improve heuristics for what makes an interesting constraint

nikomatsakis · nikomatsakis · commit 0d918a8575cf · 2018-07-25T16:36:03.000+03:00
diff --git a/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs
@@ -104,7 +104,48 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             categorized_path
         );
 
-        // Find what appears to be the most interesting path to report to the user.
+        // To find the best span to cite, we first try to look for the
+        // final constraint that is interesting and where the `sup` is
+        // not unified with the ultimate target region. The reason
+        // for this is that we have a chain of constraints that lead
+        // from the source to the target region, something like:
+        //
+        //    '0: '1 ('0 is the source)
+        //    '1: '2
+        //    '2: '3
+        //    '3: '4
+        //    '4: '5
+        //    '5: '6 ('6 is the target)
+        //
+        // Some of those regions are unified with `'6` (in the same
+        // SCC).  We want to screen those out. After that point, the
+        // "closest" constraint we have to the end is going to be the
+        // most likely to be the point where the value escapes -- but
+        // we still want to screen for an "interesting" point to
+        // highlight (e.g., a call site or something).
+        let target_scc = self.constraint_sccs.scc(target_region);
+        let best_choice = (0..path.len()).rev().find(|&i| {
+            let constraint = &self.constraints[path[i]];
+
+            let constraint_sup_scc = self.constraint_sccs.scc(constraint.sup);
+            if constraint_sup_scc == target_scc {
+                return false;
+            }
+
+            match categorized_path[i].0 {
+                ConstraintCategory::Boring => false,
+                _ => true,
+            }
+        });
+        if let Some(i) = best_choice {
+            let (category, span) = categorized_path[i];
+            return (category, span, target_region);
+        }
+
+        // If that search fails, that is.. unusual. Maybe everything
+        // is in the same SCC or something. In that case, find what
+        // appears to be the most interesting point to report to the
+        // user via an even more ad-hoc guess.
         categorized_path.sort_by(|p0, p1| p0.0.cmp(&p1.0));
         debug!("best_blame_constraint: sorted_path={:#?}", categorized_path);
 
diff --git a/src/test/ui/impl-trait/static-return-lifetime-infered.nll.stderr b/src/test/ui/impl-trait/static-return-lifetime-infered.nll.stderr
@@ -10,19 +10,21 @@ warning: not reporting region error due to nll
 LL |         self.x.iter().map(|a| a.0)
    |                ^^^^
 
-error: unsatisfied lifetime constraints
+error: borrowed data escapes outside of closure
   --> $DIR/static-return-lifetime-infered.rs:17:9
    |
 LL |     fn iter_values_anon(&self) -> impl Iterator<Item=u32> {
-   |                         - let's call the lifetime of this reference `'1`
+   |                         ----- `self` is a reference that is only valid in the closure body
 LL |         self.x.iter().map(|a| a.0)
-   |         ^^^^^^ cast requires that `'1` must outlive `'static`
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ `self` escapes the closure body here
 
-error: unsatisfied lifetime constraints
+error: borrowed data escapes outside of closure
   --> $DIR/static-return-lifetime-infered.rs:21:9
    |
+LL |     fn iter_values<'a>(&'a self) -> impl Iterator<Item=u32> {
+   |                        -------- `self` is a reference that is only valid in the closure body
 LL |         self.x.iter().map(|a| a.0)
-   |         ^^^^^^ cast requires that `'a` must outlive `'static`
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ `self` escapes the closure body here
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/issue-40288-2.nll.stderr b/src/test/ui/issue-40288-2.nll.stderr
@@ -35,22 +35,22 @@ LL |     let mut out = Struct { head: x, _tail: [()] };
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0621]: explicit lifetime required in the type of `y`
-  --> $DIR/issue-40288-2.rs:16:31
+  --> $DIR/issue-40288-2.rs:17:9
    |
 LL | fn lifetime_transmute_slice<'a, T: ?Sized>(x: &'a T, y: &T) -> &'a T {
    |                                                      - consider changing the type of `y` to `&'a T`
 ...
-LL |         let slice: &mut [_] = &mut out;
-   |                               ^^^^^^^^ lifetime `'a` required
+LL |         slice[0] = y;
+   |         ^^^^^^^^^^^^ lifetime `'a` required
 
 error[E0621]: explicit lifetime required in the type of `y`
-  --> $DIR/issue-40288-2.rs:31:41
+  --> $DIR/issue-40288-2.rs:32:9
    |
 LL | fn lifetime_transmute_struct<'a, T: ?Sized>(x: &'a T, y: &T) -> &'a T {
    |                                                       - consider changing the type of `y` to `&'a T`
 ...
-LL |         let dst: &mut Struct<_, [()]> = &mut out;
-   |                                         ^^^^^^^^ lifetime `'a` required
+LL |         dst.head = y;
+   |         ^^^^^^^^^^^^ lifetime `'a` required
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/lifetime-errors/ex3-both-anon-regions-3.nll.stderr b/src/test/ui/lifetime-errors/ex3-both-anon-regions-3.nll.stderr
@@ -11,20 +11,20 @@ LL |     z.push((x,y)); //~ ERROR lifetime mismatch
    |               ^
 
 error[E0623]: lifetime mismatch
-  --> $DIR/ex3-both-anon-regions-3.rs:11:33
+  --> $DIR/ex3-both-anon-regions-3.rs:12:5
    |
 LL | fn foo(z: &mut Vec<(&u8,&u8)>, (x, y): (&u8, &u8)) {
-   |                     ---         ^       --- these two types are declared with different lifetimes...
-   |                                 |
-   |                                 ...but data flows into `z` here
+   |                     ---                 --- these two types are declared with different lifetimes...
+LL |     z.push((x,y)); //~ ERROR lifetime mismatch
+   |     ^^^^^^^^^^^^^ ...but data flows into `z` here
 
 error[E0623]: lifetime mismatch
-  --> $DIR/ex3-both-anon-regions-3.rs:11:36
+  --> $DIR/ex3-both-anon-regions-3.rs:12:5
    |
 LL | fn foo(z: &mut Vec<(&u8,&u8)>, (x, y): (&u8, &u8)) {
-   |                         ---        ^         --- these two types are declared with different lifetimes...
-   |                                    |
-   |                                    ...but data flows into `z` here
+   |                         ---                  --- these two types are declared with different lifetimes...
+LL |     z.push((x,y)); //~ ERROR lifetime mismatch
+   |     ^^^^^^^^^^^^^ ...but data flows into `z` here
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.rs b/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.rs
@@ -52,9 +52,9 @@ fn supply<'a, 'b, 'c>(cell_a: Cell<&'a u32>, cell_b: Cell<&'b u32>, cell_c: Cell
         cell_c,
         |_outlives1, _outlives2, _outlives3, x, y| {
             // Only works if 'x: 'y:
-            let p = x.get(); //~ ERROR
+            let p = x.get();
             //~^ WARN not reporting region error due to nll
-            demand_y(x, y, p)
+            demand_y(x, y, p) //~ ERROR
         },
     );
 }
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.stderr b/src/test/ui/nll/closure-requirements/propagate-approximated-fail-no-postdom.stderr
@@ -1,28 +1,28 @@
 warning: not reporting region error due to nll
   --> $DIR/propagate-approximated-fail-no-postdom.rs:55:21
    |
-LL |             let p = x.get(); //~ ERROR
+LL |             let p = x.get();
    |                     ^^^^^^^
 
 error: unsatisfied lifetime constraints
-  --> $DIR/propagate-approximated-fail-no-postdom.rs:55:21
+  --> $DIR/propagate-approximated-fail-no-postdom.rs:57:13
    |
 LL |         |_outlives1, _outlives2, _outlives3, x, y| {
    |          ----------              ---------- lifetime `'2` appears in this argument
    |          |
    |          lifetime `'1` appears in this argument
-LL |             // Only works if 'x: 'y:
-LL |             let p = x.get(); //~ ERROR
-   |                     ^^^^^^^ argument requires that `'1` must outlive `'2`
+...
+LL |             demand_y(x, y, p) //~ ERROR
+   |             ^^^^^^^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
 
 note: No external requirements
   --> $DIR/propagate-approximated-fail-no-postdom.rs:53:9
    |
 LL | /         |_outlives1, _outlives2, _outlives3, x, y| {
 LL | |             // Only works if 'x: 'y:
-LL | |             let p = x.get(); //~ ERROR
+LL | |             let p = x.get();
 LL | |             //~^ WARN not reporting region error due to nll
-LL | |             demand_y(x, y, p)
+LL | |             demand_y(x, y, p) //~ ERROR
 LL | |         },
    | |_________^
    |
diff --git a/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.stderr b/src/test/ui/nll/closure-requirements/propagate-approximated-shorter-to-static-comparing-against-free.stderr
@@ -5,15 +5,15 @@ LL |     foo(cell, |cell_a, cell_x| {
    |     ^^^
 
 error: borrowed data escapes outside of closure
-  --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:33:20
+  --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:33:9
    |
 LL |     foo(cell, |cell_a, cell_x| {
    |                ------  ------ `cell_x` is a reference that is only valid in the closure body
    |                |
    |                `cell_a` is declared here, outside of the closure body
 LL |         //~^ WARNING not reporting region error due to nll
 LL |         cell_a.set(cell_x.get()); // forces 'x: 'a, error in closure
-   |                    ^^^^^^^^^^^^ `cell_x` escapes the closure body here
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^ `cell_x` escapes the closure body here
 
 note: No external requirements
   --> $DIR/propagate-approximated-shorter-to-static-comparing-against-free.rs:31:15
diff --git a/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.stderr b/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-no-bounds.stderr
@@ -5,15 +5,15 @@ LL |         demand_y(x, y, x.get())
    |         ^^^^^^^^^^^^^^^^^^^^^^^
 
 error: unsatisfied lifetime constraints
-  --> $DIR/propagate-fail-to-approximate-longer-no-bounds.rs:47:24
+  --> $DIR/propagate-fail-to-approximate-longer-no-bounds.rs:47:9
    |
 LL |     establish_relationships(&cell_a, &cell_b, |_outlives, x, y| {
    |                                                ---------  - lifetime `'1` appears in this argument
    |                                                |
    |                                                lifetime `'2` appears in this argument
 LL |         // Only works if 'x: 'y:
 LL |         demand_y(x, y, x.get())
-   |                        ^^^^^^^ argument requires that `'1` must outlive `'2`
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
 
 note: No external requirements
   --> $DIR/propagate-fail-to-approximate-longer-no-bounds.rs:45:47
diff --git a/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.stderr b/src/test/ui/nll/closure-requirements/propagate-fail-to-approximate-longer-wrong-bounds.stderr
@@ -5,15 +5,15 @@ LL |         demand_y(x, y, x.get())
    |         ^^^^^^^^^^^^^^^^^^^^^^^
 
 error: unsatisfied lifetime constraints
-  --> $DIR/propagate-fail-to-approximate-longer-wrong-bounds.rs:51:24
+  --> $DIR/propagate-fail-to-approximate-longer-wrong-bounds.rs:51:9
    |
 LL |     establish_relationships(&cell_a, &cell_b, |_outlives1, _outlives2, x, y| {
    |                                                ----------  ---------- lifetime `'2` appears in this argument
    |                                                |
    |                                                lifetime `'1` appears in this argument
 LL |         // Only works if 'x: 'y:
 LL |         demand_y(x, y, x.get())
-   |                        ^^^^^^^ argument requires that `'1` must outlive `'2`
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ argument requires that `'1` must outlive `'2`
 
 note: No external requirements
   --> $DIR/propagate-fail-to-approximate-longer-wrong-bounds.rs:49:47
diff --git a/src/test/ui/underscore-lifetime/dyn-trait-underscore.nll.stderr b/src/test/ui/underscore-lifetime/dyn-trait-underscore.nll.stderr
@@ -23,13 +23,16 @@ LL |     Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
 error: unsatisfied lifetime constraints
-  --> $DIR/dyn-trait-underscore.rs:18:5
+  --> $DIR/dyn-trait-underscore.rs:16:52
    |
-LL | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item=&T>> {
-   |                - let's call the lifetime of this reference `'1`
-LL |     //                      ^^^^^^^^^^^^^^^^^^^^^ bound *here* defaults to `'static`
-LL |     Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
-   |     ^^^^^^^^^^^^^^^^^^^^^^ cast requires that `'1` must outlive `'static`
+LL |   fn a<T>(items: &[T]) -> Box<dyn Iterator<Item=&T>> {
+   |  ________________-___________________________________^
+   | |                |
+   | |                let's call the lifetime of this reference `'1`
+LL | |     //                      ^^^^^^^^^^^^^^^^^^^^^ bound *here* defaults to `'static`
+LL | |     Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
+LL | | }
+   | |_^ return requires that `'1` must outlive `'static`
 
 error: aborting due to previous error
 
