Remove mutable_borrow_reservation_conflict lint and allow the code pattern

compiler/rustc_borrowck/src/borrow_set.rs

@@ -167,10 +167,6 @@ impl<'tcx> BorrowSet<'tcx> {
     crate fn get_index_of(&self, location: &Location) -> Option<BorrowIndex> {
         self.location_map.get_index_of(location).map(BorrowIndex::from)
     }
-
-    crate fn contains(&self, location: &Location) -> bool {
-        self.location_map.contains_key(location)
-    }
 }
 
 struct GatherBorrows<'a, 'tcx> {

compiler/rustc_borrowck/src/lib.rs

@@ -35,14 +35,13 @@ use rustc_middle::mir::{Field, ProjectionElem, Promoted, Rvalue, Statement, Stat
 use rustc_middle::mir::{InlineAsmOperand, Terminator, TerminatorKind};
 use rustc_middle::ty::query::Providers;
 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};
+use rustc_session::lint::builtin::UNUSED_MUT;
+use rustc_span::{Span, Symbol};
 
 use either::Either;
 use smallvec::SmallVec;
 use std::cell::RefCell;
 use std::collections::BTreeMap;
-use std::mem;
 use std::rc::Rc;
 
 use rustc_mir_dataflow::impls::{
@@ -313,7 +312,6 @@ fn do_mir_borrowck<'a, 'tcx>(
                 locals_are_invalidated_at_exit,
                 access_place_error_reported: Default::default(),
                 reservation_error_reported: Default::default(),
-                reservation_warnings: Default::default(),
                 uninitialized_error_reported: Default::default(),
                 regioncx: regioncx.clone(),
                 used_mut: Default::default(),
@@ -345,7 +343,6 @@ fn do_mir_borrowck<'a, 'tcx>(
         fn_self_span_reported: Default::default(),
         access_place_error_reported: Default::default(),
         reservation_error_reported: Default::default(),
-        reservation_warnings: Default::default(),
         uninitialized_error_reported: Default::default(),
         regioncx: Rc::clone(&regioncx),
         used_mut: Default::default(),
@@ -378,34 +375,6 @@ fn do_mir_borrowck<'a, 'tcx>(
         &mut mbcx,
     );
 
-    // Convert any reservation warnings into lints.
-    let reservation_warnings = mem::take(&mut mbcx.reservation_warnings);
-    for (_, (place, span, location, bk, borrow)) in reservation_warnings {
-        let initial_diag = mbcx.report_conflicting_borrow(location, (place, span), bk, &borrow);
-
-        let scope = mbcx.body.source_info(location).scope;
-        let lint_root = match &mbcx.body.source_scopes[scope].local_data {
-            ClearCrossCrate::Set(data) => data.lint_root,
-            _ => tcx.hir().local_def_id_to_hir_id(def.did),
-        };
-
-        // Span and message don't matter; we overwrite them below anyway
-        mbcx.infcx.tcx.struct_span_lint_hir(
-            MUTABLE_BORROW_RESERVATION_CONFLICT,
-            lint_root,
-            DUMMY_SP,
-            |lint| {
-                let mut diag = lint.build("");
-
-                diag.message = initial_diag.styled_message().clone();
-                diag.span = initial_diag.span.clone();
-
-                mbcx.buffer_non_error_diag(diag);
-            },
-        );
-        initial_diag.cancel();
-    }
-
     // For each non-user used mutable variable, check if it's been assigned from
     // a user-declared local. If so, then put that local into the used_mut set.
     // Note that this set is expected to be small - only upvars from closures
@@ -540,11 +509,6 @@ struct MirBorrowckCtxt<'cx, 'tcx> {
     /// used to report extra information for `FnSelfUse`, to avoid
     /// unnecessarily verbose errors.
     fn_self_span_reported: FxHashSet<Span>,
-    /// Migration warnings to be reported for #56254. We delay reporting these
-    /// so that we can suppress the warning if there's a corresponding error
-    /// for the activation of the borrow.
-    reservation_warnings:
-        FxHashMap<BorrowIndex, (Place<'tcx>, Span, Location, BorrowKind, BorrowData<'tcx>)>,
     /// This field keeps track of errors reported in the checking of uninitialized variables,
     /// so that we don't report seemingly duplicate errors.
     uninitialized_error_reported: FxHashSet<PlaceRef<'tcx>>,
@@ -996,12 +960,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let conflict_error =
             self.check_access_for_conflict(location, place_span, sd, rw, flow_state);
 
-        if let (Activation(_, borrow_idx), true) = (kind.1, conflict_error) {
-            // Suppress this warning when there's an error being emitted for the
-            // same borrow: fixing the error is likely to fix the warning.
-            self.reservation_warnings.remove(&borrow_idx);
-        }
-
         if conflict_error || mutability_error {
             debug!("access_place: logging error place_span=`{:?}` kind=`{:?}`", place_span, kind);
             self.access_place_error_reported.insert((place_span.0, place_span.1));
@@ -1068,6 +1026,12 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     BorrowKind::Unique | BorrowKind::Mut { .. },
                 ) => Control::Continue,
 
+                (Reservation(_), BorrowKind::Shallow | BorrowKind::Shared) => {
+                    // This used to be a future compatibility warning (to be
+                    // disallowed on NLL). See rust-lang/rust#56254
+                    Control::Continue
+                }
+
                 (Write(WriteKind::Move), BorrowKind::Shallow) => {
                     // Handled by initialization checks.
                     Control::Continue
@@ -1096,27 +1060,6 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     Control::Break
                 }
 
-                (
-                    Reservation(WriteKind::MutableBorrow(bk)),
-                    BorrowKind::Shallow | BorrowKind::Shared,
-                ) if { tcx.migrate_borrowck() && this.borrow_set.contains(&location) } => {
-                    let bi = this.borrow_set.get_index_of(&location).unwrap();
-                    debug!(
-                        "recording invalid reservation of place: {:?} with \
-                         borrow index {:?} as warning",
-                        place_span.0, bi,
-                    );
-                    // rust-lang/rust#56254 - This was previously permitted on
-                    // the 2018 edition so we emit it as a warning. We buffer
-                    // these separately so that we only emit a warning if borrow
-                    // checking was otherwise successful.
-                    this.reservation_warnings
-                        .insert(bi, (place_span.0, place_span.1, location, bk, borrow.clone()));
-
-                    // Don't suppress actual errors.
-                    Control::Continue
-                }
-
                 (Reservation(kind) | Activation(kind, _) | Write(kind), _) => {
                     match rw {
                         Reservation(..) => {

compiler/rustc_lint/src/lib.rs

@@ -491,6 +491,11 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         "converted into hard error, see RFC 2972 \
          <https://github.com/rust-lang/rfcs/blob/master/text/2972-constrained-naked.md> for more information",
     );
+    store.register_removed(
+        "mutable_borrow_reservation_conflict",
+        "now allowed, see issue #59159 \
+         <https://github.com/rust-lang/rust/issues/59159> for more information",
+    );
 }
 
 fn register_internals(store: &mut LintStore) {

compiler/rustc_lint_defs/src/builtin.rs

@@ -2345,40 +2345,6 @@ declare_lint! {
     };
 }
 
-declare_lint! {
-    /// The `mutable_borrow_reservation_conflict` lint detects the reservation
-    /// of a two-phased borrow that conflicts with other shared borrows.
-    ///
-    /// ### Example
-    ///
-    /// ```rust
-    /// let mut v = vec![0, 1, 2];
-    /// let shared = &v;
-    /// v.push(shared.len());
-    /// ```
-    ///
-    /// {{produces}}
-    ///
-    /// ### Explanation
-    ///
-    /// This is a [future-incompatible] lint to transition this to a hard error
-    /// in the future. See [issue #59159] for a complete description of the
-    /// problem, and some possible solutions.
-    ///
-    /// [issue #59159]: https://github.com/rust-lang/rust/issues/59159
-    /// [future-incompatible]: ../index.md#future-incompatible-lints
-    pub MUTABLE_BORROW_RESERVATION_CONFLICT,
-    Warn,
-    "reservation of a two-phased borrow conflicts with other shared borrows",
-    @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::Custom(
-            "this borrowing pattern was not meant to be accepted, \
-            and may become a hard error in the future"
-        ),
-        reference: "issue #59159 <https://github.com/rust-lang/rust/issues/59159>",
-    };
-}
-
 declare_lint! {
     /// The `soft_unstable` lint detects unstable features that were
     /// unintentionally allowed on stable.
@@ -3179,7 +3145,6 @@ declare_lint_pass! {
         META_VARIABLE_MISUSE,
         DEPRECATED_IN_FUTURE,
         AMBIGUOUS_ASSOCIATED_ITEMS,
-        MUTABLE_BORROW_RESERVATION_CONFLICT,
         INDIRECT_STRUCTURAL_MATCH,
         POINTER_STRUCTURAL_MATCH,
         NONTRIVIAL_STRUCTURAL_MATCH,

src/test/ui/borrowck/suggest-local-var-imm-and-mut.nll.stderr

@@ -7,23 +7,12 @@ LL |             self.foo(self.bar());
    |             |    |   mutable borrow occurs here
    |             |    immutable borrow later used by call
    |             immutable borrow occurs here
-   |
-help: try adding a local storing this argument...
-  --> $DIR/suggest-local-var-imm-and-mut.rs:12:22
-   |
-LL |             self.foo(self.bar());
-   |                      ^^^^^^^^^^
-help: ...and then using that local as the argument to this call
-  --> $DIR/suggest-local-var-imm-and-mut.rs:12:13
-   |
-LL |             self.foo(self.bar());
-   |             ^^^^^^^^^^^^^^^^^^^^
 
 error[E0502]: cannot borrow `*self` as mutable because it is also borrowed as immutable
-  --> $DIR/suggest-local-var-imm-and-mut.rs:24:39
+  --> $DIR/suggest-local-var-imm-and-mut.rs:24:29
    |
 LL |             Self::foo(self, Self::bar(self));
-   |             --------- ----            ^^^^ mutable borrow occurs here
+   |             --------- ----  ^^^^^^^^^^^^^^^ mutable borrow occurs here
    |             |         |
    |             |         immutable borrow occurs here
    |             immutable borrow later used by call

src/test/ui/borrowck/two-phase-cannot-nest-mut-self-calls.stderr

@@ -13,23 +13,6 @@ LL | |
 LL | |         0
 LL | |     });
    | |______- immutable borrow occurs here
-   |
-help: try adding a local storing this argument...
-  --> $DIR/two-phase-cannot-nest-mut-self-calls.rs:16:9
-   |
-LL |         vec.push(2);
-   |         ^^^^^^^^^^^
-help: ...and then using that local as the argument to this call
-  --> $DIR/two-phase-cannot-nest-mut-self-calls.rs:14:5
-   |
-LL | /     vec.get({
-LL | |
-LL | |         vec.push(2);
-LL | |
-LL | |
-LL | |         0
-LL | |     });
-   | |______^
 
 error: aborting due to previous error
 

src/test/ui/borrowck/two-phase-reservation-sharing-interference-2.base.stderr

@@ -0,0 +1,25 @@
+error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:19:5
+   |
+LL |     let shared = &v;
+   |                  -- immutable borrow occurs here
+LL | 
+LL |     v.extend(shared);
+   |     ^^------^^^^^^^^
+   |     | |
+   |     | immutable borrow later used by call
+   |     mutable borrow occurs here
+
+error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:27:5
+   |
+LL |     v.extend(&v);
+   |     ^^------^--^
+   |     | |      |
+   |     | |      immutable borrow occurs here
+   |     | immutable borrow later used by call
+   |     mutable borrow occurs here
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0502`.

src/test/ui/borrowck/two-phase-reservation-sharing-interference-2.migrate2015.stderr

@@ -20,22 +20,6 @@ LL |     v.extend(&v);
    |     | immutable borrow later used by call
    |     mutable borrow occurs here
 
-warning: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:40:5
-   |
-LL |     let shared = &v;
-   |                  -- immutable borrow occurs here
-LL | 
-LL |     v.push(shared.len());
-   |     ^^^^^^^------------^
-   |     |      |
-   |     |      immutable borrow later used here
-   |     mutable borrow occurs here
-   |
-   = note: `#[warn(mutable_borrow_reservation_conflict)]` on by default
-   = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
-   = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>
-
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0502`.

src/test/ui/borrowck/two-phase-reservation-sharing-interference-2.migrate2018.stderr

@@ -20,22 +20,6 @@ LL |     v.extend(&v);
    |     | immutable borrow later used by call
    |     mutable borrow occurs here
 
-warning: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:40:5
-   |
-LL |     let shared = &v;
-   |                  -- immutable borrow occurs here
-LL | 
-LL |     v.push(shared.len());
-   |     ^^^^^^^------------^
-   |     |      |
-   |     |      immutable borrow later used here
-   |     mutable borrow occurs here
-   |
-   = note: `#[warn(mutable_borrow_reservation_conflict)]` on by default
-   = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
-   = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>
-
-error: aborting due to 2 previous errors; 1 warning emitted
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0502`.

src/test/ui/borrowck/two-phase-reservation-sharing-interference-2.nll.stderr

@@ -0,0 +1,25 @@
+error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:19:5
+   |
+LL |     let shared = &v;
+   |                  -- immutable borrow occurs here
+LL | 
+LL |     v.extend(shared);
+   |     ^^------^^^^^^^^
+   |     | |
+   |     | immutable borrow later used by call
+   |     mutable borrow occurs here
+
+error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
+  --> $DIR/two-phase-reservation-sharing-interference-2.rs:27:5
+   |
+LL |     v.extend(&v);
+   |     ^^------^--^
+   |     | |      |
+   |     | |      immutable borrow occurs here
+   |     | immutable borrow later used by call
+   |     mutable borrow occurs here
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0502`.

src/test/ui/borrowck/two-phase-reservation-sharing-interference-2.nll2015.stderr

@@ -5,9 +5,9 @@ LL |     let shared = &v;
    |                  -- immutable borrow occurs here
 LL | 
 LL |     v.extend(shared);
-   |     ^^^^^^^^^------^
-   |     |        |
-   |     |        immutable borrow later used here
+   |     ^^------^^^^^^^^
+   |     | |
+   |     | immutable borrow later used by call
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
@@ -20,18 +20,6 @@ LL |     v.extend(&v);
    |     | immutable borrow later used by call
    |     mutable borrow occurs here
 
-error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:40:5
-   |
-LL |     let shared = &v;
-   |                  -- immutable borrow occurs here
-LL | 
-LL |     v.push(shared.len());
-   |     ^^^^^^^------------^
-   |     |      |
-   |     |      immutable borrow later used here
-   |     mutable borrow occurs here
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0502`.

src/test/ui/borrowck/two-phase-reservation-sharing-interference-2.nll2018.stderr

@@ -5,9 +5,9 @@ LL |     let shared = &v;
    |                  -- immutable borrow occurs here
 LL | 
 LL |     v.extend(shared);
-   |     ^^^^^^^^^------^
-   |     |        |
-   |     |        immutable borrow later used here
+   |     ^^------^^^^^^^^
+   |     | |
+   |     | immutable borrow later used by call
    |     mutable borrow occurs here
 
 error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
@@ -20,18 +20,6 @@ LL |     v.extend(&v);
    |     | immutable borrow later used by call
    |     mutable borrow occurs here
 
-error[E0502]: cannot borrow `v` as mutable because it is also borrowed as immutable
-  --> $DIR/two-phase-reservation-sharing-interference-2.rs:40:5
-   |
-LL |     let shared = &v;
-   |                  -- immutable borrow occurs here
-LL | 
-LL |     v.push(shared.len());
-   |     ^^^^^^^------------^
-   |     |      |
-   |     |      immutable borrow later used here
-   |     mutable borrow occurs here
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0502`.

src/test/ui/borrowck/two-phase-reservation-sharing-interference-2.rs

@@ -1,50 +1,39 @@
-// Test for #56254, we previously allowed the last example on the 2018
-// edition. Make sure that we now emit a warning in that case and an error for
-// everyone else.
+// Test for #56254. The last example originally failed with the ast checker, was
+// accidentally allowed under migrate/nll, then linted against in migrate mode
+// but disallowed under NLL. Now, we accept it everywhere.
 
 //ignore-compare-mode-nll
 //ignore-compare-mode-polonius
 
-//revisions: migrate2015 migrate2018 nll2015 nll2018
+//revisions: base nll
 
 //[migrate2018] edition:2018
 //[nll2018] edition:2018
 
-#![cfg_attr(any(nll2015, nll2018), feature(nll))]
+#![cfg_attr(nll, feature(nll))]
 
 fn double_conflicts() {
     let mut v = vec![0, 1, 2];
     let shared = &v;
 
     v.extend(shared);
-    //[migrate2015]~^ ERROR cannot borrow `v` as mutable
-    //[nll2015]~^^ ERROR cannot borrow `v` as mutable
-    //[migrate2018]~^^^ ERROR cannot borrow `v` as mutable
-    //[nll2018]~^^^^ ERROR cannot borrow `v` as mutable
+    //[base]~^ ERROR cannot borrow `v` as mutable
+    //[nll]~^^ ERROR cannot borrow `v` as mutable
 }
 
 fn activation_conflict() {
     let mut v = vec![0, 1, 2];
 
     v.extend(&v);
-    //[migrate2015]~^ ERROR cannot borrow `v` as mutable
-    //[nll2015]~^^ ERROR cannot borrow `v` as mutable
-    //[migrate2018]~^^^ ERROR cannot borrow `v` as mutable
-    //[nll2018]~^^^^ ERROR cannot borrow `v` as mutable
+    //[base]~^ ERROR cannot borrow `v` as mutable
+    //[nll]~^^ ERROR cannot borrow `v` as mutable
 }
 
-fn reservation_conflict() {
+fn reservation_allowed() {
     let mut v = vec![0, 1, 2];
     let shared = &v;
 
     v.push(shared.len());
-    //[nll2015]~^ ERROR cannot borrow `v` as mutable
-    //[nll2018]~^^ ERROR cannot borrow `v` as mutable
-    //[migrate2015]~^^^ WARNING cannot borrow `v` as mutable
-    //[migrate2015]~| WARNING may become a hard error in the future
-
-    //[migrate2018]~^^^^^^ WARNING cannot borrow `v` as mutable
-    //[migrate2018]~| WARNING may become a hard error in the future
 }
 
 fn main() {}

src/test/ui/error-codes/E0502.nll.stderr

@@ -1,10 +1,10 @@
 error[E0502]: cannot borrow `*a` as mutable because it is also borrowed as immutable
-  --> $DIR/E0502.rs:4:9
+  --> $DIR/E0502.rs:4:5
    |
 LL |     let ref y = a;
    |         ----- immutable borrow occurs here
 LL |     bar(a);
-   |         ^ mutable borrow occurs here
+   |     ^^^^^^ mutable borrow occurs here
 LL |     y.use_ref();
    |     ----------- immutable borrow later used here
 

src/test/ui/issues/issue-61623.rs

@@ -5,7 +5,6 @@ fn f2<P>(_: P, _: ()) {}
 fn f3<'a>(x: &'a ((), &'a mut ())) {
     f2(|| x.0, f1(x.1))
 //~^ ERROR cannot borrow `*x.1` as mutable, as it is behind a `&` reference
-//~| ERROR cannot borrow `*x.1` as mutable because it is also borrowed as immutable
 }
 
 fn main() {}

src/test/ui/issues/issue-61623.stderr

@@ -6,17 +6,6 @@ LL | fn f3<'a>(x: &'a ((), &'a mut ())) {
 LL |     f2(|| x.0, f1(x.1))
    |                   ^^^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
 
-error[E0502]: cannot borrow `*x.1` as mutable because it is also borrowed as immutable
-  --> $DIR/issue-61623.rs:6:19
-   |
-LL |     f2(|| x.0, f1(x.1))
-   |     -- -- ---     ^^^ mutable borrow occurs here
-   |     |  |  |
-   |     |  |  first borrow occurs due to use of `x` in closure
-   |     |  immutable borrow occurs here
-   |     immutable borrow later used by call
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0502, E0596.
-For more information about an error, try `rustc --explain E0502`.
+For more information about this error, try `rustc --explain E0596`.

src/test/ui/nll/lint-no-err.rs

@@ -23,6 +23,4 @@ fn main() {
     let mut conflict = Repro;
     let prev = conflict.get();
     conflict.insert(*prev + *x);
-    //~^ WARN cannot borrow `conflict` as mutable because it is also borrowed as immutable
-    //~| WARN this borrowing pattern was not meant to be accepted
 }