Fix mutability diagnostics

compiler/rustc_middle/src/ty/mod.rs

@@ -7,6 +7,7 @@ pub use self::Variance::*;
 
 use crate::hir::exports::ExportMap;
 use crate::hir::place::Place as HirPlace;
+use crate::hir::place::PlaceBase as HirPlaceBase;
 use crate::ich::StableHashingContext;
 use crate::middle::cstore::CrateStoreDyn;
 use crate::middle::resolve_lifetime::ObjectLifetimeDefault;
@@ -813,6 +814,15 @@ pub struct CapturedPlace<'tcx> {
     pub mutability: hir::Mutability,
 }
 
+impl CapturedPlace<'tcx> {
+    pub fn get_root_variable(&self) -> hir::HirId {
+        match self.place.base {
+            HirPlaceBase::Upvar(upvar_id) => upvar_id.var_path.hir_id,
+            base => bug!("Expected upvar, found={:?}", base),
+        }
+    }
+}
+
 /// Part of `MinCaptureInformationMap`; describes the capture kind (&, &mut, move)
 /// for a particular capture as well as identifying the part of the source code
 /// that triggered this capture to occur.

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

@@ -345,7 +345,9 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 };
 
                 let upvar = &self.upvars[upvar_field.unwrap().index()];
-                let upvar_hir_id = upvar.var_hir_id;
+                // FIXME(project-rfc-2229#8): Improve borrow-check diagnostics in case of precise
+                //                            capture.
+                let upvar_hir_id = upvar.place.get_root_variable();
                 let upvar_name = upvar.name;
                 let upvar_span = self.infcx.tcx.hir().span(upvar_hir_id);
 

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

@@ -61,12 +61,26 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     Place::ty_from(local, proj_base, self.body, self.infcx.tcx).ty
                 ));
 
-                item_msg = format!("`{}`", access_place_desc.unwrap());
-                if self.is_upvar_field_projection(access_place.as_ref()).is_some() {
-                    reason = ", as it is not declared as mutable".to_string();
+                // If the place is immutable then:
+                //
+                // - Either we deref a immutable ref to get to our final place.
+                //    - We don't capture derefs of raw ptrs
+                //    - If we derefed a mutable ref without deref-ing a immutable ref
+                //      to access this place then we will have mutability.
+                //    - So if we dereferenced something such that it resulted in the final
+                //      place being immutable, then we must have deref-ed a immut ref.
+                // - Or the final place is immut because the root variable of the capture
+                //   isn't marked mut and we should suggest that to the user.
+                if self.upvars[upvar_index.index()].place.place.deref_tys().next().is_none() {
+                    item_msg = format!("`{}`", access_place_desc.unwrap());
+                    if self.is_upvar_field_projection(access_place.as_ref()).is_some() {
+                        reason = ", as it is not declared as mutable".to_string();
+                    } else {
+                        let name = self.upvars[upvar_index.index()].name;
+                        reason = format!(", as `{}` is not declared as mutable", name);
+                    }
                 } else {
-                    let name = self.upvars[upvar_index.index()].name;
-                    reason = format!(", as `{}` is not declared as mutable", name);
+                    return;
                 }
             }
 
@@ -252,23 +266,38 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     Place::ty_from(local, proj_base, self.body, self.infcx.tcx).ty
                 ));
 
-                err.span_label(span, format!("cannot {ACT}", ACT = act));
-
-                let upvar_hir_id = self.upvars[upvar_index.index()].var_hir_id;
-                if let Some(Node::Binding(pat)) = self.infcx.tcx.hir().find(upvar_hir_id) {
-                    if let hir::PatKind::Binding(
-                        hir::BindingAnnotation::Unannotated,
-                        _,
-                        upvar_ident,
-                        _,
-                    ) = pat.kind
-                    {
-                        err.span_suggestion(
-                            upvar_ident.span,
-                            "consider changing this to be mutable",
-                            format!("mut {}", upvar_ident.name),
-                            Applicability::MachineApplicable,
-                        );
+                let captured_place = &self.upvars[upvar_index.index()].place;
+
+                // If the place is immutable then:
+                //
+                // - Either we deref a immutable ref to get to our final place.
+                //    - We don't capture derefs of raw ptrs
+                //    - If we derefed a mutable ref without deref-ing a immutable ref
+                //      to access this place then we will have mutability.
+                //    - So if we dereferenced something such that it resulted in the final
+                //      place being immutable, then we must have deref-ed a immut ref.
+                // - Or the final place is immut because the root variable of the capture
+                //   isn't marked mut and we should suggest that to the user.
+                if captured_place.place.deref_tys().next().is_none() {
+                    err.span_label(span, format!("cannot {ACT}", ACT = act));
+
+                    let upvar_hir_id = captured_place.get_root_variable();
+
+                    if let Some(Node::Binding(pat)) = self.infcx.tcx.hir().find(upvar_hir_id) {
+                        if let hir::PatKind::Binding(
+                            hir::BindingAnnotation::Unannotated,
+                            _,
+                            upvar_ident,
+                            _,
+                        ) = pat.kind
+                        {
+                            err.span_suggestion(
+                                upvar_ident.span,
+                                "consider changing this to be mutable",
+                                format!("mut {}", upvar_ident.name),
+                                Applicability::MachineApplicable,
+                            );
+                        }
                     }
                 }
             }

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

@@ -12,7 +12,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         tcx: TyCtxt<'tcx>,
         body: &Body<'tcx>,
         local_names: &IndexVec<Local, Option<Symbol>>,
-        upvars: &[Upvar],
+        upvars: &[Upvar<'tcx>],
         fr: RegionVid,
     ) -> Option<(Option<Symbol>, Span)> {
         debug!("get_var_name_and_span_for_region(fr={:?})", fr);
@@ -21,6 +21,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         debug!("get_var_name_and_span_for_region: attempting upvar");
         self.get_upvar_index_for_region(tcx, fr)
             .map(|index| {
+                // FIXME(project-rfc-2229#8): Use place span for diagnostics
                 let (name, span) = self.get_upvar_name_and_span_for_region(tcx, upvars, index);
                 (Some(name), span)
             })
@@ -59,10 +60,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     crate fn get_upvar_name_and_span_for_region(
         &self,
         tcx: TyCtxt<'tcx>,
-        upvars: &[Upvar],
+        upvars: &[Upvar<'tcx>],
         upvar_index: usize,
     ) -> (Symbol, Span) {
-        let upvar_hir_id = upvars[upvar_index].var_hir_id;
+        let upvar_hir_id = upvars[upvar_index].place.get_root_variable();
         debug!("get_upvar_name_and_span_for_region: upvar_hir_id={:?}", upvar_hir_id);
 
         let upvar_name = tcx.hir().name(upvar_hir_id);

compiler/rustc_mir/src/borrow_check/mod.rs

@@ -5,11 +5,10 @@ use rustc_data_structures::graph::dominators::Dominators;
 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorReported};
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
-use rustc_hir::{HirId, Node};
+use rustc_hir::Node;
 use rustc_index::bit_set::BitSet;
 use rustc_index::vec::IndexVec;
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
-use rustc_middle::hir::place::PlaceBase as HirPlaceBase;
 use rustc_middle::mir::{
     traversal, Body, ClearCrossCrate, Local, Location, Mutability, Operand, Place, PlaceElem,
     PlaceRef,
@@ -18,7 +17,7 @@ use rustc_middle::mir::{AggregateKind, BasicBlock, BorrowCheckResult, BorrowKind
 use rustc_middle::mir::{Field, ProjectionElem, Promoted, Rvalue, Statement, StatementKind};
 use rustc_middle::mir::{InlineAsmOperand, Terminator, TerminatorKind};
 use rustc_middle::ty::query::Providers;
-use rustc_middle::ty::{self, ParamEnv, RegionVid, TyCtxt};
+use rustc_middle::ty::{self, CapturedPlace, ParamEnv, RegionVid, TyCtxt};
 use rustc_session::lint::builtin::{MUTABLE_BORROW_RESERVATION_CONFLICT, UNUSED_MUT};
 use rustc_span::{Span, Symbol, DUMMY_SP};
 
@@ -73,16 +72,15 @@ crate use region_infer::RegionInferenceContext;
 
 // FIXME(eddyb) perhaps move this somewhere more centrally.
 #[derive(Debug)]
-crate struct Upvar {
+crate struct Upvar<'tcx> {
+    // FIXME(project-rfc-2229#8): ty::CapturePlace should have a to_string(), or similar
+    //                           then this should not be needed.
     name: Symbol,
 
-    // FIXME(project-rfc-2229#8): This should use Place or something similar
-    var_hir_id: HirId,
+    place: CapturedPlace<'tcx>,
 
     /// If true, the capture is behind a reference.
     by_ref: bool,
-
-    mutability: Mutability,
 }
 
 const DEREF_PROJECTION: &[PlaceElem<'_>; 1] = &[ProjectionElem::Deref];
@@ -159,21 +157,13 @@ fn do_mir_borrowck<'a, 'tcx>(
     let upvars: Vec<_> = tables
         .closure_min_captures_flattened(def.did.to_def_id())
         .map(|captured_place| {
-            let var_hir_id = match captured_place.place.base {
-                HirPlaceBase::Upvar(upvar_id) => upvar_id.var_path.hir_id,
-                _ => bug!("Expected upvar"),
-            };
+            let var_hir_id = captured_place.get_root_variable();
             let capture = captured_place.info.capture_kind;
             let by_ref = match capture {
                 ty::UpvarCapture::ByValue(_) => false,
                 ty::UpvarCapture::ByRef(..) => true,
             };
-            Upvar {
-                name: tcx.hir().name(var_hir_id),
-                var_hir_id,
-                by_ref,
-                mutability: captured_place.mutability,
-            }
+            Upvar { name: tcx.hir().name(var_hir_id), place: captured_place.clone(), by_ref }
         })
         .collect();
 
@@ -542,7 +532,7 @@ crate struct MirBorrowckCtxt<'cx, 'tcx> {
     dominators: Dominators<BasicBlock>,
 
     /// Information about upvars not necessarily preserved in types or MIR
-    upvars: Vec<Upvar>,
+    upvars: Vec<Upvar<'tcx>>,
 
     /// Names of local (user) variables (extracted from `var_debug_info`).
     local_names: IndexVec<Local, Option<Symbol>>,
@@ -2245,7 +2235,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                                  place={:?}",
                                 upvar, is_local_mutation_allowed, place
                             );
-                            match (upvar.mutability, is_local_mutation_allowed) {
+                            match (upvar.place.mutability, is_local_mutation_allowed) {
                                 (
                                     Mutability::Not,
                                     LocalMutationIsAllowed::No

compiler/rustc_mir/src/borrow_check/nll.rs

@@ -165,7 +165,7 @@ pub(in crate::borrow_check) fn compute_regions<'cx, 'tcx>(
     flow_inits: &mut ResultsCursor<'cx, 'tcx, MaybeInitializedPlaces<'cx, 'tcx>>,
     move_data: &MoveData<'tcx>,
     borrow_set: &BorrowSet<'tcx>,
-    upvars: &[Upvar],
+    upvars: &[Upvar<'tcx>],
 ) -> NllOutput<'tcx> {
     let mut all_facts = AllFacts::enabled(infcx.tcx).then_some(AllFacts::default());
 

compiler/rustc_mir/src/borrow_check/path_utils.rs

@@ -143,7 +143,7 @@ pub(super) fn borrow_of_local_data(place: Place<'_>) -> bool {
 /// of a closure type.
 pub(crate) fn is_upvar_field_projection(
     tcx: TyCtxt<'tcx>,
-    upvars: &[Upvar],
+    upvars: &[Upvar<'tcx>],
     place_ref: PlaceRef<'tcx>,
     body: &Body<'tcx>,
 ) -> Option<Field> {

compiler/rustc_mir/src/borrow_check/type_check/mod.rs

@@ -132,7 +132,7 @@ pub(crate) fn type_check<'mir, 'tcx>(
     flow_inits: &mut ResultsCursor<'mir, 'tcx, MaybeInitializedPlaces<'mir, 'tcx>>,
     move_data: &MoveData<'tcx>,
     elements: &Rc<RegionValueElements>,
-    upvars: &[Upvar],
+    upvars: &[Upvar<'tcx>],
 ) -> MirTypeckResults<'tcx> {
     let implicit_region_bound = infcx.tcx.mk_region(ty::ReVar(universal_regions.fr_fn_body));
     let mut constraints = MirTypeckRegionConstraints {
@@ -821,7 +821,7 @@ struct BorrowCheckContext<'a, 'tcx> {
     all_facts: &'a mut Option<AllFacts>,
     borrow_set: &'a BorrowSet<'tcx>,
     constraints: &'a mut MirTypeckRegionConstraints<'tcx>,
-    upvars: &'a [Upvar],
+    upvars: &'a [Upvar<'tcx>],
 }
 
 crate struct MirTypeckResults<'tcx> {
@@ -2486,7 +2486,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             body,
         );
         let category = if let Some(field) = field {
-            ConstraintCategory::ClosureUpvar(self.borrowck_context.upvars[field.index()].var_hir_id)
+            let var_hir_id = self.borrowck_context.upvars[field.index()].place.get_root_variable();
+            // FIXME(project-rfc-2229#8): Use Place for better dianostics
+            ConstraintCategory::ClosureUpvar(var_hir_id)
         } else {
             ConstraintCategory::Boring
         };

src/test/ui/closures/2229_closure_analysis/diagnostics/cant-mutate-imm-borrow.rs

@@ -14,7 +14,6 @@ fn main() {
     let mut c = || {
     //~^ ERROR: cannot borrow `z.0.0.0` as mutable, as it is behind a `&` reference
         z.0.0.0 = format!("X1");
-        //~^ ERROR: cannot assign to `z`, as it is not declared as mutable
     };
 
     c();

src/test/ui/closures/2229_closure_analysis/diagnostics/cant-mutate-imm-borrow.stderr

@@ -7,15 +7,6 @@ LL | #![feature(capture_disjoint_fields)]
    = note: `#[warn(incomplete_features)]` on by default
    = note: see issue #53488 </~https://github.com/rust-lang/rust/issues/53488> for more information
 
-error[E0594]: cannot assign to `z`, as it is not declared as mutable
-  --> $DIR/cant-mutate-imm-borrow.rs:16:9
-   |
-LL |     let z = (& y, "Z");
-   |         - help: consider changing this to be mutable: `mut z`
-...
-LL |         z.0.0.0 = format!("X1");
-   |         ^^^^^^^ cannot assign
-
 error[E0596]: cannot borrow `z.0.0.0` as mutable, as it is behind a `&` reference
   --> $DIR/cant-mutate-imm-borrow.rs:14:17
    |
@@ -25,7 +16,6 @@ LL |
 LL |         z.0.0.0 = format!("X1");
    |         - mutable borrow occurs due to use of `z.0.0.0` in closure
 
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to previous error; 1 warning emitted
 
-Some errors have detailed explanations: E0594, E0596.
-For more information about an error, try `rustc --explain E0594`.
+For more information about this error, try `rustc --explain E0596`.

src/test/ui/closures/2229_closure_analysis/diagnostics/cant-mutate-imm.rs

@@ -0,0 +1,18 @@
+#![feature(capture_disjoint_fields)]
+//~^ WARNING: the feature `capture_disjoint_fields` is incomplete
+
+// Ensure that diagnostics for mutability error (because the root variable
+// isn't mutable) work with `capture_disjoint_fields` enabled.
+
+fn main() {
+    let x = (10, 10);
+    let y = (x, 10);
+    let z = (y, 10);
+
+    let mut c = || {
+        z.0.0.0 = 20;
+        //~^ ERROR: cannot assign to `z`, as it is not declared as mutable
+    };
+
+    c();
+}

src/test/ui/closures/2229_closure_analysis/diagnostics/cant-mutate-imm.stderr

@@ -0,0 +1,21 @@
+warning: the feature `capture_disjoint_fields` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/cant-mutate-imm.rs:1:12
+   |
+LL | #![feature(capture_disjoint_fields)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #53488 </~https://github.com/rust-lang/rust/issues/53488> for more information
+
+error[E0594]: cannot assign to `z`, as it is not declared as mutable
+  --> $DIR/cant-mutate-imm.rs:13:9
+   |
+LL |     let z = (y, 10);
+   |         - help: consider changing this to be mutable: `mut z`
+...
+LL |         z.0.0.0 = 20;
+   |         ^^^^^^^^^^^^ cannot assign
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0594`.