Auto merge of #26870 - jroesch:default-typaram-fallback, r=nikomatsakis

This PR completes [RFC 213](https://github.com/rust-lang/rfcs/blob/master/text/0213-defaulted-type-params.md) by allowing default type parameters to influence inference. This is almost certainly a breaking change due to interactions between default type parameters and the old fallback algorithm used for integral and floating point literals.

The error messages still require polish but I wanted to get early review and feedback from others on the the changes, error messages, and test cases. I also imagine we will want to run anywhere from 1-3 versions of this on crater and evaluate the impact, and it would be best to get that ball rolling. 

The only outstanding issue I'm aware of is that type alias defaults don't work. It seems this may require significant restructuring, since during inference type aliases have already been expanded. @nikomatsakis might be able to provide some clarity here.

r? @nikomatsakis 

cc @eddyb @gankro @aturon @brson

src/doc/reference.md

@@ -2368,6 +2368,8 @@ The currently implemented features of the reference compiler are:
                               internally without imposing on callers
                               (i.e. making them behave like function calls in
                               terms of encapsulation).
+* - `default_type_parameter_fallback` - Allows type parameter defaults to
+                                        influence type inference.
 
 If a feature is promoted to a language feature, then all existing programs will
 start to receive compilation warnings about `#![feature]` directives which enabled

src/librustc/ast_map/mod.rs

@@ -119,6 +119,7 @@ pub enum Node<'ast> {
     NodeStructCtor(&'ast StructDef),
 
     NodeLifetime(&'ast Lifetime),
+    NodeTyParam(&'ast TyParam)
 }
 
 /// Represents an entry and its parent NodeID.
@@ -142,6 +143,7 @@ enum MapEntry<'ast> {
     EntryBlock(NodeId, &'ast Block),
     EntryStructCtor(NodeId, &'ast StructDef),
     EntryLifetime(NodeId, &'ast Lifetime),
+    EntryTyParam(NodeId, &'ast TyParam),
 
     /// Roots for node trees.
     RootCrate,
@@ -175,7 +177,8 @@ impl<'ast> MapEntry<'ast> {
             NodePat(n) => EntryPat(p, n),
             NodeBlock(n) => EntryBlock(p, n),
             NodeStructCtor(n) => EntryStructCtor(p, n),
-            NodeLifetime(n) => EntryLifetime(p, n)
+            NodeLifetime(n) => EntryLifetime(p, n),
+            NodeTyParam(n) => EntryTyParam(p, n),
         }
     }
 
@@ -194,6 +197,7 @@ impl<'ast> MapEntry<'ast> {
             EntryBlock(id, _) => id,
             EntryStructCtor(id, _) => id,
             EntryLifetime(id, _) => id,
+            EntryTyParam(id, _) => id,
             _ => return None
         })
     }
@@ -213,6 +217,7 @@ impl<'ast> MapEntry<'ast> {
             EntryBlock(_, n) => NodeBlock(n),
             EntryStructCtor(_, n) => NodeStructCtor(n),
             EntryLifetime(_, n) => NodeLifetime(n),
+            EntryTyParam(_, n) => NodeTyParam(n),
             _ => return None
         })
     }
@@ -573,6 +578,7 @@ impl<'ast> Map<'ast> {
             Some(NodePat(pat)) => pat.span,
             Some(NodeBlock(block)) => block.span,
             Some(NodeStructCtor(_)) => self.expect_item(self.get_parent(id)).span,
+            Some(NodeTyParam(ty_param)) => ty_param.span,
             _ => return None,
         };
         Some(sp)
@@ -815,6 +821,14 @@ impl<'ast> Visitor<'ast> for NodeCollector<'ast> {
         self.parent_node = parent_node;
     }
 
+    fn visit_generics(&mut self, generics: &'ast Generics) {
+        for ty_param in generics.ty_params.iter() {
+            self.insert(ty_param.id, NodeTyParam(ty_param));
+        }
+
+        visit::walk_generics(self, generics);
+    }
+
     fn visit_trait_item(&mut self, ti: &'ast TraitItem) {
         let parent_node = self.parent_node;
         self.parent_node = ti.id;
@@ -1015,7 +1029,7 @@ impl<'a> NodePrinter for pprust::State<'a> {
             NodePat(a)         => self.print_pat(&*a),
             NodeBlock(a)       => self.print_block(&*a),
             NodeLifetime(a)    => self.print_lifetime(&*a),
-
+            NodeTyParam(_)     => panic!("cannot print TyParam"),
             // these cases do not carry enough information in the
             // ast_map to reconstruct their full structure for pretty
             // printing.
@@ -1123,6 +1137,9 @@ fn node_id_to_string(map: &Map, id: NodeId, include_id: bool) -> String {
             format!("lifetime {}{}",
                     pprust::lifetime_to_string(&**l), id_str)
         }
+        Some(NodeTyParam(ref ty_param)) => {
+            format!("typaram {:?}{}", ty_param, id_str)
+        }
         None => {
             format!("unknown node{}", id_str)
         }

src/librustc/lib.rs

@@ -56,6 +56,7 @@
 #![feature(slice_splits)]
 #![feature(slice_patterns)]
 #![feature(slice_position_elem)]
+#![feature(slice_concat_ext)]
 #![feature(staged_api)]
 #![feature(str_char)]
 #![feature(str_match_indices)]

src/librustc/metadata/tydecode.rs

@@ -833,6 +833,7 @@ fn parse_type_param_def_<'a, 'tcx, F>(st: &mut PState<'a, 'tcx>, conv: &mut F)
     assert_eq!(next(st), '|');
     let index = parse_u32(st);
     assert_eq!(next(st), '|');
+    let default_def_id = parse_def_(st, NominalType, conv);
     let default = parse_opt(st, |st| parse_ty_(st, conv));
     let object_lifetime_default = parse_object_lifetime_default(st, conv);
 
@@ -841,6 +842,7 @@ fn parse_type_param_def_<'a, 'tcx, F>(st: &mut PState<'a, 'tcx>, conv: &mut F)
         def_id: def_id,
         space: space,
         index: index,
+        default_def_id: default_def_id,
         default: default,
         object_lifetime_default: object_lifetime_default,
     }

src/librustc/metadata/tyencode.rs

@@ -409,9 +409,9 @@ pub fn enc_region_bounds<'a, 'tcx>(w: &mut Encoder,
 
 pub fn enc_type_param_def<'a, 'tcx>(w: &mut Encoder, cx: &ctxt<'a, 'tcx>,
                                     v: &ty::TypeParameterDef<'tcx>) {
-    mywrite!(w, "{}:{}|{}|{}|",
+    mywrite!(w, "{}:{}|{}|{}|{}|",
              token::get_name(v.name), (cx.ds)(v.def_id),
-             v.space.to_uint(), v.index);
+             v.space.to_uint(), v.index, (cx.ds)(v.default_def_id));
     enc_opt(w, v.default, |w, t| enc_ty(w, cx, t));
     enc_object_lifetime_default(w, cx, v.object_lifetime_default);
 }

src/librustc/middle/infer/error_reporting.rs

@@ -893,8 +893,8 @@ impl<'a, 'tcx> ErrorReporting<'tcx> for InferCtxt<'a, 'tcx> {
             self.report_inference_failure(vo.clone());
         }
         self.give_suggestion(same_regions);
-        for &(ref trace, terr) in trace_origins {
-            self.report_and_explain_type_error(trace.clone(), &terr);
+        for &(ref trace, ref terr) in trace_origins {
+            self.report_and_explain_type_error(trace.clone(), terr);
         }
     }
 

src/librustc/middle/infer/mod.rs

@@ -653,6 +653,50 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
     }
 
+    /// Returns a type variable's default fallback if any exists. A default
+    /// must be attached to the variable when created, if it is created
+    /// without a default, this will return None.
+    ///
+    /// This code does not apply to integral or floating point variables,
+    /// only to use declared defaults.
+    ///
+    /// See `new_ty_var_with_default` to create a type variable with a default.
+    /// See `type_variable::Default` for details about what a default entails.
+    pub fn default(&self, ty: Ty<'tcx>) -> Option<type_variable::Default<'tcx>> {
+        match ty.sty {
+            ty::TyInfer(ty::TyVar(vid)) => self.type_variables.borrow().default(vid),
+            _ => None
+        }
+    }
+
+    pub fn unsolved_variables(&self) -> Vec<ty::Ty<'tcx>> {
+        let mut variables = Vec::new();
+
+        let unbound_ty_vars = self.type_variables
+                                  .borrow()
+                                  .unsolved_variables()
+                                  .into_iter()
+                                  .map(|t| self.tcx.mk_var(t));
+
+        let unbound_int_vars = self.int_unification_table
+                                   .borrow_mut()
+                                   .unsolved_variables()
+                                   .into_iter()
+                                   .map(|v| self.tcx.mk_int_var(v));
+
+        let unbound_float_vars = self.float_unification_table
+                                     .borrow_mut()
+                                     .unsolved_variables()
+                                     .into_iter()
+                                     .map(|v| self.tcx.mk_float_var(v));
+
+        variables.extend(unbound_ty_vars);
+        variables.extend(unbound_int_vars);
+        variables.extend(unbound_float_vars);
+
+        return variables;
+    }
+
     fn combine_fields(&'a self, a_is_expected: bool, trace: TypeTrace<'tcx>)
                       -> CombineFields<'a, 'tcx> {
         CombineFields {infcx: self,
@@ -956,13 +1000,22 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn next_ty_var_id(&self, diverging: bool) -> TyVid {
         self.type_variables
             .borrow_mut()
-            .new_var(diverging)
+            .new_var(diverging, None)
     }
 
     pub fn next_ty_var(&self) -> Ty<'tcx> {
         self.tcx.mk_var(self.next_ty_var_id(false))
     }
 
+    pub fn next_ty_var_with_default(&self,
+                                    default: Option<type_variable::Default<'tcx>>) -> Ty<'tcx> {
+        let ty_var_id = self.type_variables
+                            .borrow_mut()
+                            .new_var(false, default);
+
+        self.tcx.mk_var(ty_var_id)
+    }
+
     pub fn next_diverging_ty_var(&self) -> Ty<'tcx> {
         self.tcx.mk_var(self.next_ty_var_id(true))
     }
@@ -996,20 +1049,55 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             .collect()
     }
 
+    // We have to take `&mut Substs` in order to provide the correct substitutions for defaults
+    // along the way, for this reason we don't return them.
+    pub fn type_vars_for_defs(&self,
+                              span: Span,
+                              space: subst::ParamSpace,
+                              substs: &mut Substs<'tcx>,
+                              defs: &[ty::TypeParameterDef<'tcx>]) {
+
+        let mut vars = Vec::with_capacity(defs.len());
+
+        for def in defs.iter() {
+            let default = def.default.map(|default| {
+                type_variable::Default {
+                    ty: default.subst_spanned(self.tcx, substs, Some(span)),
+                    origin_span: span,
+                    def_id: def.default_def_id
+                }
+            });
+
+            let ty_var = self.next_ty_var_with_default(default);
+            substs.types.push(space, ty_var);
+            vars.push(ty_var)
+        }
+    }
+
     /// Given a set of generics defined on a type or impl, returns a substitution mapping each
     /// type/region parameter to a fresh inference variable.
     pub fn fresh_substs_for_generics(&self,
                                      span: Span,
                                      generics: &ty::Generics<'tcx>)
                                      -> subst::Substs<'tcx>
     {
-        let type_params =
-            generics.types.map(
-                |_| self.next_ty_var());
+        let type_params = subst::VecPerParamSpace::empty();
+
         let region_params =
             generics.regions.map(
                 |d| self.next_region_var(EarlyBoundRegion(span, d.name)));
-        subst::Substs::new(type_params, region_params)
+
+        let mut substs = subst::Substs::new(type_params, region_params);
+
+        for space in subst::ParamSpace::all().iter() {
+            self.type_vars_for_defs(
+                span,
+                *space,
+                &mut substs,
+                generics.types.get_slice(*space));
+        }
+
+        return substs;
     }
 
     /// Given a set of generics defined on a trait, returns a substitution mapping each output
@@ -1027,13 +1115,17 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         assert!(generics.regions.len(subst::SelfSpace) == 0);
         assert!(generics.regions.len(subst::FnSpace) == 0);
 
-        let type_parameter_count = generics.types.len(subst::TypeSpace);
-        let type_parameters = self.next_ty_vars(type_parameter_count);
+        let type_params = Vec::new();
 
         let region_param_defs = generics.regions.get_slice(subst::TypeSpace);
         let regions = self.region_vars_for_defs(span, region_param_defs);
 
-        subst::Substs::new_trait(type_parameters, regions, self_ty)
+        let mut substs = subst::Substs::new_trait(type_params, regions, self_ty);
+
+        let type_parameter_defs = generics.types.get_slice(subst::TypeSpace);
+        self.type_vars_for_defs(span, subst::TypeSpace, &mut substs, type_parameter_defs);
+
+        return substs;
     }
 
     pub fn fresh_bound_region(&self, debruijn: ty::DebruijnIndex) -> ty::Region {
@@ -1268,6 +1360,25 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         self.report_and_explain_type_error(trace, err);
     }
 
+    pub fn report_conflicting_default_types(&self,
+                                            span: Span,
+                                            expected: type_variable::Default<'tcx>,
+                                            actual: type_variable::Default<'tcx>) {
+        let trace = TypeTrace {
+            origin: Misc(span),
+            values: Types(ty::ExpectedFound {
+                expected: expected.ty,
+                found: actual.ty
+            })
+        };
+
+        self.report_and_explain_type_error(trace,
+            &TypeError::TyParamDefaultMismatch(ty::ExpectedFound {
+                expected: expected,
+                found: actual
+        }));
+    }
+
     pub fn replace_late_bound_regions_with_fresh_var<T>(
         &self,
         span: Span,