Rollup merge of rust-lang#67538 - varkor:lhs-assign-diagnostics, r=Centril

Improve diagnostics for invalid assignment

- Improve wording and span information for invalid assignment diagnostics.
- Link to rust-lang/rfcs#372 when it appears the user is trying a destructuring assignment.
- Make the equality constraint in `where` clauses error consistent with the invalid assignment error.

Author: Centril

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

@@ -1690,7 +1690,8 @@ 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;