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

Improve diagnostics for invalid assignment

- Improve wording and span information for invalid assignment diagnostics.
- Link to https://github.com/rust-lang/rfcs/issues/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.

5 files changed, 8 insertions, 7 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 9009faa5440..c98942abaf3 100644
--- a/src/libsyntax/ast.rs
+++ b/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`.
diff --git a/src/libsyntax/mut_visit.rs b/src/libsyntax/mut_visit.rs
index 780323d114e..f6817c713a4 100644
--- a/src/libsyntax/mut_visit.rs
+++ b/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);
         }
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index dc01f2472b7..b1b667f03be 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/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();
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index 88e3d8daf70..98af382efb0 100644
--- a/src/libsyntax/util/parser.rs
+++ b/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 }
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index d6573a06647..fbc5d133249 100644
--- a/src/libsyntax/visit.rs
+++ b/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);