Improve diagnostics for invalid assignment

src/librustc/hir/intravisit.rs

@@ -1043,9 +1043,9 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr) {
             walk_list!(visitor, visit_label, opt_label);
             visitor.visit_block(block);
         }
-        ExprKind::Assign(ref left_hand_expression, ref right_hand_expression) => {
-            visitor.visit_expr(right_hand_expression);
-            visitor.visit_expr(left_hand_expression)
+        ExprKind::Assign(ref lhs, ref rhs, _) => {
+            visitor.visit_expr(rhs);
+            visitor.visit_expr(lhs)
         }
         ExprKind::AssignOp(_, ref left_expression, ref right_expression) => {
             visitor.visit_expr(right_expression);

src/librustc/hir/lowering/expr.rs

@@ -122,8 +122,8 @@ impl LoweringContext<'_, '_> {
                 self.lower_block(blk, opt_label.is_some()),
                 self.lower_label(opt_label),
             ),
-            ExprKind::Assign(ref el, ref er) => {
-                hir::ExprKind::Assign(P(self.lower_expr(el)), P(self.lower_expr(er)))
+            ExprKind::Assign(ref el, ref er, span) => {
+                hir::ExprKind::Assign(P(self.lower_expr(el)), P(self.lower_expr(er)), span)
             }
             ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
                 self.lower_binop(op),
@@ -994,8 +994,11 @@ impl LoweringContext<'_, '_> {
             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
             let val_expr = P(self.expr_ident(pat.span, val_ident, val_pat_hid));
             let next_expr = P(self.expr_ident(pat.span, next_ident, next_pat_hid));
-            let assign =
-                P(self.expr(pat.span, hir::ExprKind::Assign(next_expr, val_expr), ThinVec::new()));
+            let assign = P(self.expr(
+                pat.span,
+                hir::ExprKind::Assign(next_expr, val_expr, pat.span),
+                ThinVec::new(),
+            ));
             let some_pat = self.pat_some(pat.span, val_pat);
             self.arm(some_pat, assign)
         };

src/librustc/hir/mod.rs

@@ -1628,7 +1628,8 @@ pub enum ExprKind {
     Block(P<Block>, Option<Label>),
 
     /// An assignment (e.g., `a = foo()`).
-    Assign(P<Expr>, P<Expr>),
+    /// The `Span` argument is the span of the `=` token.
+    Assign(P<Expr>, P<Expr>, Span),
     /// An assignment with an operator.
     ///
     /// E.g., `a += 1`.

src/librustc/hir/print.rs

@@ -1289,7 +1289,7 @@ impl<'a> State<'a> {
                 self.ibox(0);
                 self.print_block(&blk);
             }
-            hir::ExprKind::Assign(ref lhs, ref rhs) => {
+            hir::ExprKind::Assign(ref lhs, ref rhs, _) => {
                 let prec = AssocOp::Assign.precedence() as i8;
                 self.print_expr_maybe_paren(&lhs, prec + 1);
                 self.s.space();
@@ -2265,7 +2265,7 @@ fn contains_exterior_struct_lit(value: &hir::Expr) -> bool {
     match value.kind {
         hir::ExprKind::Struct(..) => true,
 
-        hir::ExprKind::Assign(ref lhs, ref rhs)
+        hir::ExprKind::Assign(ref lhs, ref rhs, _)
         | hir::ExprKind::AssignOp(_, ref lhs, ref rhs)
         | hir::ExprKind::Binary(_, ref lhs, ref rhs) => {
             // `X { y: 1 } + X { y: 2 }`

src/librustc_lint/unused.rs

@@ -490,7 +490,7 @@ impl EarlyLintPass for UnusedParens {
                 (value, "`return` value", false, Some(left), None)
             }
 
-            Assign(_, ref value) => (value, "assigned value", false, None, None),
+            Assign(_, ref value, _) => (value, "assigned value", false, None, None),
             AssignOp(.., ref value) => (value, "assigned value", false, None, None),
             // either function/method call, or something this lint doesn't care about
             ref call_or_other => {

src/librustc_mir/hair/cx/expr.rs

@@ -227,7 +227,7 @@ fn make_mirror_unadjusted<'a, 'tcx>(cx: &mut Cx<'a, 'tcx>, expr: &'tcx hir::Expr
 
         hir::ExprKind::Block(ref blk, _) => ExprKind::Block { body: &blk },
 
-        hir::ExprKind::Assign(ref lhs, ref rhs) => {
+        hir::ExprKind::Assign(ref lhs, ref rhs, _) => {
             ExprKind::Assign { lhs: lhs.to_ref(), rhs: rhs.to_ref() }
         }
 

src/librustc_parse/parser/expr.rs

@@ -281,7 +281,9 @@ impl<'a> Parser<'a> {
                     let binary = self.mk_binary(source_map::respan(cur_op_span, ast_op), lhs, rhs);
                     self.mk_expr(span, binary, AttrVec::new())
                 }
-                AssocOp::Assign => self.mk_expr(span, ExprKind::Assign(lhs, rhs), AttrVec::new()),
+                AssocOp::Assign => {
+                    self.mk_expr(span, ExprKind::Assign(lhs, rhs, cur_op_span), AttrVec::new())
+                }
                 AssocOp::AssignOp(k) => {
                     let aop = match k {
                         token::Plus => BinOpKind::Add,

src/librustc_passes/ast_validation.rs

@@ -737,8 +737,14 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
         for predicate in &generics.where_clause.predicates {
             if let WherePredicate::EqPredicate(ref predicate) = *predicate {
                 self.err_handler()
-                    .span_err(predicate.span, "equality constraints are not yet \
-                                               supported in where clauses (see #20041)");
+                    .struct_span_err(
+                        predicate.span,
+                        "equality constraints are not yet supported in `where` clauses",
+                    )
+                    .note(
+                        "for more information, see https://github.com/rust-lang/rust/issues/20041",
+                    )
+                    .emit();
             }
         }
 

src/librustc_passes/liveness.rs

@@ -1079,7 +1079,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                     .unwrap_or_else(|| span_bug!(expr.span, "continue to unknown label"))
             }
 
-            hir::ExprKind::Assign(ref l, ref r) => {
+            hir::ExprKind::Assign(ref l, ref r, _) => {
                 // see comment on places in
                 // propagate_through_place_components()
                 let succ = self.write_place(&l, succ, ACC_WRITE);
@@ -1373,7 +1373,7 @@ impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
 
 fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr) {
     match expr.kind {
-        hir::ExprKind::Assign(ref l, _) => {
+        hir::ExprKind::Assign(ref l, ..) => {
             this.check_place(&l);
         }
 

src/librustc_privacy/lib.rs

@@ -1251,7 +1251,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypePrivacyVisitor<'a, 'tcx> {
             return;
         }
         match expr.kind {
-            hir::ExprKind::Assign(.., ref rhs) | hir::ExprKind::Match(ref rhs, ..) => {
+            hir::ExprKind::Assign(_, ref rhs, _) | hir::ExprKind::Match(ref rhs, ..) => {
                 // Do not report duplicate errors for `x = y` and `match x { ... }`.
                 if self.check_expr_pat_type(rhs.hir_id, rhs.span) {
                     return;

src/librustc_typeck/check/demand.rs

@@ -492,7 +492,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             String::new()
                         };
                         if let Some(hir::Node::Expr(hir::Expr {
-                            kind: hir::ExprKind::Assign(left_expr, _),
+                            kind: hir::ExprKind::Assign(left_expr, ..),
                             ..
                         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
                         {

src/librustc_typeck/check/expr.rs

@@ -17,7 +17,7 @@ use crate::check::TupleArgumentsFlag::DontTupleArguments;
 use crate::util::common::ErrorReported;
 use crate::util::nodemap::FxHashMap;
 
-use errors::{pluralize, Applicability, DiagnosticBuilder};
+use errors::{pluralize, Applicability, DiagnosticBuilder, DiagnosticId};
 use rustc::hir;
 use rustc::hir::def::{CtorKind, DefKind, Res};
 use rustc::hir::def_id::DefId;
@@ -219,6 +219,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ExprKind::Box(ref subexpr) => self.check_expr_box(subexpr, expected),
             ExprKind::Lit(ref lit) => self.check_lit(&lit, expected),
             ExprKind::Binary(op, ref lhs, ref rhs) => self.check_binop(expr, op, lhs, rhs),
+            ExprKind::Assign(ref lhs, ref rhs, ref span) => {
+                self.check_expr_assign(expr, expected, lhs, rhs, span)
+            }
             ExprKind::AssignOp(op, ref lhs, ref rhs) => self.check_binop_assign(expr, op, lhs, rhs),
             ExprKind::Unary(unop, ref oprnd) => {
                 self.check_expr_unary(unop, oprnd, expected, needs, expr)
@@ -245,7 +248,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
             }
             ExprKind::Ret(ref expr_opt) => self.check_expr_return(expr_opt.as_deref(), expr),
-            ExprKind::Assign(ref lhs, ref rhs) => self.check_expr_assign(expr, expected, lhs, rhs),
             ExprKind::Loop(ref body, _, source) => {
                 self.check_expr_loop(body, source, expected, expr)
             }
@@ -723,6 +725,40 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         );
     }
 
+    fn is_destructuring_place_expr(&self, expr: &'tcx hir::Expr) -> bool {
+        match &expr.kind {
+            ExprKind::Array(comps) | ExprKind::Tup(comps) => {
+                comps.iter().all(|e| self.is_destructuring_place_expr(e))
+            }
+            ExprKind::Struct(_path, fields, rest) => {
+                rest.as_ref().map(|e| self.is_destructuring_place_expr(e)).unwrap_or(true)
+                    && fields.iter().all(|f| self.is_destructuring_place_expr(&f.expr))
+            }
+            _ => expr.is_syntactic_place_expr(),
+        }
+    }
+
+    pub(crate) fn check_lhs_assignable(
+        &self,
+        lhs: &'tcx hir::Expr,
+        err_code: &'static str,
+        expr_span: &Span,
+    ) {
+        if !lhs.is_syntactic_place_expr() {
+            let mut err = self.tcx.sess.struct_span_err_with_code(
+                *expr_span,
+                "invalid left-hand side of assignment",
+                DiagnosticId::Error(err_code.into()),
+            );
+            err.span_label(lhs.span, "cannot assign to this expression");
+            if self.is_destructuring_place_expr(lhs) {
+                err.note("destructuring assignments are not currently supported");
+                err.note("for more information, see https://github.com/rust-lang/rfcs/issues/372");
+            }
+            err.emit();
+        }
+    }
+
     /// Type check assignment expression `expr` of form `lhs = rhs`.
     /// The expected type is `()` and is passsed to the function for the purposes of diagnostics.
     fn check_expr_assign(
@@ -731,6 +767,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Expectation<'tcx>,
         lhs: &'tcx hir::Expr,
         rhs: &'tcx hir::Expr,
+        span: &Span,
     ) -> Ty<'tcx> {
         let lhs_ty = self.check_expr_with_needs(&lhs, Needs::MutPlace);
         let rhs_ty = self.check_expr_coercable_to_type(&rhs, lhs_ty);
@@ -752,10 +789,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 err.help(msg);
             }
             err.emit();
-        } else if !lhs.is_syntactic_place_expr() {
-            struct_span_err!(self.tcx.sess, expr.span, E0070, "invalid left-hand side expression")
-                .span_label(expr.span, "left-hand of expression not valid")
-                .emit();
+        } else {
+            self.check_lhs_assignable(lhs, "E0070", span);
         }
 
         self.require_type_is_sized(lhs_ty, lhs.span, traits::AssignmentLhsSized);

src/librustc_typeck/check/op.rs

@@ -19,30 +19,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         expr: &'tcx hir::Expr,
         op: hir::BinOp,
-        lhs_expr: &'tcx hir::Expr,
-        rhs_expr: &'tcx hir::Expr,
+        lhs: &'tcx hir::Expr,
+        rhs: &'tcx hir::Expr,
     ) -> Ty<'tcx> {
         let (lhs_ty, rhs_ty, return_ty) =
-            self.check_overloaded_binop(expr, lhs_expr, rhs_expr, op, IsAssign::Yes);
+            self.check_overloaded_binop(expr, lhs, rhs, op, IsAssign::Yes);
 
         let ty =
             if !lhs_ty.is_ty_var() && !rhs_ty.is_ty_var() && is_builtin_binop(lhs_ty, rhs_ty, op) {
-                self.enforce_builtin_binop_types(lhs_expr, lhs_ty, rhs_expr, rhs_ty, op);
+                self.enforce_builtin_binop_types(lhs, lhs_ty, rhs, rhs_ty, op);
                 self.tcx.mk_unit()
             } else {
                 return_ty
             };
 
-        if !lhs_expr.is_syntactic_place_expr() {
-            struct_span_err!(
-                self.tcx.sess,
-                lhs_expr.span,
-                E0067,
-                "invalid left-hand side expression"
-            )
-            .span_label(lhs_expr.span, "invalid expression for left-hand side")
-            .emit();
-        }
+        self.check_lhs_assignable(lhs, "E0067", &op.span);
+
         ty
     }
 

src/librustc_typeck/expr_use_visitor.rs

@@ -286,7 +286,7 @@ impl<'a, 'tcx> ExprUseVisitor<'a, 'tcx> {
                 }
             }
 
-            hir::ExprKind::Assign(ref lhs, ref rhs) => {
+            hir::ExprKind::Assign(ref lhs, ref rhs, _) => {
                 self.mutate_expr(lhs);
                 self.consume_expr(rhs);
             }

src/libsyntax/ast.rs

@@ -1216,7 +1216,8 @@ pub enum ExprKind {
     TryBlock(P<Block>),
 
     /// An assignment (`a = foo()`).
-    Assign(P<Expr>, P<Expr>),
+    /// The `Span` argument is the span of the `=` token.
+    Assign(P<Expr>, P<Expr>, Span),
     /// An assignment with an operator.
     ///
     /// E.g., `a += 1`.

src/libsyntax/mut_visit.rs

@@ -1168,7 +1168,7 @@ pub fn noop_visit_expr<T: MutVisitor>(Expr { kind, id, span, attrs }: &mut Expr,
             vis.visit_block(body);
         }
         ExprKind::Await(expr) => vis.visit_expr(expr),
-        ExprKind::Assign(el, er) => {
+        ExprKind::Assign(el, er, _) => {
             vis.visit_expr(el);
             vis.visit_expr(er);
         }

src/libsyntax/print/pprust.rs

@@ -2041,7 +2041,7 @@ impl<'a> State<'a> {
                 self.print_expr_maybe_paren(expr, parser::PREC_POSTFIX);
                 self.s.word(".await");
             }
-            ast::ExprKind::Assign(ref lhs, ref rhs) => {
+            ast::ExprKind::Assign(ref lhs, ref rhs, _) => {
                 let prec = AssocOp::Assign.precedence() as i8;
                 self.print_expr_maybe_paren(lhs, prec + 1);
                 self.s.space();

src/libsyntax/util/parser.rs

@@ -378,7 +378,7 @@ pub fn contains_exterior_struct_lit(value: &ast::Expr) -> bool {
     match value.kind {
         ast::ExprKind::Struct(..) => true,
 
-        ast::ExprKind::Assign(ref lhs, ref rhs)
+        ast::ExprKind::Assign(ref lhs, ref rhs, _)
         | ast::ExprKind::AssignOp(_, ref lhs, ref rhs)
         | ast::ExprKind::Binary(_, ref lhs, ref rhs) => {
             // X { y: 1 } + X { y: 2 }

src/libsyntax/visit.rs

@@ -766,9 +766,9 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             visitor.visit_block(body);
         }
         ExprKind::Await(ref expr) => visitor.visit_expr(expr),
-        ExprKind::Assign(ref left_hand_expression, ref right_hand_expression) => {
-            visitor.visit_expr(left_hand_expression);
-            visitor.visit_expr(right_hand_expression);
+        ExprKind::Assign(ref lhs, ref rhs, _) => {
+            visitor.visit_expr(lhs);
+            visitor.visit_expr(rhs);
         }
         ExprKind::AssignOp(_, ref left_expression, ref right_expression) => {
             visitor.visit_expr(left_expression);

src/test/ui-fulldeps/pprust-expr-roundtrip.rs

@@ -126,8 +126,8 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
                                           DUMMY_SP)));
             },
             12 => {
-                iter_exprs(depth - 1, &mut |e| g(ExprKind::Assign(e, make_x())));
-                iter_exprs(depth - 1, &mut |e| g(ExprKind::Assign(make_x(), e)));
+                iter_exprs(depth - 1, &mut |e| g(ExprKind::Assign(e, make_x(), DUMMY_SP)));
+                iter_exprs(depth - 1, &mut |e| g(ExprKind::Assign(make_x(), e, DUMMY_SP)));
             },
             13 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Field(e, Ident::from_str("f"))));

src/test/ui/bad/bad-expr-lhs.rs

@@ -1,10 +1,10 @@
 fn main() {
-    1 = 2; //~ ERROR invalid left-hand side expression
-    1 += 2; //~ ERROR invalid left-hand side expression
-    (1, 2) = (3, 4); //~ ERROR invalid left-hand side expression
+    1 = 2; //~ ERROR invalid left-hand side of assignment
+    1 += 2; //~ ERROR invalid left-hand side of assignment
+    (1, 2) = (3, 4); //~ ERROR invalid left-hand side of assignment
 
     let (a, b) = (1, 2);
-    (a, b) = (3, 4); //~ ERROR invalid left-hand side expression
+    (a, b) = (3, 4); //~ ERROR invalid left-hand side of assignment
 
-    None = Some(3); //~ ERROR invalid left-hand side expression
+    None = Some(3); //~ ERROR invalid left-hand side of assignment
 }