[E0506]: cannot assign to `self.container.container_field` because it is borrowed
+  --> $DIR/issue-81365-2.rs:25:9
+   |
+LL |         let first = &self.container.target_field;
+   |                      -------------- borrow of `self.container.container_field` occurs here
+LL |         self.container.container_field = true;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ assignment to borrowed `self.container.container_field` occurs here
+LL |         first;
+   |         ----- borrow later used here
+   |
+   = note: borrow occurs due to deref coercion to `DerefTarget`
+note: deref defined here
+  --> $DIR/issue-81365-2.rs:12:5
+   |
+LL |     type Target = DerefTarget;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0506`.

src/test/ui/borrowck/issue-81365-3.rs

@@ -0,0 +1,37 @@
+use std::ops::Deref;
+
+struct DerefTarget {
+    target_field: bool,
+}
+struct Container {
+    target: DerefTarget,
+    container_field: bool,
+}
+
+impl Deref for Container {
+    type Target = DerefTarget;
+    fn deref(&self) -> &Self::Target {
+        &self.target
+    }
+}
+
+struct Outer {
+    container: Container,
+}
+
+impl Deref for Outer {
+    type Target = Container;
+    fn deref(&self) -> &Self::Target {
+        &self.container
+    }
+}
+
+impl Outer {
+    fn bad_borrow(&mut self) {
+        let first = &self.target_field;
+        self.container.container_field = true; //~ ERROR E0506
+        first;
+    }
+}
+
+fn main() {}

src/test/ui/borrowck/issue-81365-3.stderr

@@ -0,0 +1,20 @@
+error[E0506]: cannot assign to `self.container.container_field` because it is borrowed
+  --> $DIR/issue-81365-3.rs:32:9
+   |
+LL |         let first = &self.target_field;
+   |                      ---- borrow of `self.container.container_field` occurs here
+LL |         self.container.container_field = true;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ assignment to borrowed `self.container.container_field` occurs here
+LL |         first;
+   |         ----- borrow later used here
+   |
+   = note: borrow occurs due to deref coercion to `Container`
+note: deref defined here
+  --> $DIR/issue-81365-3.rs:23:5
+   |
+LL |     type Target = Container;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0506`.

src/test/ui/borrowck/issue-81365-4.rs

@@ -0,0 +1,38 @@
+use std::ops::Deref;
+
+struct DerefTarget {
+    target_field: bool,
+}
+struct Container {
+    target: DerefTarget,
+    container_field: bool,
+}
+
+impl Deref for Container {
+    type Target = DerefTarget;
+    fn deref(&self) -> &Self::Target {
+        &self.target
+    }
+}
+
+struct Outer {
+    container: Container,
+    outer_field: bool,
+}
+
+impl Deref for Outer {
+    type Target = Container;
+    fn deref(&self) -> &Self::Target {
+        &self.container
+    }
+}
+
+impl Outer {
+    fn bad_borrow(&mut self) {
+        let first = &self.target_field;
+        self.outer_field = true; //~ ERROR E0506
+        first;
+    }
+}
+
+fn main() {}

src/test/ui/borrowck/issue-81365-4.stderr

@@ -0,0 +1,20 @@
+error[E0506]: cannot assign to `self.outer_field` because it is borrowed
+  --> $DIR/issue-81365-4.rs:33:9
+   |
+LL |         let first = &self.target_field;
+   |                      ---- borrow of `self.outer_field` occurs here
+LL |         self.outer_field = true;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ assignment to borrowed `self.outer_field` occurs here
+LL |         first;
+   |         ----- borrow later used here
+   |
+   = note: borrow occurs due to deref coercion to `Container`
+note: deref defined here
+  --> $DIR/issue-81365-4.rs:24:5
+   |
+LL |     type Target = Container;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0506`.

src/test/ui/borrowck/issue-81365-5.rs

@@ -0,0 +1,33 @@
+use std::ops::Deref;
+
+struct DerefTarget {
+    target_field: bool,
+}
+
+impl DerefTarget {
+    fn get(&self) -> &bool {
+        &self.target_field
+    }
+}
+
+struct Container {
+    target: DerefTarget,
+    container_field: bool,
+}
+
+impl Deref for Container {
+    type Target = DerefTarget;
+    fn deref(&self) -> &Self::Target {
+        &self.target
+    }
+}
+
+impl Container {
+    fn bad_borrow(&mut self) {
+        let first = self.get();
+        self.container_field = true; //~ ERROR E0506
+        first;
+    }
+}
+
+fn main() {}