Overloaded augmented assignments

Author: Jorge Aparicio

src/libcore/ops.rs

@@ -525,11 +525,17 @@ impl<'d,'t,'a,'tcx> ExprUseVisitor<'d,'t,'a,'tcx> {
                 self.consume_expr(&**base);
             }
 
-            hir::ExprAssignOp(_, ref lhs, ref rhs) => {
-                // This will have to change if/when we support
-                // overloaded operators for `+=` and so forth.
-                self.mutate_expr(expr, &**lhs, WriteAndRead);
-                self.consume_expr(&**rhs);
+            hir::ExprAssignOp(op, ref lhs, ref rhs) => {
+                let pass_args = if ::rustc_front::util::is_by_value_binop(op.node) {
+                    PassArgs::ByValue
+                } else {
+                    PassArgs::ByRef
+                };
+
+                if !self.walk_overloaded_operator(expr, &**lhs, vec![&**rhs], pass_args) {
+                    self.mutate_expr(expr, &**lhs, WriteAndRead);
+                    self.consume_expr(&**rhs);
+                }
             }
 
             hir::ExprRepeat(ref base, ref count) => {

src/librustc/middle/expr_use_visitor.rs

@@ -286,6 +286,16 @@ lets_do_this! {
     BitOrTraitLangItem,              "bitor",                   bitor_trait;
     ShlTraitLangItem,                "shl",                     shl_trait;
     ShrTraitLangItem,                "shr",                     shr_trait;
+    AddAssignTraitLangItem,          "add_assign",              add_assign_trait;
+    SubAssignTraitLangItem,          "sub_assign",              sub_assign_trait;
+    MulAssignTraitLangItem,          "mul_assign",              mul_assign_trait;
+    DivAssignTraitLangItem,          "div_assign",              div_assign_trait;
+    RemAssignTraitLangItem,          "rem_assign",              rem_assign_trait;
+    BitXorAssignTraitLangItem,       "bitxor_assign",           bitxor_assign_trait;
+    BitAndAssignTraitLangItem,       "bitand_assign",           bitand_assign_trait;
+    BitOrAssignTraitLangItem,        "bitor_assign",            bitor_assign_trait;
+    ShlAssignTraitLangItem,          "shl_assign",              shl_assign_trait;
+    ShrAssignTraitLangItem,          "shr_assign",              shr_assign_trait;
     IndexTraitLangItem,              "index",                   index_trait;
     IndexMutTraitLangItem,           "index_mut",               index_mut_trait;
     RangeStructLangItem,             "range",                   range_struct;

src/librustc/middle/lang_items.rs

@@ -1018,7 +1018,20 @@ fn trans_rvalue_stmt_unadjusted<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
             }
         }
         hir::ExprAssignOp(op, ref dst, ref src) => {
-            trans_assign_op(bcx, expr, op, &**dst, &**src)
+            let has_method_map = bcx.tcx()
+                                    .tables
+                                    .borrow()
+                                    .method_map
+                                    .contains_key(&MethodCall::expr(expr.id));
+
+            if has_method_map {
+                let dst = unpack_datum!(bcx, trans(bcx, &**dst));
+                let src_datum = unpack_datum!(bcx, trans(bcx, &**src));
+                trans_overloaded_op(bcx, expr, MethodCall::expr(expr.id), dst,
+                                    Some((src_datum, src.id)), None, false).bcx
+            } else {
+                trans_assign_op(bcx, expr, op, &**dst, &**src)
+            }
         }
         hir::ExprInlineAsm(ref a) => {
             asm::trans_inline_asm(bcx, a)
@@ -1207,8 +1220,11 @@ fn trans_rvalue_dps_unadjusted<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
             // Trait casts used to come this way, now they should be coercions.
             bcx.tcx().sess.span_bug(expr.span, "DPS expr_cast (residual trait cast?)")
         }
-        hir::ExprAssignOp(op, ref dst, ref src) => {
-            trans_assign_op(bcx, expr, op, &**dst, &**src)
+        hir::ExprAssignOp(op, _, _) => {
+            bcx.tcx().sess.span_bug(
+                expr.span,
+                &format!("augmented assignment `{}=` should always be a rvalue_stmt",
+                         rustc_front::util::binop_to_string(op.node)))
         }
         _ => {
             bcx.tcx().sess.span_bug(

src/librustc_trans/trans/expr.rs

@@ -17,10 +17,8 @@ use super::{
     demand,
     method,
     FnCtxt,
-    structurally_resolved_type,
 };
 use middle::def_id::DefId;
-use middle::traits;
 use middle::ty::{Ty, HasTypeFlags, PreferMutLvalue};
 use syntax::ast;
 use syntax::parse::token;
@@ -34,34 +32,24 @@ pub fn check_binop_assign<'a,'tcx>(fcx: &FnCtxt<'a,'tcx>,
                                    lhs_expr: &'tcx hir::Expr,
                                    rhs_expr: &'tcx hir::Expr)
 {
-    let tcx = fcx.ccx.tcx;
-
     check_expr_with_lvalue_pref(fcx, lhs_expr, PreferMutLvalue);
-    check_expr(fcx, rhs_expr);
 
-    let lhs_ty = structurally_resolved_type(fcx, lhs_expr.span, fcx.expr_ty(lhs_expr));
-    let rhs_ty = structurally_resolved_type(fcx, rhs_expr.span, fcx.expr_ty(rhs_expr));
+    let lhs_ty = fcx.resolve_type_vars_if_possible(fcx.expr_ty(lhs_expr));
+    let (rhs_ty, return_ty) =
+        check_overloaded_binop(fcx, expr, lhs_expr, lhs_ty, rhs_expr, op, true);
+    let rhs_ty = fcx.resolve_type_vars_if_possible(rhs_ty);
 
-    if is_builtin_binop(lhs_ty, rhs_ty, op) {
+    if !lhs_ty.is_ty_var() && !rhs_ty.is_ty_var() && is_builtin_binop(lhs_ty, rhs_ty, op) {
         enforce_builtin_binop_types(fcx, lhs_expr, lhs_ty, rhs_expr, rhs_ty, op);
         fcx.write_nil(expr.id);
     } else {
-        // error types are considered "builtin"
-        assert!(!lhs_ty.references_error() || !rhs_ty.references_error());
-        span_err!(tcx.sess, lhs_expr.span, E0368,
-                  "binary assignment operation `{}=` cannot be applied to types `{}` and `{}`",
-                  hir_util::binop_to_string(op.node),
-                  lhs_ty,
-                  rhs_ty);
-        fcx.write_error(expr.id);
+        fcx.write_ty(expr.id, return_ty);
     }
 
     let tcx = fcx.tcx();
     if !tcx.expr_is_lval(lhs_expr) {
         span_err!(tcx.sess, lhs_expr.span, E0067, "invalid left-hand side expression");
     }
-
-    fcx.require_expr_have_sized_type(lhs_expr, traits::AssignmentLhsSized);
 }
 
 /// Check a potentially overloaded binary operator.
@@ -95,7 +83,7 @@ pub fn check_binop<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
             // overloaded. This is the way to be most flexible w/r/t
             // types that get inferred.
             let (rhs_ty, return_ty) =
-                check_overloaded_binop(fcx, expr, lhs_expr, lhs_ty, rhs_expr, op);
+                check_overloaded_binop(fcx, expr, lhs_expr, lhs_ty, rhs_expr, op, false);
 
             // Supply type inference hints if relevant. Probably these
             // hints should be enforced during select as part of the
@@ -167,14 +155,16 @@ fn check_overloaded_binop<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                     lhs_expr: &'tcx hir::Expr,
                                     lhs_ty: Ty<'tcx>,
                                     rhs_expr: &'tcx hir::Expr,
-                                    op: hir::BinOp)
+                                    op: hir::BinOp,
+                                    assign: bool)
                                     -> (Ty<'tcx>, Ty<'tcx>)
 {
-    debug!("check_overloaded_binop(expr.id={}, lhs_ty={:?})",
+    debug!("check_overloaded_binop(expr.id={}, lhs_ty={:?}, assign={})",
            expr.id,
-           lhs_ty);
+           lhs_ty,
+           assign);
 
-    let (name, trait_def_id) = name_and_trait_def_id(fcx, op);
+    let (name, trait_def_id) = name_and_trait_def_id(fcx, op, assign);
 
     // NB: As we have not yet type-checked the RHS, we don't have the
     // type at hand. Make a variable to represent it. The whole reason
@@ -191,10 +181,17 @@ fn check_overloaded_binop<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
         Err(()) => {
             // error types are considered "builtin"
             if !lhs_ty.references_error() {
-                span_err!(fcx.tcx().sess, lhs_expr.span, E0369,
-                          "binary operation `{}` cannot be applied to type `{}`",
-                          hir_util::binop_to_string(op.node),
-                          lhs_ty);
+                if assign {
+                    span_err!(fcx.tcx().sess, lhs_expr.span, E0368,
+                              "binary assignment operation `{}=` cannot be applied to type `{}`",
+                              hir_util::binop_to_string(op.node),
+                              lhs_ty);
+                } else {
+                    span_err!(fcx.tcx().sess, lhs_expr.span, E0369,
+                              "binary operation `{}` cannot be applied to type `{}`",
+                              hir_util::binop_to_string(op.node),
+                              lhs_ty);
+                }
             }
             fcx.tcx().types.err
         }
@@ -231,27 +228,51 @@ pub fn check_user_unop<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
     }
 }
 
-fn name_and_trait_def_id(fcx: &FnCtxt, op: hir::BinOp) -> (&'static str, Option<DefId>) {
+fn name_and_trait_def_id(fcx: &FnCtxt,
+                         op: hir::BinOp,
+                         assign: bool)
+                         -> (&'static str, Option<DefId>) {
     let lang = &fcx.tcx().lang_items;
-    match op.node {
-        hir::BiAdd => ("add", lang.add_trait()),
-        hir::BiSub => ("sub", lang.sub_trait()),
-        hir::BiMul => ("mul", lang.mul_trait()),
-        hir::BiDiv => ("div", lang.div_trait()),
-        hir::BiRem => ("rem", lang.rem_trait()),
-        hir::BiBitXor => ("bitxor", lang.bitxor_trait()),
-        hir::BiBitAnd => ("bitand", lang.bitand_trait()),
-        hir::BiBitOr => ("bitor", lang.bitor_trait()),
-        hir::BiShl => ("shl", lang.shl_trait()),
-        hir::BiShr => ("shr", lang.shr_trait()),
-        hir::BiLt => ("lt", lang.ord_trait()),
-        hir::BiLe => ("le", lang.ord_trait()),
-        hir::BiGe => ("ge", lang.ord_trait()),
-        hir::BiGt => ("gt", lang.ord_trait()),
-        hir::BiEq => ("eq", lang.eq_trait()),
-        hir::BiNe => ("ne", lang.eq_trait()),
-        hir::BiAnd | hir::BiOr => {
-            fcx.tcx().sess.span_bug(op.span, "&& and || are not overloadable")
+
+    if assign {
+        match op.node {
+            hir::BiAdd => ("add_assign", lang.add_assign_trait()),
+            hir::BiSub => ("sub_assign", lang.sub_assign_trait()),
+            hir::BiMul => ("mul_assign", lang.mul_assign_trait()),
+            hir::BiDiv => ("div_assign", lang.div_assign_trait()),
+            hir::BiRem => ("rem_assign", lang.rem_assign_trait()),
+            hir::BiBitXor => ("bitxor_assign", lang.bitxor_assign_trait()),
+            hir::BiBitAnd => ("bitand_assign", lang.bitand_assign_trait()),
+            hir::BiBitOr => ("bitor_assign", lang.bitor_assign_trait()),
+            hir::BiShl => ("shl_assign", lang.shl_assign_trait()),
+            hir::BiShr => ("shr_assign", lang.shr_assign_trait()),
+            hir::BiLt | hir::BiLe | hir::BiGe | hir::BiGt | hir::BiEq | hir::BiNe | hir::BiAnd |
+            hir::BiOr => {
+                fcx.tcx().sess.span_bug(op.span, &format!("impossible assignment operation: {}=",
+                                        hir_util::binop_to_string(op.node)))
+            }
+        }
+    } else {
+        match op.node {
+            hir::BiAdd => ("add", lang.add_trait()),
+            hir::BiSub => ("sub", lang.sub_trait()),
+            hir::BiMul => ("mul", lang.mul_trait()),
+            hir::BiDiv => ("div", lang.div_trait()),
+            hir::BiRem => ("rem", lang.rem_trait()),
+            hir::BiBitXor => ("bitxor", lang.bitxor_trait()),
+            hir::BiBitAnd => ("bitand", lang.bitand_trait()),
+            hir::BiBitOr => ("bitor", lang.bitor_trait()),
+            hir::BiShl => ("shl", lang.shl_trait()),
+            hir::BiShr => ("shr", lang.shr_trait()),
+            hir::BiLt => ("lt", lang.ord_trait()),
+            hir::BiLe => ("le", lang.ord_trait()),
+            hir::BiGe => ("ge", lang.ord_trait()),
+            hir::BiGt => ("gt", lang.ord_trait()),
+            hir::BiEq => ("eq", lang.eq_trait()),
+            hir::BiNe => ("ne", lang.eq_trait()),
+            hir::BiAnd | hir::BiOr => {
+                fcx.tcx().sess.span_bug(op.span, "&& and || are not overloadable")
+            }
         }
     }
 }

src/librustc_typeck/check/op.rs

@@ -695,7 +695,7 @@ fn visit_expr(rcx: &mut Rcx, expr: &hir::Expr) {
         hir::ExprAssignOp(_, ref lhs, ref rhs) => {
             if has_method_map {
                 constrain_call(rcx, expr, Some(&**lhs),
-                               Some(&**rhs).into_iter(), true);
+                               Some(&**rhs).into_iter(), false);
             }
 
             visit::walk_expr(rcx, expr);

src/librustc_typeck/check/regionck.rs

@@ -17,7 +17,7 @@ use astconv::AstConv;
 use check::FnCtxt;
 use middle::def_id::DefId;
 use middle::pat_util;
-use middle::ty::{self, Ty, MethodCall, MethodCallee};
+use middle::ty::{self, Ty, MethodCall, MethodCallee, HasTypeFlags};
 use middle::ty::adjustment;
 use middle::ty::fold::{TypeFolder,TypeFoldable};
 use middle::infer;
@@ -91,24 +91,51 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
     // we observe that something like `a+b` is (known to be)
     // operating on scalars, we clear the overload.
     fn fix_scalar_binary_expr(&mut self, e: &hir::Expr) {
-        if let hir::ExprBinary(ref op, ref lhs, ref rhs) = e.node {
-            let lhs_ty = self.fcx.node_ty(lhs.id);
-            let lhs_ty = self.fcx.infcx().resolve_type_vars_if_possible(&lhs_ty);
-
-            let rhs_ty = self.fcx.node_ty(rhs.id);
-            let rhs_ty = self.fcx.infcx().resolve_type_vars_if_possible(&rhs_ty);
-
-            if lhs_ty.is_scalar() && rhs_ty.is_scalar() {
-                self.fcx.inh.tables.borrow_mut().method_map.remove(&MethodCall::expr(e.id));
-
-                // weird but true: the by-ref binops put an
-                // adjustment on the lhs but not the rhs; the
-                // adjustment for rhs is kind of baked into the
-                // system.
-                if !hir_util::is_by_value_binop(op.node) {
-                    self.fcx.inh.tables.borrow_mut().adjustments.remove(&lhs.id);
+        match e.node {
+            hir::ExprBinary(ref op, ref lhs, ref rhs) |
+                hir::ExprAssignOp(ref op, ref lhs, ref rhs) => {
+                    let lhs_ty = self.fcx.node_ty(lhs.id);
+                    let lhs_ty = self.fcx.infcx().resolve_type_vars_if_possible(&lhs_ty);
+
+                    let rhs_ty = self.fcx.node_ty(rhs.id);
+                    let rhs_ty = self.fcx.infcx().resolve_type_vars_if_possible(&rhs_ty);
+
+                    if lhs_ty.is_scalar() && rhs_ty.is_scalar() {
+                        self.fcx.inh.tables.borrow_mut().method_map.remove(&MethodCall::expr(e.id));
+
+                        // weird but true: the by-ref binops put an
+                        // adjustment on the lhs but not the rhs; the
+                        // adjustment for rhs is kind of baked into the
+                        // system.
+                        match e.node {
+                            hir::ExprBinary(..) => {
+                                if !hir_util::is_by_value_binop(op.node) {
+                                    self.fcx.inh.tables.borrow_mut().adjustments.remove(&lhs.id);
+                                }
+                            },
+                            hir::ExprAssignOp(..) => {
+                                self.fcx.inh.tables.borrow_mut().adjustments.remove(&lhs.id);
+                            },
+                            _ => {},
+                        }
+                    } else {
+                        let tcx = self.tcx();
+
+                        if let hir::ExprAssignOp(..) = e.node {
+                            if !tcx.sess.features.borrow().augmented_assignments &&
+                                !self.fcx.expr_ty(e).references_error() {
+                                tcx.sess.span_err(
+                                    e.span,
+                                    "overloaded augmented assignments are not stable");
+                                fileline_help!(
+                                    tcx.sess, e.span,
+                                    "add #![feature(augmented_assignments)] to the crate features \
+                                     to enable");
+                            }
+                        }
+                    }
                 }
-            }
+            _ => {},
         }
     }
 }

src/librustc_typeck/check/writeback.rs

@@ -2862,14 +2862,27 @@ impl <T: Foo> Drop for MyStructWrapper<T> {
 
 E0368: r##"
 This error indicates that a binary assignment operator like `+=` or `^=` was
-applied to the wrong types. For example:
+applied to a type that doesn't support it. For example:
 
 ```
-let mut x: u16 = 5;
-x ^= true; // error, `^=` cannot be applied to types `u16` and `bool`
-x += ();   // error, `+=` cannot be applied to types `u16` and `()`
+let mut x = 12f32; // error: binary operation `<<` cannot be applied to
+               //        type `f32`
+
+x <<= 2;
+```
+
+To fix this error, please check that this type implements this binary
+operation. Example:
+
+```
+let x = 12u32; // the `u32` type does implement the `ShlAssign` trait
+
+x <<= 2; // ok!
 ```
 
+It is also possible to overload most operators for your own type by
+implementing the `[OP]Assign` traits from `std::ops`.
+
 Another problem you might be facing is this: suppose you've overloaded the `+`
 operator for some type `Foo` by implementing the `std::ops::Add` trait for
 `Foo`, but you find that using `+=` does not work, as in this example:
@@ -2889,15 +2902,12 @@ impl Add for Foo {
 
 fn main() {
     let mut x: Foo = Foo(5);
-    x += Foo(7); // error, `+= cannot be applied to types `Foo` and `Foo`
+    x += Foo(7); // error, `+= cannot be applied to the type `Foo`
 }
 ```
 
-This is because the binary assignment operators currently do not work off of
-traits, so it is not possible to overload them. See [RFC 953] for a proposal
-to change this.
-
-[RFC 953]: https://github.com/rust-lang/rfcs/pull/953
+This is because `AddAssign` is not automatically implemented, so you need to
+manually implement it for your type.
 "##,
 
 E0369: r##"