Auto merge of #40008 - eddyb:lazy-12, r=nikomatsakis

[12/12] On-demand type-checking, const-evaluation, MIR building & const-qualification.

_This is the last of a series ([prev](#38813)) of patches designed to rework rustc into an out-of-order on-demand pipeline model for both better feature support (e.g. [MIR-based](https://github.com/solson/miri) early constant evaluation) and incremental execution of compiler passes (e.g. type-checking), with beneficial consequences to IDE support as well.
If any motivation is unclear, please ask for additional PR description clarifications or code comments._

<hr>

As this contains all of the changes that didn't fit neatly into other PRs, I'll be elaborating a bit:

### User-facing changes
* when determining whether an `impl Trait` type implements an auto-trait (e.g. `Send` or `Sync`), the function the `impl Trait` came from has to be inferred and type-checking, disallowing cycles
  * this results from not having an obvious place to put the "deferred obligation" in on-demand atm
  * while we could model side-effects like that and "post-processing passes" better, it's still more limiting than being able to know the result in the original function (e.g. specialization) *and* there are serious problems around region-checking (if a `Send` impl required `'static`, it wasn't enforced)
* early const-eval requires type-checking and const-qualification to be performed first, which means:
  * you get the intended errors before (if any) constant evaluation error that is simply fallout
  * associated consts should always work now, and `const fn` type parameters are properly tracked
    * don't get too excited, array lengths still can't depend on type parameters
* #38864 works as intended now, with `Self` being allowed in `impl` bounds
* #32205 is largely improved, with associated types being limited to "exact match" `impl`s (as opposed to traversing the specialization graph to resolve unspecified type parameters to their defaults in another `impl` or in the `trait`) *while* checking for overlaps building the specialization graph for that trait - once all the trait impls' have been checked for coherence (including ahead-of-time/on-demand), it's uniform
* [crater report](https://gist.github.com/eddyb/bbb869072468c7e08d6d808e75938051) looks clean (aside from `clippy` which broke due to `rustc` internal changes)

### Compiler-internal changes
* `ty::Generics`
  * no longer contains the actual type parameter defaults, instead they're associated with the type parameter's `DefId`, like associated types in a trait definition
    * this allows computing `ty::Generics` as a leaf (reading only its own HIR)
  * holds a mapping from `DefIndex` of type parameters to their indices
* `ty::AdtDef`
  * only tracks `#[repr(simd)]` in its `ReprOptions` `repr` field
  * doesn't contain `enum` discriminant values, but instead each variant either refers to either an explicit value for its discriminant, or the distance from the last explicit discriminant, if any
    * the `.discriminants(tcx)` method produces an iterator of `ConstInt` values, looking up explicit discriminants in a separate map, if necessary
    * this allows computing `ty::AdtDef` as a leaf (reading only its own HIR)
* Small note: the two above (`Generics`, `AdtDef`), `TraitDef` and `AssociatedItem` should probably end up as part of the HIR, eventually, as they're trivially constructed from it
* `ty::FnSig`
  * now also holds ABI and unsafety, alongside argument types, return type and C variadicity
  * `&ty::BareFnTy` and `ty::ClosureTy` have been replaced with `PolyFnSig = Binder<FnSig>`
    * `BareFnTy` was interned and `ClosureTy` was treated as non-trivial to `Clone` because they had a `PolyFnSig` and so used to contain a `Vec<Ty>` (now `&[Ty]`)
* `ty::maps`
  * all the `DepTrackingMap`s have been grouped in a structure available at `tcx.maps`
  * when creating the `tcx`, a set of `Providers` (one `fn` pointer per map) is required for the local crate, and one for all other crates (i.e. metadata loading), `librustc_driver` plugging the various crates (e.g. `librustc_metadata`, `librustc_typeck`, `librustc_mir`) into it
  * when a map is queried and the value is missing, the appropriate `fn` pointer from the `Providers` of that crate is called with the `TyCtxt` and the key being queried, to produce the value on-demand
* `rustc_const_eval`
  * demands both `typeck_tables` and `mir_const_qualif` (in preparation for miri)
  * tracks `Substs` in `ConstVal::Function` for `const fn` calls
  * returns `TypeckError` if type-checking has failed (or cases that can only be reached if it had)
    * this error kind is never reported, resulting in less noisy/redundant diagnostics
  * fixes #39548 (testcase by @larsluthman, taken from #39812, which this supersedes)
* on-demand has so far been hooked up to:
  * `rustc_metadata::cstore_impl`: `ty`, `generics`, `predicates`, `super_predicates`, `trait_def`, `adt_def`, `variances`, `associated_item_def_ids`, `associated_item`, `impl_trait_ref`, `custom_coerce_unsized_kind`, `mir`, `mir_const_qualif`, `typeck_tables`, `closure_kind`, `closure_type`
  * `rustc_typeck::collect`: `ty`, `generics`, `predicates`, `super_predicates`, `type_param_predicates`, `trait_def`, `adt_def`, `impl_trait_ref`
  * `rustc_typeck::coherence`: `coherent_trait`, `coherent_inherent_impls`
  * `rustc_typeck::check`: `typeck_tables`, `closure_type`, `closure_kind`
  * `rustc_mir::mir_map`: `mir`
  * `rustc_mir::transform::qualify_consts`: `mir_const_qualif`

src/librustc/dep_graph/dep_node.rs

@@ -70,6 +70,7 @@ pub enum DepNode<D: Clone + Debug> {
     Resolve,
     EntryPoint,
     CheckEntryFn,
+    CoherenceCheckTrait(D),
     CoherenceCheckImpl(D),
     CoherenceOverlapCheck(D),
     CoherenceOverlapCheckSpecial(D),
@@ -109,11 +110,13 @@ pub enum DepNode<D: Clone + Debug> {
     // predicates for an item wind up in `ItemSignature`).
     AssociatedItems(D),
     ItemSignature(D),
+    TypeParamPredicates((D, D)),
     SizedConstraint(D),
     AssociatedItemDefIds(D),
     InherentImpls(D),
     TypeckTables(D),
     UsedTraitImports(D),
+    MonomorphicConstEval(D),
 
     // The set of impls for a given trait. Ultimately, it would be
     // nice to get more fine-grained here (e.g., to include a
@@ -239,6 +242,7 @@ impl<D: Clone + Debug> DepNode<D> {
             MetaData(ref d) => op(d).map(MetaData),
             CollectItem(ref d) => op(d).map(CollectItem),
             CollectItemSig(ref d) => op(d).map(CollectItemSig),
+            CoherenceCheckTrait(ref d) => op(d).map(CoherenceCheckTrait),
             CoherenceCheckImpl(ref d) => op(d).map(CoherenceCheckImpl),
             CoherenceOverlapCheck(ref d) => op(d).map(CoherenceOverlapCheck),
             CoherenceOverlapCheckSpecial(ref d) => op(d).map(CoherenceOverlapCheckSpecial),
@@ -258,11 +262,15 @@ impl<D: Clone + Debug> DepNode<D> {
             TransInlinedItem(ref d) => op(d).map(TransInlinedItem),
             AssociatedItems(ref d) => op(d).map(AssociatedItems),
             ItemSignature(ref d) => op(d).map(ItemSignature),
+            TypeParamPredicates((ref item, ref param)) => {
+                Some(TypeParamPredicates((try_opt!(op(item)), try_opt!(op(param)))))
+            }
             SizedConstraint(ref d) => op(d).map(SizedConstraint),
             AssociatedItemDefIds(ref d) => op(d).map(AssociatedItemDefIds),
             InherentImpls(ref d) => op(d).map(InherentImpls),
             TypeckTables(ref d) => op(d).map(TypeckTables),
             UsedTraitImports(ref d) => op(d).map(UsedTraitImports),
+            MonomorphicConstEval(ref d) => op(d).map(MonomorphicConstEval),
             TraitImpls(ref d) => op(d).map(TraitImpls),
             TraitItems(ref d) => op(d).map(TraitItems),
             ReprHints(ref d) => op(d).map(ReprHints),

src/librustc/dep_graph/dep_tracking_map.rs

@@ -11,6 +11,7 @@
 use hir::def_id::DefId;
 use rustc_data_structures::fx::FxHashMap;
 use std::cell::RefCell;
+use std::collections::hash_map::Entry;
 use std::ops::Index;
 use std::hash::Hash;
 use std::marker::PhantomData;
@@ -67,6 +68,11 @@ impl<M: DepTrackingMapConfig> DepTrackingMap<M> {
         assert!(old_value.is_none());
     }
 
+    pub fn entry(&mut self, k: M::Key) -> Entry<M::Key, M::Value> {
+        self.write(&k);
+        self.map.entry(k)
+    }
+
     pub fn contains_key(&self, k: &M::Key) -> bool {
         self.read(k);
         self.map.contains_key(k)

src/librustc/diagnostics.rs

@@ -390,44 +390,6 @@ RFC. It is, however, [currently unimplemented][iss15872].
 [iss15872]: https://github.com/rust-lang/rust/issues/15872
 "##,
 
-E0109: r##"
-You tried to give a type parameter to a type which doesn't need it. Erroneous
-code example:
-
-```compile_fail,E0109
-type X = u32<i32>; // error: type parameters are not allowed on this type
-```
-
-Please check that you used the correct type and recheck its definition. Perhaps
-it doesn't need the type parameter.
-
-Example:
-
-```
-type X = u32; // this compiles
-```
-
-Note that type parameters for enum-variant constructors go after the variant,
-not after the enum (Option::None::<u32>, not Option::<u32>::None).
-"##,
-
-E0110: r##"
-You tried to give a lifetime parameter to a type which doesn't need it.
-Erroneous code example:
-
-```compile_fail,E0110
-type X = u32<'static>; // error: lifetime parameters are not allowed on
-                       //        this type
-```
-
-Please check that the correct type was used and recheck its definition; perhaps
-it doesn't need the lifetime parameter. Example:
-
-```
-type X = u32; // ok!
-```
-"##,
-
 E0133: r##"
 Unsafe code was used outside of an unsafe function or block.
 
@@ -627,41 +589,6 @@ attributes:
 See also https://doc.rust-lang.org/book/no-stdlib.html
 "##,
 
-E0229: r##"
-An associated type binding was done outside of the type parameter declaration
-and `where` clause. Erroneous code example:
-
-```compile_fail,E0229
-pub trait Foo {
-    type A;
-    fn boo(&self) -> <Self as Foo>::A;
-}
-
-struct Bar;
-
-impl Foo for isize {
-    type A = usize;
-    fn boo(&self) -> usize { 42 }
-}
-
-fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
-// error: associated type bindings are not allowed here
-```
-
-To solve this error, please move the type bindings in the type parameter
-declaration:
-
-```ignore
-fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
-```
-
-Or in the `where` clause:
-
-```ignore
-fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}
-```
-"##,
-
 E0261: r##"
 When using a lifetime like `'a` in a type, it must be declared before being
 used.
@@ -1390,6 +1317,23 @@ error. To resolve it, add an `else` block having the same type as the `if`
 block.
 "##,
 
+E0391: r##"
+This error indicates that some types or traits depend on each other
+and therefore cannot be constructed.
+
+The following example contains a circular dependency between two traits:
+
+```compile_fail,E0391
+trait FirstTrait : SecondTrait {
+
+}
+
+trait SecondTrait : FirstTrait {
+
+}
+```
+"##,
+
 E0398: r##"
 In Rust 1.3, the default object lifetime bounds are expected to change, as
 described in RFC #1156 [1]. You are getting a warning because the compiler

src/librustc/hir/lowering.rs

@@ -80,6 +80,9 @@ pub struct LoweringContext<'a> {
     impl_items: BTreeMap<hir::ImplItemId, hir::ImplItem>,
     bodies: FxHashMap<hir::BodyId, hir::Body>,
 
+    trait_impls: BTreeMap<DefId, Vec<NodeId>>,
+    trait_default_impl: BTreeMap<DefId, NodeId>,
+
     loop_scopes: Vec<NodeId>,
     is_in_loop_condition: bool,
 
@@ -116,6 +119,8 @@ pub fn lower_crate(sess: &Session,
         trait_items: BTreeMap::new(),
         impl_items: BTreeMap::new(),
         bodies: FxHashMap(),
+        trait_impls: BTreeMap::new(),
+        trait_default_impl: BTreeMap::new(),
         loop_scopes: Vec::new(),
         is_in_loop_condition: false,
         type_def_lifetime_params: DefIdMap(),
@@ -201,6 +206,8 @@ impl<'a> LoweringContext<'a> {
             trait_items: self.trait_items,
             impl_items: self.impl_items,
             bodies: self.bodies,
+            trait_impls: self.trait_impls,
+            trait_default_impl: self.trait_default_impl,
         }
     }
 
@@ -525,7 +532,7 @@ impl<'a> LoweringContext<'a> {
                         return n;
                     }
                     assert!(!def_id.is_local());
-                    let (n, _) = self.sess.cstore.item_generics_own_param_counts(def_id);
+                    let n = self.sess.cstore.item_generics_cloned(def_id).regions.len();
                     self.type_def_lifetime_params.insert(def_id, n);
                     n
                 });
@@ -1089,14 +1096,27 @@ impl<'a> LoweringContext<'a> {
                 hir::ItemUnion(vdata, self.lower_generics(generics))
             }
             ItemKind::DefaultImpl(unsafety, ref trait_ref) => {
+                let trait_ref = self.lower_trait_ref(trait_ref);
+
+                if let Def::Trait(def_id) = trait_ref.path.def {
+                    self.trait_default_impl.insert(def_id, id);
+                }
+
                 hir::ItemDefaultImpl(self.lower_unsafety(unsafety),
-                                     self.lower_trait_ref(trait_ref))
+                                     trait_ref)
             }
             ItemKind::Impl(unsafety, polarity, ref generics, ref ifce, ref ty, ref impl_items) => {
                 let new_impl_items = impl_items.iter()
                                                .map(|item| self.lower_impl_item_ref(item))
                                                .collect();
                 let ifce = ifce.as_ref().map(|trait_ref| self.lower_trait_ref(trait_ref));
+
+                if let Some(ref trait_ref) = ifce {
+                    if let Def::Trait(def_id) = trait_ref.path.def {
+                        self.trait_impls.entry(def_id).or_insert(vec![]).push(id);
+                    }
+                }
+
                 hir::ItemImpl(self.lower_unsafety(unsafety),
                               self.lower_impl_polarity(polarity),
                               self.lower_generics(generics),

src/librustc/hir/map/mod.rs

@@ -437,6 +437,50 @@ impl<'hir> Map<'hir> {
         self.local_def_id(self.body_owner(id))
     }
 
+    pub fn ty_param_owner(&self, id: NodeId) -> NodeId {
+        match self.get(id) {
+            NodeItem(&Item { node: ItemTrait(..), .. }) => id,
+            NodeTyParam(_) => self.get_parent_node(id),
+            _ => {
+                bug!("ty_param_owner: {} not a type parameter",
+                    self.node_to_string(id))
+            }
+        }
+    }
+
+    pub fn ty_param_name(&self, id: NodeId) -> Name {
+        match self.get(id) {
+            NodeItem(&Item { node: ItemTrait(..), .. }) => {
+                keywords::SelfType.name()
+            }
+            NodeTyParam(tp) => tp.name,
+            _ => {
+                bug!("ty_param_name: {} not a type parameter",
+                    self.node_to_string(id))
+            }
+        }
+    }
+
+    pub fn trait_impls(&self, trait_did: DefId) -> &'hir [NodeId] {
+        self.dep_graph.read(DepNode::TraitImpls(trait_did));
+
+        // NB: intentionally bypass `self.forest.krate()` so that we
+        // do not trigger a read of the whole krate here
+        self.forest.krate.trait_impls.get(&trait_did).map_or(&[], |xs| &xs[..])
+    }
+
+    pub fn trait_default_impl(&self, trait_did: DefId) -> Option<NodeId> {
+        self.dep_graph.read(DepNode::TraitImpls(trait_did));
+
+        // NB: intentionally bypass `self.forest.krate()` so that we
+        // do not trigger a read of the whole krate here
+        self.forest.krate.trait_default_impl.get(&trait_did).cloned()
+    }
+
+    pub fn trait_is_auto(&self, trait_did: DefId) -> bool {
+        self.trait_default_impl(trait_did).is_some()
+    }
+
     /// Get the attributes on the krate. This is preferable to
     /// invoking `krate.attrs` because it registers a tighter
     /// dep-graph access.

src/librustc/hir/mod.rs

@@ -410,6 +410,9 @@ pub struct Crate {
     pub trait_items: BTreeMap<TraitItemId, TraitItem>,
     pub impl_items: BTreeMap<ImplItemId, ImplItem>,
     pub bodies: FxHashMap<BodyId, Body>,
+
+    pub trait_impls: BTreeMap<DefId, Vec<NodeId>>,
+    pub trait_default_impl: BTreeMap<DefId, NodeId>,
 }
 
 impl Crate {

src/librustc/infer/mod.rs

@@ -505,7 +505,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'gcx> {
             evaluation_cache: traits::EvaluationCache::new(),
             projection_cache: RefCell::new(traits::ProjectionCache::new()),
             reported_trait_errors: RefCell::new(FxHashSet()),
-            projection_mode: Reveal::NotSpecializable,
+            projection_mode: Reveal::UserFacing,
             tainted_by_errors_flag: Cell::new(false),
             err_count_on_creation: self.sess.err_count(),
             obligations_in_snapshot: Cell::new(false),
@@ -600,7 +600,7 @@ impl_trans_normalize!('gcx,
     Ty<'gcx>,
     &'gcx Substs<'gcx>,
     ty::FnSig<'gcx>,
-    &'gcx ty::BareFnTy<'gcx>,
+    ty::PolyFnSig<'gcx>,
     ty::ClosureSubsts<'gcx>,
     ty::PolyTraitRef<'gcx>,
     ty::ExistentialTraitRef<'gcx>
@@ -1197,16 +1197,19 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     /// as the substitutions for the default, `(T, U)`.
     pub fn type_var_for_def(&self,
                             span: Span,
-                            def: &ty::TypeParameterDef<'tcx>,
+                            def: &ty::TypeParameterDef,
                             substs: &[Kind<'tcx>])
                             -> Ty<'tcx> {
-        let default = def.default.map(|default| {
-            type_variable::Default {
+        let default = if def.has_default {
+            let default = self.tcx.item_type(def.def_id);
+            Some(type_variable::Default {
                 ty: default.subst_spanned(self.tcx, substs, Some(span)),
                 origin_span: span,
-                def_id: def.default_def_id
-            }
-        });
+                def_id: def.def_id
+            })
+        } else {
+            None
+        };
 
 
         let ty_var_id = self.type_variables
@@ -1646,20 +1649,16 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         Some(self.tcx.closure_kind(def_id))
     }
 
-    pub fn closure_type(&self,
-                        def_id: DefId,
-                        substs: ty::ClosureSubsts<'tcx>)
-                        -> ty::ClosureTy<'tcx>
-    {
+    pub fn closure_type(&self, def_id: DefId) -> ty::PolyFnSig<'tcx> {
         if let InferTables::InProgress(tables) = self.tables {
             if let Some(id) = self.tcx.hir.as_local_node_id(def_id) {
-                if let Some(ty) = tables.borrow().closure_tys.get(&id) {
-                    return ty.subst(self.tcx, substs.substs);
+                if let Some(&ty) = tables.borrow().closure_tys.get(&id) {
+                    return ty;
                 }
             }
         }
 
-        self.tcx.closure_type(def_id, substs)
+        self.tcx.closure_type(def_id)
     }
 }
 

src/librustc/lib.rs

@@ -39,6 +39,7 @@
 #![feature(rustc_diagnostic_macros)]
 #![feature(rustc_private)]
 #![feature(slice_patterns)]
+#![feature(specialization)]
 #![feature(staged_api)]
 #![feature(unboxed_closures)]
 
@@ -75,7 +76,6 @@ pub mod infer;
 pub mod lint;
 
 pub mod middle {
-    pub mod astconv_util;
     pub mod expr_use_visitor;
     pub mod const_val;
     pub mod cstore;