Auto merge of #52106 - PramodBisht:issue/52049, r=oli-obk

Don't suggest `let` bindings if they don't help with borrows

@oli-obk I have added a condition to address #52049, right now, this is on WIP because I think code change is also required on `error_reporting.rs`. Plus I need to check if any test cases fail.
I will ping you again if everything passes

r? @oli-obk

src/librustc_borrowck/borrowck/mod.rs

@@ -1011,7 +1011,10 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     let node_id = scope.node_id(self.tcx, &self.region_scope_tree);
                     match self.tcx.hir.find(node_id) {
                         Some(hir_map::NodeStmt(_)) => {
-                            db.note("consider using a `let` binding to increase its lifetime");
+                            if *sub_scope != ty::ReStatic {
+                                db.note("consider using a `let` binding to increase its lifetime");
+                            }
+
                         }
                         _ => {}
                     }

src/test/ui/suggestions/issue-52049.nll.stderr

@@ -0,0 +1,13 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/issue-52049.rs:16:10
+   |
+LL |     foo(&unpromotable(5u32));
+   |          ^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
+LL | }
+   | - temporary value only lives until here
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.

src/test/ui/suggestions/issue-52049.rs

@@ -0,0 +1,18 @@
+// 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.
+
+fn foo(_: &'static u32) {}
+
+fn unpromotable<T>(t: T) -> T { t }
+
+fn main() {
+    foo(&unpromotable(5u32));
+}
+//~^^ ERROR borrowed value does not live long enough

src/test/ui/suggestions/issue-52049.stderr

@@ -0,0 +1,13 @@
+error[E0597]: borrowed value does not live long enough
+  --> $DIR/issue-52049.rs:16:10
+   |
+LL |     foo(&unpromotable(5u32));
+   |          ^^^^^^^^^^^^^^^^^^ - temporary value only lives until here
+   |          |
+   |          temporary value does not live long enough
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.