Explain why a closure is FnOnce in closure errors.

src/librustc/infer/mod.rs

@@ -1682,7 +1682,11 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     {
         if let InferTables::InProgress(tables) = self.tables {
             if let Some(id) = self.tcx.hir.as_local_node_id(def_id) {
-                return tables.borrow().closure_kinds.get(&id).cloned();
+                return tables.borrow()
+                             .closure_kinds
+                             .get(&id)
+                             .cloned()
+                             .map(|(kind, _)| kind);
             }
         }
 

src/librustc/ty/context.rs

@@ -58,6 +58,7 @@ use syntax::abi;
 use syntax::ast::{self, Name, NodeId};
 use syntax::attr;
 use syntax::symbol::{Symbol, keywords};
+use syntax_pos::Span;
 
 use hir;
 
@@ -229,8 +230,9 @@ pub struct TypeckTables<'tcx> {
     /// Records the type of each closure.
     pub closure_tys: NodeMap<ty::PolyFnSig<'tcx>>,
 
-    /// Records the kind of each closure.
-    pub closure_kinds: NodeMap<ty::ClosureKind>,
+    /// Records the kind of each closure and the span and name of the variable
+    /// that caused the closure to be this kind.
+    pub closure_kinds: NodeMap<(ty::ClosureKind, Option<(Span, ast::Name)>)>,
 
     /// For each fn, records the "liberated" types of its arguments
     /// and return type. Liberated means that all bound regions

src/librustc_borrowck/borrowck/mod.rs

@@ -39,8 +39,6 @@ use rustc::middle::free_region::RegionRelations;
 use rustc::ty::{self, TyCtxt};
 use rustc::ty::maps::Providers;
 
-use syntax_pos::DUMMY_SP;
-
 use std::fmt;
 use std::rc::Rc;
 use std::hash::{Hash, Hasher};
@@ -594,9 +592,15 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     verb, msg, nl);
                 let need_note = match lp.ty.sty {
                     ty::TypeVariants::TyClosure(id, _) => {
-                        if let Ok(ty::ClosureKind::FnOnce) =
-                           ty::queries::closure_kind::try_get(self.tcx, DUMMY_SP, id) {
-                            err.help("closure was moved because it only implements `FnOnce`");
+                        let node_id = self.tcx.hir.as_local_node_id(id).unwrap();
+                        if let Some(&(ty::ClosureKind::FnOnce, Some((span, name)))) =
+                            self.tables.closure_kinds.get(&node_id)
+                        {
+                            err.span_note(span, &format!(
+                                "closure cannot be invoked more than once because \
+                                it moves the variable `{}` out of its environment",
+                                name
+                            ));
                             false
                         } else {
                             true

src/librustc_typeck/check/closure.rs

@@ -103,7 +103,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         self.tables.borrow_mut().closure_tys.insert(expr.id, sig);
         match opt_kind {
             Some(kind) => {
-                self.tables.borrow_mut().closure_kinds.insert(expr.id, kind);
+                self.tables.borrow_mut().closure_kinds.insert(expr.id, (kind, None));
             }
             None => {}
         }

src/librustc_typeck/check/method/probe.rs

@@ -802,7 +802,7 @@ impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
 
             let closure_kinds = &self.tables.borrow().closure_kinds;
             let closure_kind = match closure_kinds.get(&closure_id) {
-                Some(&k) => k,
+                Some(&(k, _)) => k,
                 None => {
                     return Err(MethodError::ClosureAmbiguity(trait_def_id));
                 }

src/librustc_typeck/check/mod.rs

@@ -702,7 +702,7 @@ fn closure_kind<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                           def_id: DefId)
                           -> ty::ClosureKind {
     let node_id = tcx.hir.as_local_node_id(def_id).unwrap();
-    tcx.typeck_tables_of(def_id).closure_kinds[&node_id]
+    tcx.typeck_tables_of(def_id).closure_kinds[&node_id].0
 }
 
 fn adt_destructor<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,

src/librustc_typeck/check/upvar.rs

@@ -74,7 +74,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 
 struct SeedBorrowKind<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
-    temp_closure_kinds: NodeMap<ty::ClosureKind>,
+    temp_closure_kinds: NodeMap<(ty::ClosureKind, Option<(Span, ast::Name)>)>,
 }
 
 impl<'a, 'gcx, 'tcx> Visitor<'gcx> for SeedBorrowKind<'a, 'gcx, 'tcx> {
@@ -107,7 +107,7 @@ impl<'a, 'gcx, 'tcx> SeedBorrowKind<'a, 'gcx, 'tcx> {
                      capture_clause: hir::CaptureClause)
     {
         if !self.fcx.tables.borrow().closure_kinds.contains_key(&expr.id) {
-            self.temp_closure_kinds.insert(expr.id, ty::ClosureKind::Fn);
+            self.temp_closure_kinds.insert(expr.id, (ty::ClosureKind::Fn, None));
             debug!("check_closure: adding closure {:?} as Fn", expr.id);
         }
 
@@ -143,12 +143,12 @@ impl<'a, 'gcx, 'tcx> SeedBorrowKind<'a, 'gcx, 'tcx> {
 
 struct AdjustBorrowKind<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
-    temp_closure_kinds: NodeMap<ty::ClosureKind>,
+    temp_closure_kinds: NodeMap<(ty::ClosureKind, Option<(Span, ast::Name)>)>,
 }
 
 impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
     fn new(fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
-           temp_closure_kinds: NodeMap<ty::ClosureKind>)
+           temp_closure_kinds: NodeMap<(ty::ClosureKind, Option<(Span, ast::Name)>)>)
            -> AdjustBorrowKind<'a, 'gcx, 'tcx> {
         AdjustBorrowKind { fcx: fcx, temp_closure_kinds: temp_closure_kinds }
     }
@@ -211,8 +211,8 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
 
         // If we are also inferred the closure kind here, update the
         // main table and process any deferred resolutions.
-        if let Some(&kind) = self.temp_closure_kinds.get(&id) {
-            self.fcx.tables.borrow_mut().closure_kinds.insert(id, kind);
+        if let Some(&(kind, context)) = self.temp_closure_kinds.get(&id) {
+            self.fcx.tables.borrow_mut().closure_kinds.insert(id, (kind, context));
             let closure_def_id = self.fcx.tcx.hir.local_def_id(id);
             debug!("closure_kind({:?}) = {:?}", closure_def_id, kind);
 
@@ -272,6 +272,8 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
             euv::Move(_) => { }
         }
 
+        let tcx = self.fcx.tcx;
+
         // watch out for a move of the deref of a borrowed pointer;
         // for that to be legal, the upvar would have to be borrowed
         // by value instead
@@ -289,7 +291,9 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
 
                         // to move out of an upvar, this must be a FnOnce closure
                         self.adjust_closure_kind(upvar_id.closure_expr_id,
-                                                 ty::ClosureKind::FnOnce);
+                                                 ty::ClosureKind::FnOnce,
+                                                 guarantor.span,
+                                                 tcx.hir.name(upvar_id.var_id));
 
                         let upvar_capture_map =
                             &mut self.fcx.tables.borrow_mut().upvar_capture_map;
@@ -303,7 +307,9 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
                         // to be a FnOnce closure to permit moves out
                         // of the environment.
                         self.adjust_closure_kind(upvar_id.closure_expr_id,
-                                                 ty::ClosureKind::FnOnce);
+                                                 ty::ClosureKind::FnOnce,
+                                                 guarantor.span,
+                                                 tcx.hir.name(upvar_id.var_id));
                     }
                     mc::NoteNone => {
                     }
@@ -331,7 +337,7 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
 
             Categorization::Deref(base, _, mc::BorrowedPtr(..)) |
             Categorization::Deref(base, _, mc::Implicit(..)) => {
-                if !self.try_adjust_upvar_deref(&cmt.note, ty::MutBorrow) {
+                if !self.try_adjust_upvar_deref(cmt, ty::MutBorrow) {
                     // assignment to deref of an `&mut`
                     // borrowed pointer implies that the
                     // pointer itself must be unique, but not
@@ -365,7 +371,7 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
 
             Categorization::Deref(base, _, mc::BorrowedPtr(..)) |
             Categorization::Deref(base, _, mc::Implicit(..)) => {
-                if !self.try_adjust_upvar_deref(&cmt.note, ty::UniqueImmBorrow) {
+                if !self.try_adjust_upvar_deref(cmt, ty::UniqueImmBorrow) {
                     // for a borrowed pointer to be unique, its
                     // base must be unique
                     self.adjust_upvar_borrow_kind_for_unique(base);
@@ -382,7 +388,7 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
     }
 
     fn try_adjust_upvar_deref(&mut self,
-                              note: &mc::Note,
+                              cmt: mc::cmt<'tcx>,
                               borrow_kind: ty::BorrowKind)
                               -> bool
     {
@@ -394,7 +400,9 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
             ty::ImmBorrow => false,
         });
 
-        match *note {
+        let tcx = self.fcx.tcx;
+
+        match cmt.note {
             mc::NoteUpvarRef(upvar_id) => {
                 // if this is an implicit deref of an
                 // upvar, then we need to modify the
@@ -407,15 +415,21 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
                 }
 
                 // also need to be in an FnMut closure since this is not an ImmBorrow
-                self.adjust_closure_kind(upvar_id.closure_expr_id, ty::ClosureKind::FnMut);
+                self.adjust_closure_kind(upvar_id.closure_expr_id,
+                                         ty::ClosureKind::FnMut,
+                                         cmt.span,
+                                         tcx.hir.name(upvar_id.var_id));
 
                 true
             }
             mc::NoteClosureEnv(upvar_id) => {
                 // this kind of deref occurs in a `move` closure, or
                 // for a by-value upvar; in either case, to mutate an
                 // upvar, we need to be an FnMut closure
-                self.adjust_closure_kind(upvar_id.closure_expr_id, ty::ClosureKind::FnMut);
+                self.adjust_closure_kind(upvar_id.closure_expr_id,
+                                         ty::ClosureKind::FnMut,
+                                         cmt.span,
+                                         tcx.hir.name(upvar_id.var_id));
 
                 true
             }
@@ -462,11 +476,13 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
 
     fn adjust_closure_kind(&mut self,
                            closure_id: ast::NodeId,
-                           new_kind: ty::ClosureKind) {
-        debug!("adjust_closure_kind(closure_id={}, new_kind={:?})",
-               closure_id, new_kind);
+                           new_kind: ty::ClosureKind,
+                           upvar_span: Span,
+                           var_name: ast::Name) {
+        debug!("adjust_closure_kind(closure_id={}, new_kind={:?}, upvar_span={:?}, var_name={})",
+               closure_id, new_kind, upvar_span, var_name);
 
-        if let Some(&existing_kind) = self.temp_closure_kinds.get(&closure_id) {
+        if let Some(&(existing_kind, _)) = self.temp_closure_kinds.get(&closure_id) {
             debug!("adjust_closure_kind: closure_id={}, existing_kind={:?}, new_kind={:?}",
                    closure_id, existing_kind, new_kind);
 
@@ -482,7 +498,10 @@ impl<'a, 'gcx, 'tcx> AdjustBorrowKind<'a, 'gcx, 'tcx> {
                 (ty::ClosureKind::Fn, ty::ClosureKind::FnOnce) |
                 (ty::ClosureKind::FnMut, ty::ClosureKind::FnOnce) => {
                     // new kind is stronger than the old kind
-                    self.temp_closure_kinds.insert(closure_id, new_kind);
+                    self.temp_closure_kinds.insert(
+                        closure_id,
+                        (new_kind, Some((upvar_span, var_name)))
+                    );
                 }
             }
         }

src/test/ui/fn_once-moved.rs

@@ -20,5 +20,6 @@ fn main() {
     debug_dump_dict();
     debug_dump_dict();
     //~^ ERROR use of moved value: `debug_dump_dict`
-    //~| NOTE closure was moved because it only implements `FnOnce`
+    //~| NOTE closure cannot be invoked more than once because it moves the
+    //~| variable `dict` out of its environment
 }

src/test/ui/fn_once-moved.stderr

@@ -6,7 +6,11 @@ error[E0382]: use of moved value: `debug_dump_dict`
 21 |     debug_dump_dict();
    |     ^^^^^^^^^^^^^^^ value used here after move
    |
-   = help: closure was moved because it only implements `FnOnce`
+note: closure cannot be invoked more than once because it moves the variable `dict` out of its environment
+  --> $DIR/fn_once-moved.rs:16:29
+   |
+16 |         for (key, value) in dict {
+   |                             ^^^^
 
 error: aborting due to previous error