Auto merge of #45337 - Zoxc:gen-static, r=nikomatsakis

Immovable generators

This adds support for immovable generators which allow you to borrow local values inside generator across suspension points. These are declared using a `static` keyword:
```rust
let mut generator = static || {
    let local = &Vec::new();
    yield;
    local.push(0i8);
};
generator.resume();

// ERROR moving the generator after it has resumed would invalidate the interior reference
// drop(generator);
```

Region inference is no longer affected by the types stored in generators so the regions inside should be similar to other code (and unaffected by the presence of `yield` expressions). The borrow checker is extended to pick up the slack so interior references still result in errors for movable generators. This fixes #44197, #45259 and #45093.

This PR depends on [PR #44917 (immovable types)](#44917), I suggest potential reviewers ignore the first commit as it adds immovable types.

Author: bors

src/librustc/diagnostics.rs

@@ -2118,4 +2118,6 @@ register_diagnostics! {
     E0657, // `impl Trait` can only capture lifetimes bound at the fn level
     E0687, // in-band lifetimes cannot be used in `fn`/`Fn` syntax
     E0688, // in-band lifetimes cannot be mixed with explicit lifetime binders
+
+    E0906, // closures cannot be static
 }

src/librustc/hir/lowering.rs

@@ -2773,7 +2773,7 @@ impl<'a> LoweringContext<'a> {
                                arms.iter().map(|x| self.lower_arm(x)).collect(),
                                hir::MatchSource::Normal)
             }
-            ExprKind::Closure(capture_clause, ref decl, ref body, fn_decl_span) => {
+            ExprKind::Closure(capture_clause, movability, ref decl, ref body, fn_decl_span) => {
                 self.with_new_scopes(|this| {
                     this.with_parent_def(e.id, |this| {
                         let mut is_generator = false;
@@ -2782,16 +2782,28 @@ impl<'a> LoweringContext<'a> {
                             is_generator = this.is_generator;
                             e
                         });
-                        if is_generator && !decl.inputs.is_empty() {
-                            span_err!(this.sess, fn_decl_span, E0628,
-                                      "generators cannot have explicit arguments");
-                            this.sess.abort_if_errors();
-                        }
+                        let generator_option = if is_generator {
+                            if !decl.inputs.is_empty() {
+                                span_err!(this.sess, fn_decl_span, E0628,
+                                        "generators cannot have explicit arguments");
+                                this.sess.abort_if_errors();
+                            }
+                            Some(match movability {
+                                Movability::Movable => hir::GeneratorMovability::Movable,
+                                Movability::Static => hir::GeneratorMovability::Static,
+                            })
+                        } else {
+                            if movability == Movability::Static {
+                                span_err!(this.sess, fn_decl_span, E0906,
+                                        "closures cannot be static");
+                            }
+                            None
+                        };
                         hir::ExprClosure(this.lower_capture_clause(capture_clause),
                                          this.lower_fn_decl(decl, None, false),
                                          body_id,
                                          fn_decl_span,
-                                         is_generator)
+                                         generator_option)
                     })
                 })
             }

src/librustc/hir/mod.rs

@@ -1290,7 +1290,7 @@ pub enum Expr_ {
     ///
     /// This may also be a generator literal, indicated by the final boolean,
     /// in that case there is an GeneratorClause.
-    ExprClosure(CaptureClause, P<FnDecl>, BodyId, Span, bool),
+    ExprClosure(CaptureClause, P<FnDecl>, BodyId, Span, Option<GeneratorMovability>),
     /// A block (`{ ... }`)
     ExprBlock(P<Block>),
 
@@ -1466,6 +1466,12 @@ pub struct Destination {
     pub target_id: ScopeTarget,
 }
 
+#[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug, Copy)]
+pub enum GeneratorMovability {
+    Static,
+    Movable,
+}
+
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug, Copy)]
 pub enum CaptureClause {
     CaptureByValue,

src/librustc/ich/impls_hir.rs

@@ -606,6 +606,11 @@ impl<'gcx> HashStable<StableHashingContext<'gcx>> for hir::MatchSource {
     }
 }
 
+impl_stable_hash_for!(enum hir::GeneratorMovability {
+    Static,
+    Movable
+});
+
 impl_stable_hash_for!(enum hir::CaptureClause {
     CaptureByValue,
     CaptureByRef

src/librustc/ich/impls_ty.rs

@@ -431,7 +431,7 @@ for ::middle::const_val::ErrKind<'gcx> {
 
 impl_stable_hash_for!(struct ty::ClosureSubsts<'tcx> { substs });
 
-impl_stable_hash_for!(struct ty::GeneratorInterior<'tcx> { witness });
+impl_stable_hash_for!(struct ty::GeneratorInterior<'tcx> { witness, movable });
 
 impl_stable_hash_for!(struct ty::GenericPredicates<'tcx> {
     parent,
@@ -656,6 +656,9 @@ for ty::TypeVariants<'gcx>
                 closure_substs.hash_stable(hcx, hasher);
                 interior.hash_stable(hcx, hasher);
             }
+            TyGeneratorWitness(types) => {
+                types.hash_stable(hcx, hasher)
+            }
             TyTuple(inner_tys, from_diverging_type_var) => {
                 inner_tys.hash_stable(hcx, hasher);
                 from_diverging_type_var.hash_stable(hcx, hasher);

src/librustc/infer/error_reporting/nice_region_error/outlives_closure.rs

@@ -60,7 +60,7 @@ impl<'a, 'gcx, 'tcx> NiceRegionError<'a, 'gcx, 'tcx> {
                 if let Some(node_id) = hir.as_local_node_id(free_region.scope) {
                     match hir.get(node_id) {
                         NodeExpr(Expr {
-                            node: ExprClosure(_, _, _, closure_span, false),
+                            node: ExprClosure(_, _, _, closure_span, None),
                             ..
                         }) => {
                             let sup_sp = sup_origin.span();

src/librustc/infer/freshen.rs

@@ -192,6 +192,7 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for TypeFreshener<'a, 'gcx, 'tcx> {
             ty::TyForeign(..) |
             ty::TyParam(..) |
             ty::TyClosure(..) |
+            ty::TyGeneratorWitness(..) |
             ty::TyAnon(..) => {
                 t.super_fold_with(self)
             }

src/librustc/middle/region.rs

@@ -453,6 +453,43 @@ struct RegionResolutionVisitor<'a, 'tcx: 'a> {
     terminating_scopes: FxHashSet<hir::ItemLocalId>,
 }
 
+struct ExprLocatorVisitor {
+    id: ast::NodeId,
+    result: Option<usize>,
+    expr_and_pat_count: usize,
+}
+
+// This visitor has to have the same visit_expr calls as RegionResolutionVisitor
+// since `expr_count` is compared against the results there.
+impl<'tcx> Visitor<'tcx> for ExprLocatorVisitor {
+    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
+        NestedVisitorMap::None
+    }
+
+    fn visit_pat(&mut self, pat: &'tcx Pat) {
+        self.expr_and_pat_count += 1;
+
+        intravisit::walk_pat(self, pat);
+    }
+
+    fn visit_expr(&mut self, expr: &'tcx Expr) {
+        debug!("ExprLocatorVisitor - pre-increment {} expr = {:?}",
+               self.expr_and_pat_count,
+               expr);
+
+        intravisit::walk_expr(self, expr);
+
+        self.expr_and_pat_count += 1;
+
+        debug!("ExprLocatorVisitor - post-increment {} expr = {:?}",
+               self.expr_and_pat_count,
+               expr);
+
+        if expr.id == self.id {
+            self.result = Some(self.expr_and_pat_count);
+        }
+    }
+}
 
 impl<'tcx> ScopeTree {
     pub fn record_scope_parent(&mut self, child: Scope, parent: Option<Scope>) {
@@ -612,6 +649,20 @@ impl<'tcx> ScopeTree {
         return true;
     }
 
+    /// Returns the id of the innermost containing body
+    pub fn containing_body(&self, mut scope: Scope)-> Option<hir::ItemLocalId> {
+        loop {
+            if let ScopeData::CallSite(id) = scope.data() {
+                return Some(id);
+            }
+
+            match self.opt_encl_scope(scope) {
+                None => return None,
+                Some(parent) => scope = parent,
+            }
+        }
+    }
+
     /// Finds the nearest common ancestor (if any) of two scopes.  That is, finds the smallest
     /// scope which is greater than or equal to both `scope_a` and `scope_b`.
     pub fn nearest_common_ancestor(&self,
@@ -768,6 +819,28 @@ impl<'tcx> ScopeTree {
         self.yield_in_scope.get(&scope).cloned()
     }
 
+    /// Checks whether the given scope contains a `yield` and if that yield could execute
+    /// after `expr`. If so, it returns the span of that `yield`.
+    /// `scope` must be inside the body.
+    pub fn yield_in_scope_for_expr(&self,
+                                   scope: Scope,
+                                   expr: ast::NodeId,
+                                   body: &'tcx hir::Body) -> Option<Span> {
+        self.yield_in_scope(scope).and_then(|(span, count)| {
+            let mut visitor = ExprLocatorVisitor {
+                id: expr,
+                result: None,
+                expr_and_pat_count: 0,
+            };
+            visitor.visit_body(body);
+            if count >= visitor.result.unwrap() {
+                Some(span)
+            } else {
+                None
+            }
+        })
+    }
+
     /// Gives the number of expressions visited in a body.
     /// Used to sanity check visit_expr call count when
     /// calculating generator interiors.
@@ -872,9 +945,13 @@ fn resolve_pat<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>, pat: &
         record_var_lifetime(visitor, pat.hir_id.local_id, pat.span);
     }
 
+    debug!("resolve_pat - pre-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
+
     intravisit::walk_pat(visitor, pat);
 
     visitor.expr_and_pat_count += 1;
+
+    debug!("resolve_pat - post-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
 }
 
 fn resolve_stmt<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>, stmt: &'tcx hir::Stmt) {
@@ -897,7 +974,7 @@ fn resolve_stmt<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>, stmt:
 }
 
 fn resolve_expr<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>, expr: &'tcx hir::Expr) {
-    debug!("resolve_expr(expr.id={:?})", expr.id);
+    debug!("resolve_expr - pre-increment {} expr = {:?}", visitor.expr_and_pat_count, expr);
 
     let prev_cx = visitor.cx;
     visitor.enter_node_scope_with_dtor(expr.hir_id.local_id);
@@ -982,6 +1059,8 @@ fn resolve_expr<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>, expr:
 
     visitor.expr_and_pat_count += 1;
 
+    debug!("resolve_expr post-increment {}, expr = {:?}", visitor.expr_and_pat_count, expr);
+
     if let hir::ExprYield(..) = expr.node {
         // Mark this expr's scope and all parent scopes as containing `yield`.
         let mut scope = Scope::Node(expr.hir_id.local_id);
@@ -1077,12 +1156,13 @@ fn resolve_local<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>,
         }
     }
 
-    if let Some(pat) = pat {
-        visitor.visit_pat(pat);
-    }
+    // Make sure we visit the initializer first, so expr_and_pat_count remains correct
     if let Some(expr) = init {
         visitor.visit_expr(expr);
     }
+    if let Some(pat) = pat {
+        visitor.visit_pat(pat);
+    }
 
     /// True if `pat` match the `P&` nonterminal:
     ///

src/librustc/traits/coherence.rs

@@ -451,7 +451,10 @@ fn ty_is_local_constructor(ty: Ty, in_crate: InCrate) -> bool {
             true
         }
 
-        ty::TyClosure(..) | ty::TyGenerator(..) | ty::TyAnon(..) => {
+        ty::TyClosure(..) |
+        ty::TyGenerator(..) |
+        ty::TyGeneratorWitness(..) |
+        ty::TyAnon(..) => {
             bug!("ty_is_local invoked on unexpected type: {:?}", ty)
         }
     }

src/librustc/traits/error_reporting.rs

@@ -262,6 +262,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 },
                 ty::TyGenerator(..) => Some(18),
                 ty::TyForeign(..) => Some(19),
+                ty::TyGeneratorWitness(..) => Some(20),
                 ty::TyInfer(..) | ty::TyError => None
             }
         }