Rollup merge of rust-lang#36171 - jonathandturner:temporary_value, r=nikomatsakis

Update lifetime errors to specifically note temporaries

This PR updates the error message we give in the case of a temporary value not living long enough.

Before:

<img width="497" alt="screen shot 2016-08-31 at 10 02 47 am" src="https://cloud.githubusercontent.com/assets/547158/18138551/27a06794-6f62-11e6-9ee2-bdf8bed75ca7.png">

Now:

<img width="488" alt="screen shot 2016-08-31 at 10 03 01 am" src="https://cloud.githubusercontent.com/assets/547158/18138557/2e5cf322-6f62-11e6-9047-4a78abf3d78c.png">

Specifically, it makes the following changes:

* Detects if a temporary is being used.  If so, it changes the labels to mention that a temporary value specifically is in question
* Simplifies wording of the existing labels to focus on lifetimes rather than values being valid
* Changes the help to a note, since the help+span wasn't as helpful (and sometimes more confusing) than just a note.

r? @nikomatsakis

Author: Jonathan Turner

src/librustc_borrowck/borrowck/mod.rs

@@ -1028,6 +1028,12 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
             }
 
             err_out_of_scope(super_scope, sub_scope, cause) => {
+                let (value_kind, value_msg) = match err.cmt.cat {
+                    mc::Categorization::Rvalue(_) =>
+                        ("temporary value", "temporary value created here"),
+                    _ =>
+                        ("borrowed value", "does not live long enough")
+                };
                 match cause {
                     euv::ClosureCapture(s) => {
                         // The primary span starts out as the closure creation point.
@@ -1038,13 +1044,13 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                             Some(primary) => {
                                 db.span = MultiSpan::from_span(s);
                                 db.span_label(primary, &format!("capture occurs here"));
-                                db.span_label(s, &format!("does not live long enough"));
+                                db.span_label(s, &value_msg);
                             }
                             None => ()
                         }
                     }
                     _ => {
-                        db.span_label(error_span, &format!("does not live long enough"));
+                        db.span_label(error_span, &value_msg);
                     }
                 }
 
@@ -1053,14 +1059,15 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
 
                 match (sub_span, super_span) {
                     (Some(s1), Some(s2)) if s1 == s2 => {
-                        db.span_label(s1, &"borrowed value dropped before borrower");
+                        db.span_label(s1, &format!("{} dropped before borrower", value_kind));
                         db.note("values in a scope are dropped in the opposite order \
                                 they are created");
                     }
                     _ => {
                         match sub_span {
                             Some(s) => {
-                                db.span_label(s, &"borrowed value must be valid until here");
+                                db.span_label(s, &format!("{} needs to live until here",
+                                                          value_kind));
                             }
                             None => {
                                 self.tcx.note_and_explain_region(
@@ -1072,7 +1079,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                         }
                         match super_span {
                             Some(s) => {
-                                db.span_label(s, &"borrowed value only valid until here");
+                                db.span_label(s, &format!("{} only lives until here", value_kind));
                             }
                             None => {
                                 self.tcx.note_and_explain_region(
@@ -1085,9 +1092,8 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     }
                 }
 
-                if let Some(span) = statement_scope_span(self.tcx, super_scope) {
-                    db.span_help(span,
-                                 "consider using a `let` binding to increase its lifetime");
+                if let Some(_) = statement_scope_span(self.tcx, super_scope) {
+                    db.note("consider using a `let` binding to increase its lifetime");
                 }
             }
 

src/test/compile-fail/borrowck/borrowck-let-suggestion-suffixes.rs

@@ -25,36 +25,36 @@ fn f() {
 
     v3.push(&'x');           // statement 6
     //~^ ERROR borrowed value does not live long enough
-    //~| NOTE does not live long enough
-    //~| NOTE borrowed value only valid until here
-    //~| HELP consider using a `let` binding to increase its lifetime
+    //~| NOTE temporary value created here
+    //~| NOTE temporary value only lives until here
+    //~| NOTE consider using a `let` binding to increase its lifetime
 
     {
 
         let mut v4 = Vec::new(); // (sub) statement 0
 
         v4.push(&'y');
         //~^ ERROR borrowed value does not live long enough
-        //~| NOTE does not live long enough
-        //~| NOTE borrowed value only valid until here
-        //~| HELP consider using a `let` binding to increase its lifetime
+        //~| NOTE temporary value created here
+        //~| NOTE temporary value only lives until here
+        //~| NOTE consider using a `let` binding to increase its lifetime
 
     }                       // (statement 7)
-    //~^ NOTE borrowed value must be valid until here
+    //~^ NOTE temporary value needs to live until here
 
     let mut v5 = Vec::new(); // statement 8
 
     v5.push(&'z');
     //~^ ERROR borrowed value does not live long enough
-    //~| NOTE does not live long enough
-    //~| NOTE borrowed value only valid until here
-    //~| HELP consider using a `let` binding to increase its lifetime
+    //~| NOTE temporary value created here
+    //~| NOTE temporary value only lives until here
+    //~| NOTE consider using a `let` binding to increase its lifetime
 
     v1.push(&old[0]);
 }
 //~^ NOTE borrowed value dropped before borrower
-//~| NOTE borrowed value must be valid until here
-//~| NOTE borrowed value must be valid until here
+//~| NOTE temporary value needs to live until here
+//~| NOTE temporary value needs to live until here
 
 fn main() {
     f();

src/test/compile-fail/regions-escape-loop-via-vec.rs

@@ -24,8 +24,8 @@ fn broken() {
         x += 1; //~ ERROR cannot assign
         //~^ NOTE assignment to borrowed `x` occurs here
     }
-    //~^ NOTE borrowed value only valid until here
+    //~^ NOTE borrowed value only lives until here
 }
-//~^ NOTE borrowed value must be valid until here
+//~^ NOTE borrowed value needs to live until here
 
 fn main() { }

src/test/compile-fail/borrowck/borrowck-let-suggestion.rs src/test/ui/lifetimes/borrowck-let-suggestion.rs

@@ -10,12 +10,7 @@
 
 fn f() {
     let x = [1].iter();
-    //~^ ERROR borrowed value does not live long enough
-    //~| NOTE does not live long enough
-    //~| NOTE borrowed value only valid until here
-    //~| HELP consider using a `let` binding to increase its lifetime
 }
-//~^ borrowed value must be valid until here
 
 fn main() {
     f();

src/test/ui/lifetimes/borrowck-let-suggestion.stderr

@@ -0,0 +1,14 @@
+error: borrowed value does not live long enough
+  --> $DIR/borrowck-let-suggestion.rs:12:13
+   |
+12 |     let x = [1].iter();
+   |             ^^^       - temporary value only lives until here
+   |             |
+   |             temporary value created here
+13 | }
+   | - temporary value needs to live until here
+   |
+   = note: consider using a `let` binding to increase its lifetime
+
+error: aborting due to previous error
+

src/test/ui/span/issue-11925.stderr

@@ -5,10 +5,10 @@ error: `x` does not live long enough
    |                                    ^
    |                                    |
    |                                    does not live long enough
-   |                                    borrowed value only valid until here
+   |                                    borrowed value only lives until here
 ...
 23 | }
-   | - borrowed value must be valid until here
+   | - borrowed value needs to live until here
 
 error: aborting due to previous error