trace span info for constraints and report errors

Author: nikomatsakis

src/librustc_mir/transform/nll/constraint_generation.rs

@@ -191,14 +191,16 @@ impl<'cx, 'gcx, 'tcx> ConstraintGeneration<'cx, 'gcx, 'tcx> {
         _borrowed_lv: &Lvalue<'tcx>,
     ) {
         let tcx = self.infcx.tcx;
+        let span = self.mir.source_info(location).span;
         let destination_ty = destination_lv.ty(self.mir, tcx).to_ty(tcx);
 
         let destination_region = match destination_ty.sty {
             ty::TyRef(r, _) => r,
             _ => bug!()
         };
 
-        self.regioncx.add_outlives(borrow_region.to_region_index(),
+        self.regioncx.add_outlives(span,
+                                   borrow_region.to_region_index(),
                                    destination_region.to_region_index(),
                                    location.successor_within_block());
     }
@@ -226,7 +228,9 @@ impl<'cx, 'gcx, 'tcx> ConstraintGeneration<'cx, 'gcx, 'tcx> {
                         },
                     }
 
-                    self.regioncx.add_outlives(base_region.to_region_index(),
+                    let span = self.mir.source_info(location).span;
+                    self.regioncx.add_outlives(span,
+                                               base_region.to_region_index(),
                                                borrow_region.to_region_index(),
                                                location.successor_within_block());
                 }
@@ -259,8 +263,9 @@ impl<'cx, 'gcx, 'tcx> Visitor<'tcx> for ConstraintGeneration<'cx, 'gcx, 'tcx> {
             let destination_ty = destination_lv.ty(self.mir, tcx).to_ty(tcx);
             let rv_ty = rv.ty(self.mir, tcx);
 
+            let span = self.mir.source_info(location).span;
             for (a, b) in subtype::outlives_pairs(tcx, rv_ty, destination_ty) {
-                self.regioncx.add_outlives(a, b, location.successor_within_block());
+                self.regioncx.add_outlives(span, a, b, location.successor_within_block());
             }
         }
 

src/librustc_mir/transform/nll/region_infer.rs

@@ -17,6 +17,7 @@ use rustc_data_structures::indexed_vec::{Idx, IndexVec};
 use rustc_data_structures::fx::FxHashSet;
 use std::collections::BTreeSet;
 use std::fmt;
+use syntax_pos::Span;
 
 pub struct RegionInferenceContext<'tcx> {
     /// Contains the definition for every region variable.  Region
@@ -70,10 +71,11 @@ struct Region {
 
 impl fmt::Debug for Region {
     fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
-        formatter.debug_set()
-                 .entries(&self.points)
-                 .entries(&self.free_regions)
-                 .finish()
+        formatter
+            .debug_set()
+            .entries(&self.points)
+            .entries(&self.free_regions)
+            .finish()
     }
 }
 
@@ -93,8 +95,16 @@ impl Region {
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Constraint {
-    sub: RegionIndex,
+    /// Where did this constraint arise?
+    span: Span,
+
+    /// The region SUP must outlive SUB...
     sup: RegionIndex,
+
+    /// Region that must be outlived.
+    sub: RegionIndex,
+
+    /// At this location.
     point: Location,
 }
 
@@ -210,56 +220,116 @@ impl<'a, 'gcx, 'tcx> RegionInferenceContext<'tcx> {
     pub(super) fn add_live_point(&mut self, v: RegionIndex, point: Location) {
         debug!("add_live_point({:?}, {:?})", v, point);
         let definition = &mut self.definitions[v];
-        definition.value.add_point(point);
+        if !definition.constant {
+            definition.value.add_point(point);
+        } else {
+            // Constants are used for free regions, which already
+            // contain all the points in the control-flow graph.
+            assert!(definition.value.contains_point(point));
+        }
     }
 
     /// Indicates that the region variable `sup` must outlive `sub` is live at the point `point`.
-    pub(super) fn add_outlives(&mut self, sup: RegionIndex, sub: RegionIndex, point: Location) {
+    pub(super) fn add_outlives(
+        &mut self,
+        span: Span,
+        sup: RegionIndex,
+        sub: RegionIndex,
+        point: Location,
+    ) {
         debug!("add_outlives({:?}: {:?} @ {:?}", sup, sub, point);
-        self.constraints.push(Constraint { sup, sub, point });
+        self.constraints.push(Constraint {
+            span,
+            sup,
+            sub,
+            point,
+        });
     }
 
     /// Perform region inference.
     pub(super) fn solve(&mut self, infcx: &InferCtxt<'a, 'gcx, 'tcx>, mir: &Mir<'tcx>) {
-        self.propagate_constraints(infcx, mir);
+        let errors = self.propagate_constraints(mir);
+
+        // worst error msg ever
+        for (fr1, span, fr2) in errors {
+            infcx.tcx.sess.span_err(
+                span,
+                &format!(
+                    "free region `{}` does not outlive `{}`",
+                    self.definitions[fr1].name.unwrap(),
+                    self.definitions[fr2].name.unwrap()
+                ),
+            );
+        }
     }
 
     /// Propagate the region constraints: this will grow the values
     /// for each region variable until all the constraints are
     /// satisfied. Note that some values may grow **too** large to be
     /// feasible, but we check this later.
-    fn propagate_constraints(&mut self, infcx: &InferCtxt<'a, 'gcx, 'tcx>, mir: &Mir<'tcx>) {
+    fn propagate_constraints(
+        &mut self,
+        mir: &Mir<'tcx>,
+    ) -> Vec<(RegionIndex, Span, RegionIndex)> {
         let mut changed = true;
-        let mut dfs = Dfs::new(infcx, mir);
+        let mut dfs = Dfs::new(mir);
+        let mut error_regions = FxHashSet();
+        let mut errors = vec![];
         while changed {
             changed = false;
             for constraint in &self.constraints {
+                debug!("constraint: {:?}", constraint);
                 let sub = &self.definitions[constraint.sub].value.clone();
                 let sup_def = &mut self.definitions[constraint.sup];
-                debug!("constraint: {:?}", constraint);
+
                 debug!("    sub (before): {:?}", sub);
                 debug!("    sup (before): {:?}", sup_def.value);
 
-                if dfs.copy(sub, &mut sup_def.value, constraint.point) {
-                    changed = true;
+                if !sup_def.constant {
+                    // If this is not a constant, then grow the value as needed to
+                    // accommodate the outlives constraint.
+
+                    if dfs.copy(sub, &mut sup_def.value, constraint.point) {
+                        changed = true;
+                    }
+
+                    debug!("    sup (after) : {:?}", sup_def.value);
+                    debug!("    changed     : {:?}", changed);
+                } else {
+                    // If this is a constant, check whether it *would
+                    // have* to grow in order for the constraint to be
+                    // satisfied. If so, create an error.
+
+                    let mut sup_value = sup_def.value.clone();
+                    if dfs.copy(sub, &mut sup_value, constraint.point) {
+                        // Constant values start out with the entire
+                        // CFG, so it must be some new free region
+                        // that was added. Find one.
+                        let &new_region = sup_value
+                            .free_regions
+                            .difference(&sup_def.value.free_regions)
+                            .next()
+                            .unwrap();
+                        debug!("    new_region : {:?}", new_region);
+                        if error_regions.insert(constraint.sup) {
+                            errors.push((constraint.sup, constraint.span, new_region));
+                        }
+                    }
                 }
-
-                debug!("    sup (after) : {:?}", sup_def.value);
-                debug!("    changed     : {:?}", changed);
             }
             debug!("\n");
         }
+        errors
     }
 }
 
-struct Dfs<'a, 'gcx: 'tcx + 'a, 'tcx: 'a> {
-    #[allow(dead_code)] infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
+struct Dfs<'a, 'tcx: 'a> {
     mir: &'a Mir<'tcx>,
 }
 
-impl<'a, 'gcx: 'tcx, 'tcx: 'a> Dfs<'a, 'gcx, 'tcx> {
-    fn new(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>, mir: &'a Mir<'tcx>) -> Self {
-        Self { infcx, mir }
+impl<'a, 'tcx> Dfs<'a, 'tcx> {
+    fn new(mir: &'a Mir<'tcx>) -> Self {
+        Self { mir }
     }
 
     fn copy(
@@ -316,7 +386,10 @@ impl<'a, 'gcx: 'tcx, 'tcx: 'a> Dfs<'a, 'gcx, 'tcx> {
                 // over any skolemized end points in the `from_region`
                 // and make sure they are included in the `to_region`.
 
-                to_region.free_regions.extend(&from_region.free_regions);
+                debug!("        dfs: free_regions={:?}", from_region.free_regions);
+                for &fr in &from_region.free_regions {
+                    changed |= to_region.free_regions.insert(fr);
+                }
             } else {
                 stack.extend(successor_points);
             }

src/test/compile-fail/nll/region-error.rs

@@ -0,0 +1,22 @@
+// Copyright 2016 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.
+
+// Basic test for free regions in the NLL code. This test ought to
+// report an error due to a reborrowing constraint. Right now, we get
+// a variety of errors from the older, AST-based machinery (notably
+// borrowck), and then we get the NLL error at the end.
+
+// compile-flags:-Znll
+
+fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'b u32 {
+    &*x
+}
+
+fn main() { }

src/test/ui/nll/named-region-basic.rs

@@ -0,0 +1,29 @@
+warning: not reporting region error due to -Znll: SubSupConflict(AddrOfRegion($DIR/named-region-basic.rs:19:5: 19:8), Reborrow($DIR/named-region-basic.rs:19:5: 19:8), ReFree(DefId { krate: CrateNum(0), index: DefIndex(0:3) => named_region_basic[317d]::foo[0] }, BrNamed(CrateNum(0):DefIndex(1:10), 'b(88))), Reborrow($DIR/named-region-basic.rs:19:5: 19:8), ReFree(DefId { krate: CrateNum(0), index: DefIndex(0:3) => named_region_basic[317d]::foo[0] }, BrNamed(CrateNum(0):DefIndex(1:9), 'a(86))))
+  --> $DIR/named-region-basic.rs:19:5
+   |
+19 |     &*x
+   |     ^^^
+
+error[E0597]: `*x` does not live long enough
+  --> $DIR/named-region-basic.rs:19:6
+   |
+19 |     &*x
+   |      ^^ does not live long enough
+   |
+   = note: borrowed value must be valid for the static lifetime...
+note: ...but borrowed value is only valid for the lifetime 'a as defined on the function body at 18:1
+  --> $DIR/named-region-basic.rs:18:1
+   |
+18 | / fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'b u32 {
+19 | |     &*x
+20 | | }
+   | |_^
+
+error: free region `'a` does not outlive `'b`
+  --> $DIR/named-region-basic.rs:19:5
+   |
+19 |     &*x
+   |     ^^^
+
+error: aborting due to 2 previous errors
+