Auto merge of #16669 - ShoyuVanilla:merge-borrow-kind-unique, r=Veykril

Merge `BorrowKind::Unique` into `BorrowKind::Mut`

Resolves #15170

7 files changed, 68 insertions, 83 deletions

diff --git a/crates/hir-ty/src/infer/closure.rs b/crates/hir-ty/src/infer/closure.rs
index 22a70f951ea..000248c2ce2 100644
--- a/crates/hir-ty/src/infer/closure.rs
+++ b/crates/hir-ty/src/infer/closure.rs
@@ -23,7 +23,7 @@ use stdx::never;
 use crate::{
     db::{HirDatabase, InternedClosure},
     from_placeholder_idx, make_binders,
-    mir::{BorrowKind, MirSpan, ProjectionElem},
+    mir::{BorrowKind, MirSpan, MutBorrowKind, ProjectionElem},
     static_lifetime, to_chalk_trait_id,
     traits::FnTrait,
     utils::{self, generics, Generics},
@@ -142,9 +142,13 @@ impl HirPlace {
         mut current_capture: CaptureKind,
         len: usize,
     ) -> CaptureKind {
-        if let CaptureKind::ByRef(BorrowKind::Mut { .. }) = current_capture {
+        if let CaptureKind::ByRef(BorrowKind::Mut {
+            kind: MutBorrowKind::Default | MutBorrowKind::TwoPhasedBorrow,
+        }) = current_capture
+        {
             if self.projections[len..].iter().any(|it| *it == ProjectionElem::Deref) {
-                current_capture = CaptureKind::ByRef(BorrowKind::Unique);
+                current_capture =
+                    CaptureKind::ByRef(BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture });
             }
         }
         current_capture
@@ -377,7 +381,7 @@ impl InferenceContext<'_> {
         if let Some(place) = self.place_of_expr(expr) {
             self.add_capture(
                 place,
-                CaptureKind::ByRef(BorrowKind::Mut { allow_two_phase_borrow: false }),
+                CaptureKind::ByRef(BorrowKind::Mut { kind: MutBorrowKind::Default }),
                 expr.into(),
             );
         }
@@ -426,9 +430,7 @@ impl InferenceContext<'_> {
 
     fn ref_capture_with_adjusts(&mut self, m: Mutability, tgt_expr: ExprId, rest: &[Adjustment]) {
         let capture_kind = match m {
-            Mutability::Mut => {
-                CaptureKind::ByRef(BorrowKind::Mut { allow_two_phase_borrow: false })
-            }
+            Mutability::Mut => CaptureKind::ByRef(BorrowKind::Mut { kind: MutBorrowKind::Default }),
             Mutability::Not => CaptureKind::ByRef(BorrowKind::Shared),
         };
         if let Some(place) = self.place_of_expr_without_adjust(tgt_expr) {
@@ -648,7 +650,7 @@ impl InferenceContext<'_> {
         self.walk_pat_inner(
             pat,
             &mut update_result,
-            BorrowKind::Mut { allow_two_phase_borrow: false },
+            BorrowKind::Mut { kind: MutBorrowKind::Default },
         );
     }
 
@@ -699,7 +701,7 @@ impl InferenceContext<'_> {
             },
         }
         if self.result.pat_adjustments.get(&p).map_or(false, |it| !it.is_empty()) {
-            for_mut = BorrowKind::Unique;
+            for_mut = BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture };
         }
         self.body.walk_pats_shallow(p, |p| self.walk_pat_inner(p, update_result, for_mut));
     }
@@ -880,7 +882,7 @@ impl InferenceContext<'_> {
                     }
                     BindingMode::Ref(Mutability::Not) => BorrowKind::Shared,
                     BindingMode::Ref(Mutability::Mut) => {
-                        BorrowKind::Mut { allow_two_phase_borrow: false }
+                        BorrowKind::Mut { kind: MutBorrowKind::Default }
                     }
                 };
                 self.add_capture(place, CaptureKind::ByRef(capture_kind), pat.into());
@@ -930,9 +932,7 @@ impl InferenceContext<'_> {
             r = cmp::min(
                 r,
                 match &it.kind {
-                    CaptureKind::ByRef(BorrowKind::Unique | BorrowKind::Mut { .. }) => {
-                        FnTrait::FnMut
-                    }
+                    CaptureKind::ByRef(BorrowKind::Mut { .. }) => FnTrait::FnMut,
                     CaptureKind::ByRef(BorrowKind::Shallow | BorrowKind::Shared) => FnTrait::Fn,
                     CaptureKind::ByValue => FnTrait::FnOnce,
                 },
@@ -949,8 +949,12 @@ impl InferenceContext<'_> {
         };
         self.consume_expr(*body);
         for item in &self.current_captures {
-            if matches!(item.kind, CaptureKind::ByRef(BorrowKind::Mut { .. }))
-                && !item.place.projections.contains(&ProjectionElem::Deref)
+            if matches!(
+                item.kind,
+                CaptureKind::ByRef(BorrowKind::Mut {
+                    kind: MutBorrowKind::Default | MutBorrowKind::TwoPhasedBorrow
+                })
+            ) && !item.place.projections.contains(&ProjectionElem::Deref)
             {
                 // FIXME: remove the `mutated_bindings_in_closure` completely and add proper fake reads in
                 // MIR. I didn't do that due duplicate diagnostics.
diff --git a/crates/hir-ty/src/mir.rs b/crates/hir-ty/src/mir.rs
index 494f1850b88..cfaef2a392c 100644
--- a/crates/hir-ty/src/mir.rs
+++ b/crates/hir-ty/src/mir.rs
@@ -659,66 +659,33 @@ pub enum BorrowKind {
     /// We can also report errors with this kind of borrow differently.
     Shallow,
 
-    /// Data must be immutable but not aliasable. This kind of borrow
-    /// cannot currently be expressed by the user and is used only in
-    /// implicit closure bindings. It is needed when the closure is
-    /// borrowing or mutating a mutable referent, e.g.:
-    /// ```
-    /// let mut z = 3;
-    /// let x: &mut isize = &mut z;
-    /// let y = || *x += 5;
-    /// ```
-    /// If we were to try to translate this closure into a more explicit
-    /// form, we'd encounter an error with the code as written:
-    /// ```compile_fail,E0594
-    /// struct Env<'a> { x: &'a &'a mut isize }
-    /// let mut z = 3;
-    /// let x: &mut isize = &mut z;
-    /// let y = (&mut Env { x: &x }, fn_ptr);  // Closure is pair of env and fn
-    /// fn fn_ptr(env: &mut Env) { **env.x += 5; }
-    /// ```
-    /// This is then illegal because you cannot mutate an `&mut` found
-    /// in an aliasable location. To solve, you'd have to translate with
-    /// an `&mut` borrow:
-    /// ```compile_fail,E0596
-    /// struct Env<'a> { x: &'a mut &'a mut isize }
-    /// let mut z = 3;
-    /// let x: &mut isize = &mut z;
-    /// let y = (&mut Env { x: &mut x }, fn_ptr); // changed from &x to &mut x
-    /// fn fn_ptr(env: &mut Env) { **env.x += 5; }
-    /// ```
-    /// Now the assignment to `**env.x` is legal, but creating a
-    /// mutable pointer to `x` is not because `x` is not mutable. We
-    /// could fix this by declaring `x` as `let mut x`. This is ok in
-    /// user code, if awkward, but extra weird for closures, since the
-    /// borrow is hidden.
-    ///
-    /// So we introduce a "unique imm" borrow -- the referent is
-    /// immutable, but not aliasable. This solves the problem. For
-    /// simplicity, we don't give users the way to express this
-    /// borrow, it's just used when translating closures.
-    Unique,
-
     /// Data is mutable and not aliasable.
-    Mut {
-        /// `true` if this borrow arose from method-call auto-ref
-        /// (i.e., `adjustment::Adjust::Borrow`).
-        allow_two_phase_borrow: bool,
-    },
+    Mut { kind: MutBorrowKind },
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Copy, PartialOrd, Ord)]
+pub enum MutBorrowKind {
+    Default,
+    /// This borrow arose from method-call auto-ref
+    /// (i.e., adjustment::Adjust::Borrow).
+    TwoPhasedBorrow,
+    /// Data must be immutable but not aliasable. This kind of borrow cannot currently
+    /// be expressed by the user and is used only in implicit closure bindings.
+    ClosureCapture,
 }
 
 impl BorrowKind {
     fn from_hir(m: hir_def::type_ref::Mutability) -> Self {
         match m {
             hir_def::type_ref::Mutability::Shared => BorrowKind::Shared,
-            hir_def::type_ref::Mutability::Mut => BorrowKind::Mut { allow_two_phase_borrow: false },
+            hir_def::type_ref::Mutability::Mut => BorrowKind::Mut { kind: MutBorrowKind::Default },
         }
     }
 
     fn from_chalk(m: Mutability) -> Self {
         match m {
             Mutability::Not => BorrowKind::Shared,
-            Mutability::Mut => BorrowKind::Mut { allow_two_phase_borrow: false },
+            Mutability::Mut => BorrowKind::Mut { kind: MutBorrowKind::Default },
         }
     }
 }
diff --git a/crates/hir-ty/src/mir/borrowck.rs b/crates/hir-ty/src/mir/borrowck.rs
index 63fa87ad662..8b6936f8bc0 100644
--- a/crates/hir-ty/src/mir/borrowck.rs
+++ b/crates/hir-ty/src/mir/borrowck.rs
@@ -19,8 +19,8 @@ use crate::{
 };
 
 use super::{
-    BasicBlockId, BorrowKind, LocalId, MirBody, MirLowerError, MirSpan, Place, ProjectionElem,
-    Rvalue, StatementKind, TerminatorKind,
+    BasicBlockId, BorrowKind, LocalId, MirBody, MirLowerError, MirSpan, MutBorrowKind, Place,
+    ProjectionElem, Rvalue, StatementKind, TerminatorKind,
 };
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -540,7 +540,13 @@ fn mutability_of_locals(
                         }
                         Rvalue::ShallowInitBox(_, _) | Rvalue::ShallowInitBoxWithAlloc(_) => (),
                     }
-                    if let Rvalue::Ref(BorrowKind::Mut { .. }, p) = value {
+                    if let Rvalue::Ref(
+                        BorrowKind::Mut {
+                            kind: MutBorrowKind::Default | MutBorrowKind::TwoPhasedBorrow,
+                        },
+                        p,
+                    ) = value
+                    {
                         if place_case(db, body, p) != ProjectionCase::Indirect {
                             push_mut_span(p.local, statement.span, &mut result);
                         }
diff --git a/crates/hir-ty/src/mir/lower/as_place.rs b/crates/hir-ty/src/mir/lower/as_place.rs
index afe33607d46..be81915bb40 100644
--- a/crates/hir-ty/src/mir/lower/as_place.rs
+++ b/crates/hir-ty/src/mir/lower/as_place.rs
@@ -1,5 +1,7 @@
 //! MIR lowering for places
 
+use crate::mir::MutBorrowKind;
+
 use super::*;
 use hir_def::FunctionId;
 use hir_expand::name;
@@ -328,7 +330,7 @@ impl MirLowerCtx<'_> {
                 Mutability::Mut,
                 LangItem::DerefMut,
                 name![deref_mut],
-                BorrowKind::Mut { allow_two_phase_borrow: false },
+                BorrowKind::Mut { kind: MutBorrowKind::Default },
             )
         };
         let ty_ref = TyKind::Ref(chalk_mut, static_lifetime(), source_ty.clone()).intern(Interner);
diff --git a/crates/hir-ty/src/mir/lower/pattern_matching.rs b/crates/hir-ty/src/mir/lower/pattern_matching.rs
index 85c8d1685b8..90cbd13a6c6 100644
--- a/crates/hir-ty/src/mir/lower/pattern_matching.rs
+++ b/crates/hir-ty/src/mir/lower/pattern_matching.rs
@@ -3,12 +3,15 @@
 use hir_def::{hir::LiteralOrConst, resolver::HasResolver, AssocItemId};
 
 use crate::{
-    mir::lower::{
-        BasicBlockId, BinOp, BindingId, BorrowKind, Either, Expr, FieldId, Idx, Interner,
-        MemoryMap, MirLowerCtx, MirLowerError, MirSpan, Mutability, Operand, Pat, PatId, Place,
-        PlaceElem, ProjectionElem, RecordFieldPat, ResolveValueResult, Result, Rvalue,
-        Substitution, SwitchTargets, TerminatorKind, TupleFieldId, TupleId, TyBuilder, TyKind,
-        ValueNs, VariantData, VariantId,
+    mir::{
+        lower::{
+            BasicBlockId, BinOp, BindingId, BorrowKind, Either, Expr, FieldId, Idx, Interner,
+            MemoryMap, MirLowerCtx, MirLowerError, MirSpan, Mutability, Operand, Pat, PatId, Place,
+            PlaceElem, ProjectionElem, RecordFieldPat, ResolveValueResult, Result, Rvalue,
+            Substitution, SwitchTargets, TerminatorKind, TupleFieldId, TupleId, TyBuilder, TyKind,
+            ValueNs, VariantData, VariantId,
+        },
+        MutBorrowKind,
     },
     BindingMode,
 };
@@ -450,7 +453,7 @@ impl MirLowerCtx<'_> {
                 BindingMode::Move => Operand::Copy(cond_place).into(),
                 BindingMode::Ref(Mutability::Not) => Rvalue::Ref(BorrowKind::Shared, cond_place),
                 BindingMode::Ref(Mutability::Mut) => {
-                    Rvalue::Ref(BorrowKind::Mut { allow_two_phase_borrow: false }, cond_place)
+                    Rvalue::Ref(BorrowKind::Mut { kind: MutBorrowKind::Default }, cond_place)
                 }
             },
             span,
diff --git a/crates/hir-ty/src/mir/pretty.rs b/crates/hir-ty/src/mir/pretty.rs
index 23fc2713554..0c641d7c6c2 100644
--- a/crates/hir-ty/src/mir/pretty.rs
+++ b/crates/hir-ty/src/mir/pretty.rs
@@ -18,7 +18,8 @@ use crate::{
 };
 
 use super::{
-    AggregateKind, BasicBlockId, BorrowKind, LocalId, MirBody, Operand, Place, Rvalue, UnOp,
+    AggregateKind, BasicBlockId, BorrowKind, LocalId, MirBody, MutBorrowKind, Operand, Place,
+    Rvalue, UnOp,
 };
 
 macro_rules! w {
@@ -366,8 +367,10 @@ impl<'a> MirPrettyCtx<'a> {
                 match r {
                     BorrowKind::Shared => w!(self, "&"),
                     BorrowKind::Shallow => w!(self, "&shallow "),
-                    BorrowKind::Unique => w!(self, "&uniq "),
-                    BorrowKind::Mut { .. } => w!(self, "&mut "),
+                    BorrowKind::Mut { kind: MutBorrowKind::ClosureCapture } => w!(self, "&uniq "),
+                    BorrowKind::Mut {
+                        kind: MutBorrowKind::Default | MutBorrowKind::TwoPhasedBorrow,
+                    } => w!(self, "&mut "),
                 }
                 self.place(p);
             }
diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index 2d8811cf5eb..a0237e3b90b 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -68,7 +68,7 @@ use hir_ty::{
     known_const_to_ast,
     layout::{Layout as TyLayout, RustcEnumVariantIdx, RustcFieldIdx, TagEncoding},
     method_resolution::{self, TyFingerprint},
-    mir::interpret_mir,
+    mir::{interpret_mir, MutBorrowKind},
     primitive::UintTy,
     traits::FnTrait,
     AliasTy, CallableDefId, CallableSig, Canonical, CanonicalVarKinds, Cast, ClosureId, GenericArg,
@@ -3754,12 +3754,12 @@ impl ClosureCapture {
             hir_ty::CaptureKind::ByRef(
                 hir_ty::mir::BorrowKind::Shallow | hir_ty::mir::BorrowKind::Shared,
             ) => CaptureKind::SharedRef,
-            hir_ty::CaptureKind::ByRef(hir_ty::mir::BorrowKind::Unique) => {
-                CaptureKind::UniqueSharedRef
-            }
-            hir_ty::CaptureKind::ByRef(hir_ty::mir::BorrowKind::Mut { .. }) => {
-                CaptureKind::MutableRef
-            }
+            hir_ty::CaptureKind::ByRef(hir_ty::mir::BorrowKind::Mut {
+                kind: MutBorrowKind::ClosureCapture,
+            }) => CaptureKind::UniqueSharedRef,
+            hir_ty::CaptureKind::ByRef(hir_ty::mir::BorrowKind::Mut {
+                kind: MutBorrowKind::Default | MutBorrowKind::TwoPhasedBorrow,
+            }) => CaptureKind::MutableRef,
             hir_ty::CaptureKind::ByValue => CaptureKind::Move,
         }
     }