Auto merge of rust-lang#52021 - nikomatsakis:nll-region-errors, r=estebank

refactor and cleanup region errors for NLL

This is a WIP commit. It simplifies some of the code from rust-lang#51536 and extends a few more steps towards the errors that @davidtwco and I were shooting for. These are intended as a replacement for the general "unable to infer lifetime" messages -- one that is actually actionable. We're certainly not there yet, but the overall shape hopefully gets a bit clearer.

I'm thinking about trying to open up an internals thread to sketch out the overall plan and perhaps discuss how to get the wording right, which special cases to handle, etc.

r? @estebank
cc @davidtwco

Author: bors

src/librustc/ich/impls_mir.rs

@@ -30,7 +30,7 @@ impl_stable_hash_for!(struct mir::LocalDecl<'tcx> {
     internal,
     is_user_variable
 });
-impl_stable_hash_for!(struct mir::UpvarDecl { debug_name, by_ref, mutability });
+impl_stable_hash_for!(struct mir::UpvarDecl { debug_name, var_hir_id, by_ref, mutability });
 impl_stable_hash_for!(struct mir::BasicBlockData<'tcx> { statements, terminator, is_cleanup });
 impl_stable_hash_for!(struct mir::UnsafetyViolation { source_info, description, kind });
 impl_stable_hash_for!(struct mir::UnsafetyCheckResult { violations, unsafe_blocks });

src/librustc/mir/mod.rs

@@ -15,7 +15,7 @@
 use graphviz::IntoCow;
 use hir::def::CtorKind;
 use hir::def_id::DefId;
-use hir::{self, InlineAsm};
+use hir::{self, HirId, InlineAsm};
 use middle::region;
 use mir::interpret::{EvalErrorKind, Scalar, Value};
 use mir::visit::MirVisitable;
@@ -380,6 +380,15 @@ pub enum ClearCrossCrate<T> {
     Set(T),
 }
 
+impl<T> ClearCrossCrate<T> {
+    pub fn assert_crate_local(self) -> T {
+        match self {
+            ClearCrossCrate::Clear => bug!("unwrapping cross-crate data"),
+            ClearCrossCrate::Set(v) => v,
+        }
+    }
+}
+
 impl<T: serialize::Encodable> serialize::UseSpecializedEncodable for ClearCrossCrate<T> {}
 impl<T: serialize::Decodable> serialize::UseSpecializedDecodable for ClearCrossCrate<T> {}
 
@@ -785,6 +794,9 @@ impl<'tcx> LocalDecl<'tcx> {
 pub struct UpvarDecl {
     pub debug_name: Name,
 
+    /// `HirId` of the captured variable
+    pub var_hir_id: ClearCrossCrate<HirId>,
+
     /// If true, the capture is behind a reference.
     pub by_ref: bool,
 

src/librustc/ty/fold.rs

@@ -253,13 +253,34 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         value.fold_with(&mut RegionFolder::new(self, skipped_regions, &mut f))
     }
 
-    pub fn for_each_free_region<T,F>(self,
-                                     value: &T,
-                                     callback: F)
-        where F: FnMut(ty::Region<'tcx>),
-              T: TypeFoldable<'tcx>,
-    {
-        value.visit_with(&mut RegionVisitor {
+    /// Invoke `callback` on every region appearing free in `value`.
+    pub fn for_each_free_region(
+        self,
+        value: &impl TypeFoldable<'tcx>,
+        mut callback: impl FnMut(ty::Region<'tcx>),
+    ) {
+        self.any_free_region_meets(value, |r| {
+            callback(r);
+            false
+        });
+    }
+
+    /// True if `callback` returns true for every region appearing free in `value`.
+    pub fn all_free_regions_meet(
+        self,
+        value: &impl TypeFoldable<'tcx>,
+        mut callback: impl FnMut(ty::Region<'tcx>) -> bool,
+    ) -> bool {
+        !self.any_free_region_meets(value, |r| !callback(r))
+    }
+
+    /// True if `callback` returns true for some region appearing free in `value`.
+    pub fn any_free_region_meets(
+        self,
+        value: &impl TypeFoldable<'tcx>,
+        callback: impl FnMut(ty::Region<'tcx>) -> bool,
+    ) -> bool {
+        return value.visit_with(&mut RegionVisitor {
             outer_index: ty::INNERMOST,
             callback
         });
@@ -287,25 +308,22 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         }
 
         impl<'tcx, F> TypeVisitor<'tcx> for RegionVisitor<F>
-            where F : FnMut(ty::Region<'tcx>)
+            where F: FnMut(ty::Region<'tcx>) -> bool
         {
             fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &Binder<T>) -> bool {
                 self.outer_index.shift_in(1);
-                t.skip_binder().visit_with(self);
+                let result = t.skip_binder().visit_with(self);
                 self.outer_index.shift_out(1);
-
-                false // keep visiting
+                result
             }
 
             fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
                 match *r {
                     ty::ReLateBound(debruijn, _) if debruijn < self.outer_index => {
-                        /* ignore bound regions */
+                        false // ignore bound regions, keep visiting
                     }
                     _ => (self.callback)(r),
                 }
-
-                false // keep visiting
             }
         }
     }

src/librustc_mir/borrow_check/mod.rs

@@ -128,6 +128,8 @@ fn do_mir_borrowck<'a, 'gcx, 'tcx>(
     input_mir: &Mir<'gcx>,
     def_id: DefId,
 ) -> BorrowCheckResult<'gcx> {
+    debug!("do_mir_borrowck(def_id = {:?})", def_id);
+
     let tcx = infcx.tcx;
     let attributes = tcx.get_attrs(def_id);
     let param_env = tcx.param_env(def_id);
@@ -319,10 +321,14 @@ fn do_mir_borrowck<'a, 'gcx, 'tcx>(
         }
     }
 
-    BorrowCheckResult {
+    let result = BorrowCheckResult {
         closure_requirements: opt_closure_req,
         used_mut_upvars: mbcx.used_mut_upvars,
-    }
+    };
+
+    debug!("do_mir_borrowck: result = {:#?}", result);
+
+    result
 }
 
 #[allow(dead_code)]