First stab at fixing #54505

src/librustc_typeck/check/demand.rs

@@ -310,9 +310,10 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 if self.can_coerce(ref_ty, expected) {
                     if let Ok(src) = cm.span_to_snippet(sp) {
                         let sugg_expr = match expr.node { // parenthesize if needed (Issue #46756)
-                            hir::ExprKind::Cast(_, _) |
-                            hir::ExprKind::Binary(_, _, _) => format!("({})", src),
-                            _ => src,
+                            hir::ExprKind::Cast(_, _)        |
+                            hir::ExprKind::Binary(_, _, _)   |
+                            _ if self.is_range_literal(expr) => format!("({})", src),
+                            _                                => src,
                         };
                         if let Some(sugg) = self.can_use_as_ref(expr) {
                             return Some(sugg);
@@ -374,6 +375,53 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         None
     }
 
+    // This function checks if the specified expression is a built-in range literal
+    // (See: librustc/hir/lowering.rs::LoweringContext::lower_expr() )
+    fn is_range_literal(&self, expr: &hir::Expr) -> bool {
+        use hir::{Path, QPath, ExprKind, TyKind};
+
+        // TODO how to work out std vs core here?
+        let ops_path = ["{{root}}", "std", "ops"];
+
+        let is_range_path = |path: &Path| {
+            let ident_names: Vec<_> = path.segments
+                .iter()
+                .map(|seg| seg.ident.as_str())
+                .collect();
+
+            if let Some((last, preceding)) = ident_names.split_last() {
+                last.starts_with("Range") &&
+                    preceding.len() == 3 &&
+                    preceding.iter()
+                        .zip(ops_path.iter())
+                        .all(|(a, b)| a == b)
+            } else {
+                false
+            }
+        };
+
+        match expr.node {
+            ExprKind::Struct(QPath::Resolved(None, ref path), _, _) |
+            ExprKind::Path(QPath::Resolved(None, ref path)) => {
+                return is_range_path(&path);
+            }
+
+            ExprKind::Call(ref func, _) => {
+                if let ExprKind::Path(QPath::TypeRelative(ref ty, ref segment)) = func.node {
+                    if let TyKind::Path(QPath::Resolved(None, ref path)) = ty.node {
+                        let calls_new = segment.ident.as_str() == "new";
+
+                        return is_range_path(&path) && calls_new;
+                    }
+                }
+            }
+
+            _ => {}
+        }
+
+        false
+    }
+
     pub fn check_for_cast(&self,
                       err: &mut DiagnosticBuilder<'tcx>,
                       expr: &hir::Expr,

src/test/ui/range/issue-54505-no-literals.rs

@@ -0,0 +1,85 @@
+// Copyright 2018 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.
+
+// run-rustfix
+
+// Regression test for changes introduced while fixing #54505
+
+// This test uses non-literals for Ranges
+// (expecting no parens with borrow suggestion)
+
+use std::ops::RangeBounds;
+
+
+// take a reference to any built-in range
+fn take_range(_r: &impl RangeBounds<i8>) {}
+
+
+fn main() {
+    take_range(std::ops::Range { start: 0, end: 1 });
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &std::ops::Range { start: 0, end: 1 }
+
+    take_range(::std::ops::Range { start: 0, end: 1 });
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &::std::ops::Range { start: 0, end: 1 }
+
+    take_range(std::ops::RangeFrom { start: 1 });
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &std::ops::RangeFrom { start: 1 }
+
+    take_range(::std::ops::RangeFrom { start: 1 });
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &::std::ops::RangeFrom { start: 1 }
+
+    take_range(std::ops::RangeFull {});
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &std::ops::RangeFull {}
+
+    take_range(::std::ops::RangeFull {});
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &::std::ops::RangeFull {}
+
+    take_range(std::ops::RangeInclusive::new(0, 1));
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &std::ops::RangeInclusive::new(0, 1)
+
+    take_range(::std::ops::RangeInclusive::new(0, 1));
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &::std::ops::RangeInclusive::new(0, 1)
+
+    take_range(std::ops::RangeTo { end: 5 });
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &std::ops::RangeTo { end: 5 }
+
+    take_range(::std::ops::RangeTo { end: 5 });
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &::std::ops::RangeTo { end: 5 }
+
+    take_range(std::ops::RangeToInclusive { end: 5 });
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &std::ops::RangeToInclusive { end: 5 }
+
+    take_range(::std::ops::RangeToInclusive { end: 5 });
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &::std::ops::RangeToInclusive { end: 5 }
+}

src/test/ui/range/issue-54505-no-std.rs

@@ -0,0 +1,61 @@
+// Copyright 2018 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.
+
+// run-rustfix
+// error-pattern: error: `#[panic_handler]` function required, but not found
+// error-pattern: language item required, but not found: `panic_info`
+
+
+// Regression test for #54505 - range borrowing suggestion had
+// incorrect syntax (missing parentheses).
+
+// This test doesn't use std
+// (so all Ranges resolve to core::ops::Range...)
+
+#![no_std]
+
+use core::ops::RangeBounds;
+
+
+// take a reference to any built-in range
+fn take_range(_r: &impl RangeBounds<i8>) {}
+
+
+fn main() {
+    take_range(0..1);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(0..1)
+
+    take_range(1..);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(1..)
+
+    take_range(..);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(..)
+
+    take_range(0..=1);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(0..=1)
+
+    take_range(..5);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(..5)
+
+    take_range(..=42);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(..=42)
+}

src/test/ui/range/issue-54505.fixed

@@ -0,0 +1,53 @@
+// Copyright 2018 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.
+
+// run-rustfix
+
+// Regression test for #54505 - range borrowing suggestion had
+// incorrect syntax (missing parentheses).
+
+use std::ops::RangeBounds;
+
+
+// take a reference to any built-in range
+fn take_range(_r: &impl RangeBounds<i8>) {}
+
+
+fn main() {
+    take_range(&(0..1));
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(0..1)
+
+    take_range(&(1..));
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(1..)
+
+    take_range(&(..));
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(..)
+
+    take_range(&(0..=1));
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(0..=1)
+
+    take_range(&(..5));
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(..5)
+
+    take_range(&(..=42));
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(..=42)
+}

src/test/ui/range/issue-54505.rs

@@ -0,0 +1,53 @@
+// Copyright 2018 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.
+
+// run-rustfix
+
+// Regression test for #54505 - range borrowing suggestion had
+// incorrect syntax (missing parentheses).
+
+use std::ops::RangeBounds;
+
+
+// take a reference to any built-in range
+fn take_range(_r: &impl RangeBounds<i8>) {}
+
+
+fn main() {
+    take_range(0..1);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(0..1)
+
+    take_range(1..);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(1..)
+
+    take_range(..);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(..)
+
+    take_range(0..=1);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(0..=1)
+
+    take_range(..5);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(..5)
+
+    take_range(..=42);
+    //~^ ERROR mismatched types [E0308]
+    //~| HELP consider borrowing here
+    //~| SUGGESTION &(..=42)
+}

src/test/ui/range/issue-54505.stderr

@@ -0,0 +1,75 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-54505.rs:24:16
+   |
+LL |     take_range(0..1);
+   |                ^^^^
+   |                |
+   |                expected reference, found struct `std::ops::Range`
+   |                help: consider borrowing here: `&(0..1)`
+   |
+   = note: expected type `&_`
+              found type `std::ops::Range<{integer}>`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-54505.rs:29:16
+   |
+LL |     take_range(1..);
+   |                ^^^
+   |                |
+   |                expected reference, found struct `std::ops::RangeFrom`
+   |                help: consider borrowing here: `&(1..)`
+   |
+   = note: expected type `&_`
+              found type `std::ops::RangeFrom<{integer}>`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-54505.rs:34:16
+   |
+LL |     take_range(..);
+   |                ^^
+   |                |
+   |                expected reference, found struct `std::ops::RangeFull`
+   |                help: consider borrowing here: `&(..)`
+   |
+   = note: expected type `&_`
+              found type `std::ops::RangeFull`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-54505.rs:39:16
+   |
+LL |     take_range(0..=1);
+   |                ^^^^^
+   |                |
+   |                expected reference, found struct `std::ops::RangeInclusive`
+   |                help: consider borrowing here: `&(0..=1)`
+   |
+   = note: expected type `&_`
+              found type `std::ops::RangeInclusive<{integer}>`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-54505.rs:44:16
+   |
+LL |     take_range(..5);
+   |                ^^^
+   |                |
+   |                expected reference, found struct `std::ops::RangeTo`
+   |                help: consider borrowing here: `&(..5)`
+   |
+   = note: expected type `&_`
+              found type `std::ops::RangeTo<{integer}>`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-54505.rs:49:16
+   |
+LL |     take_range(..=42);
+   |                ^^^^^
+   |                |
+   |                expected reference, found struct `std::ops::RangeToInclusive`
+   |                help: consider borrowing here: `&(..=42)`
+   |
+   = note: expected type `&_`
+              found type `std::ops::RangeToInclusive<{integer}>`
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0308`.