Pretty print parens around casts on the LHS of </<<

src/librustc/hir/print.rs

@@ -1253,6 +1253,15 @@ impl<'a> State<'a> {
             Fixity::None => (prec + 1, prec + 1),
         };
 
+        let left_prec = match (&lhs.node, op.node) {
+            // These cases need parens: `x as i32 < y` has the parser thinking that `i32 < y` is
+            // the beginning of a path type. It starts trying to parse `x as (i32 < y ...` instead
+            // of `(x as i32) < ...`. We need to convince it _not_ to do that.
+            (&hir::ExprCast { .. }, hir::BinOp_::BiLt) |
+            (&hir::ExprCast { .. }, hir::BinOp_::BiShl) => parser::PREC_FORCE_PAREN,
+            _ => left_prec,
+        };
+
         self.print_expr_maybe_paren(lhs, left_prec)?;
         self.s.space()?;
         self.word_space(op.node.as_str())?;

src/libsyntax/print/pprust.rs

@@ -1986,6 +1986,15 @@ impl<'a> State<'a> {
             Fixity::None => (prec + 1, prec + 1),
         };
 
+        let left_prec = match (&lhs.node, op.node) {
+            // These cases need parens: `x as i32 < y` has the parser thinking that `i32 < y` is
+            // the beginning of a path type. It starts trying to parse `x as (i32 < y ...` instead
+            // of `(x as i32) < ...`. We need to convince it _not_ to do that.
+            (&ast::ExprKind::Cast { .. }, ast::BinOpKind::Lt) |
+            (&ast::ExprKind::Cast { .. }, ast::BinOpKind::Shl) => parser::PREC_FORCE_PAREN,
+            _ => left_prec,
+        };
+
         self.print_expr_maybe_paren(lhs, left_prec)?;
         self.s.space()?;
         self.word_space(op.node.to_string())?;

src/test/pretty/cast-lt.pp

@@ -0,0 +1,24 @@
+#![feature(prelude_import)]
+#![no_std]
+#[prelude_import]
+use std::prelude::v1::*;
+#[macro_use]
+extern crate std as std;
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// pretty-compare-only
+// pretty-mode:expanded
+// pp-exact:cast-lt.pp
+
+macro_rules! negative(( $ e : expr ) => { $ e < 0 });
+
+fn main() { (1 as i32) < 0; }
+

src/test/pretty/cast-lt.rs

@@ -0,0 +1,22 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// pretty-compare-only
+// pretty-mode:expanded
+// pp-exact:cast-lt.pp
+
+macro_rules! negative {
+      ($e:expr) => { $e < 0 }
+}
+
+fn main() {
+      negative!(1 as i32);
+}
+