Bring back suggestion for splitting <- into < -

Closes #62632

src/libsyntax/parse/parser/expr.rs

@@ -224,6 +224,10 @@ impl<'a> Parser<'a> {
                 self.err_dotdotdot_syntax(self.token.span);
             }
 
+            if self.token == token::LArrow {
+                self.err_larrow_operator(self.token.span);
+            }
+
             self.bump();
             if op.is_comparison() {
                 self.check_no_chained_comparison(&lhs, &op);
@@ -1702,6 +1706,19 @@ impl<'a> Parser<'a> {
             .emit();
     }
 
+    fn err_larrow_operator(&self, span: Span) {
+        self.struct_span_err(
+            span,
+            "unexpected token: `<-`"
+        ).span_suggestion(
+            span,
+            "if you meant to write a comparison against a negative value, add a \
+             space in between `<` and `-`",
+            "< -".to_string(),
+            Applicability::MaybeIncorrect
+        ).emit();
+    }
+
     fn mk_assign_op(&self, binop: BinOp, lhs: P<Expr>, rhs: P<Expr>) -> ExprKind {
         ExprKind::AssignOp(binop, lhs, rhs)
     }

src/libsyntax/util/parser.rs

@@ -97,6 +97,8 @@ impl AssocOp {
             // DotDotDot is no longer supported, but we need some way to display the error
             token::DotDotDot => Some(DotDotEq),
             token::Colon => Some(Colon),
+            // `<-` should probably be `< -`
+            token::LArrow => Some(Less),
             _ if t.is_keyword(kw::As) => Some(As),
             _ => None
         }

src/test/ui/obsolete-in-place/bad.rs

@@ -2,7 +2,7 @@
 
 fn foo() {
     let (x, y) = (0, 0);
-    x <- y; //~ ERROR expected one of
+    x <- y; //~ ERROR unexpected token: `<-`
 }
 
 fn main() {

src/test/ui/obsolete-in-place/bad.stderr

@@ -1,8 +1,12 @@
-error: expected one of `!`, `.`, `::`, `;`, `?`, `{`, `}`, or an operator, found `<-`
+error: unexpected token: `<-`
   --> $DIR/bad.rs:5:7
    |
 LL |     x <- y;
-   |       ^^ expected one of 8 possible tokens here
+   |       ^^
+help: if you meant to write a comparison against a negative value, add a space in between `<` and `-`
+   |
+LL |     x < - y;
+   |       ^^^
 
 error: expected expression, found keyword `in`
   --> $DIR/bad.rs:10:5

src/test/ui/placement-syntax.rs

@@ -1,6 +1,6 @@
 fn main() {
     let x = -5;
-    if x<-1 { //~ ERROR expected `{`, found `<-`
+    if x<-1 { //~ ERROR unexpected token: `<-`
         println!("ok");
     }
 }

src/test/ui/placement-syntax.stderr

@@ -1,10 +1,12 @@
-error: expected `{`, found `<-`
+error: unexpected token: `<-`
   --> $DIR/placement-syntax.rs:3:9
    |
 LL |     if x<-1 {
-   |     --  ^^ expected `{`
+   |         ^^
-   |     |
+help: if you meant to write a comparison against a negative value, add a space in between `<` and `-`
-   |     this `if` statement has a condition, but no block
+   |
+LL |     if x< -1 {
+   |         ^^^
 
 error: aborting due to previous error