Rollup merge of rust-lang#40398 - eddyb:struct-hint, r=nikomatsakis

Propagate expected type hints through struct literals.

Partial fix for rust-lang#31260 to maximize backwards-compatibility, i.e. the hint is provided but not coerced to.

The added test works because `{...; x}` with a hint of `T` coerces `x` to `T`, and the reasoning why that is slightly different has to do with DSTs: `&Struct { tail: [x] }: &Struct<[T]>` has a hint of `[T]` for `[x]`, but the inferred type should be `[T; 1]` to succeed later, so `[x]` shouldn't be *forced* to be `[T]`.

*However*, implementing that complete behavior in a backwards-compatible way may be non-trivial, and has not yet been fully investigated, while this PR fixes rust-lang#40355 and can be backported.

r? @nikomatsakis

src/librustc_typeck/check/callee.rs

@@ -254,7 +254,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 
         // Call the generic checker.
         let expected_arg_tys =
-            self.expected_types_for_fn_args(call_expr.span,
+            self.expected_inputs_for_expected_output(call_expr.span,
                                             expected,
                                             fn_sig.output(),
                                             fn_sig.inputs());
@@ -280,7 +280,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         // do know the types expected for each argument and the return
         // type.
 
-        let expected_arg_tys = self.expected_types_for_fn_args(call_expr.span,
+        let expected_arg_tys = self.expected_inputs_for_expected_output(call_expr.span,
                                                                expected,
                                                                fn_sig.output().clone(),
                                                                fn_sig.inputs());

src/librustc_typeck/check/mod.rs

@@ -2292,7 +2292,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             match method_fn_ty.sty {
                 ty::TyFnDef(def_id, .., ref fty) => {
                     // HACK(eddyb) ignore self in the definition (see above).
-                    let expected_arg_tys = self.expected_types_for_fn_args(
+                    let expected_arg_tys = self.expected_inputs_for_expected_output(
                         sp,
                         expected,
                         fty.0.output(),
@@ -2645,14 +2645,14 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         TypeAndSubsts { substs: substs, ty: substd_ty }
     }
 
-    /// Unifies the return type with the expected type early, for more coercions
-    /// and forward type information on the argument expressions.
-    fn expected_types_for_fn_args(&self,
-                                  call_span: Span,
-                                  expected_ret: Expectation<'tcx>,
-                                  formal_ret: Ty<'tcx>,
-                                  formal_args: &[Ty<'tcx>])
-                                  -> Vec<Ty<'tcx>> {
+    /// Unifies the output type with the expected type early, for more coercions
+    /// and forward type information on the input expressions.
+    fn expected_inputs_for_expected_output(&self,
+                                           call_span: Span,
+                                           expected_ret: Expectation<'tcx>,
+                                           formal_ret: Ty<'tcx>,
+                                           formal_args: &[Ty<'tcx>])
+                                           -> Vec<Ty<'tcx>> {
         let expected_args = expected_ret.only_has_type(self).and_then(|ret_ty| {
             self.fudge_regions_if_ok(&RegionVariableOrigin::Coercion(call_span), || {
                 // Attempt to apply a subtyping relationship between the formal
@@ -2675,7 +2675,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 }).collect())
             }).ok()
         }).unwrap_or(vec![]);
-        debug!("expected_types_for_fn_args(formal={:?} -> {:?}, expected={:?} -> {:?})",
+        debug!("expected_inputs_for_expected_output(formal={:?} -> {:?}, expected={:?} -> {:?})",
                formal_args, formal_ret,
                expected_args, expected_ret);
         expected_args
@@ -3032,14 +3032,22 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 
     fn check_expr_struct_fields(&self,
                                 adt_ty: Ty<'tcx>,
+                                expected: Expectation<'tcx>,
                                 expr_id: ast::NodeId,
                                 span: Span,
                                 variant: &'tcx ty::VariantDef,
                                 ast_fields: &'gcx [hir::Field],
                                 check_completeness: bool) {
         let tcx = self.tcx;
-        let (substs, adt_kind, kind_name) = match adt_ty.sty {
-            ty::TyAdt(adt, substs) => (substs, adt.adt_kind(), adt.variant_descr()),
+
+        let adt_ty_hint =
+            self.expected_inputs_for_expected_output(span, expected, adt_ty, &[adt_ty])
+                .get(0).cloned().unwrap_or(adt_ty);
+
+        let (substs, hint_substs, adt_kind, kind_name) = match (&adt_ty.sty, &adt_ty_hint.sty) {
+            (&ty::TyAdt(adt, substs), &ty::TyAdt(_, hint_substs)) => {
+                (substs, hint_substs, adt.adt_kind(), adt.variant_descr())
+            }
             _ => span_bug!(span, "non-ADT passed to check_expr_struct_fields")
         };
 
@@ -3054,10 +3062,12 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 
         // Typecheck each field.
         for field in ast_fields {
-            let expected_field_type;
+            let final_field_type;
+            let field_type_hint;
 
             if let Some(v_field) = remaining_fields.remove(&field.name.node) {
-                expected_field_type = self.field_ty(field.span, v_field, substs);
+                final_field_type = self.field_ty(field.span, v_field, substs);
+                field_type_hint = self.field_ty(field.span, v_field, hint_substs);
 
                 seen_fields.insert(field.name.node, field.span);
 
@@ -3069,7 +3079,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 }
             } else {
                 error_happened = true;
-                expected_field_type = tcx.types.err;
+                final_field_type = tcx.types.err;
+                field_type_hint = tcx.types.err;
                 if let Some(_) = variant.find_field_named(field.name.node) {
                     let mut err = struct_span_err!(self.tcx.sess,
                                                 field.name.span,
@@ -3091,7 +3102,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 
             // Make sure to give a type to the field even if there's
             // an error, so we can continue typechecking
-            self.check_expr_coercable_to_type(&field.expr, expected_field_type);
+            let ty = self.check_expr_with_hint(&field.expr, field_type_hint);
+            self.demand_coerce(&field.expr, ty, final_field_type);
         }
 
         // Make sure the programmer specified correct number of fields.
@@ -3201,6 +3213,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 
     fn check_expr_struct(&self,
                          expr: &hir::Expr,
+                         expected: Expectation<'tcx>,
                          qpath: &hir::QPath,
                          fields: &'gcx [hir::Field],
                          base_expr: &'gcx Option<P<hir::Expr>>) -> Ty<'tcx>
@@ -3219,7 +3232,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             hir::QPath::TypeRelative(ref qself, _) => qself.span
         };
 
-        self.check_expr_struct_fields(struct_ty, expr.id, path_span, variant, fields,
+        self.check_expr_struct_fields(struct_ty, expected, expr.id, path_span, variant, fields,
                                       base_expr.is_none());
         if let &Some(ref base_expr) = base_expr {
             self.check_expr_has_type(base_expr, struct_ty);
@@ -3764,7 +3777,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             }
           }
           hir::ExprStruct(ref qpath, ref fields, ref base_expr) => {
-            self.check_expr_struct(expr, qpath, fields, base_expr)
+            self.check_expr_struct(expr, expected, qpath, fields, base_expr)
           }
           hir::ExprField(ref base, ref field) => {
             self.check_field(expr, lvalue_pref, &base, field)

src/test/run-pass/issue-31260.rs

@@ -0,0 +1,20 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub struct Struct<K: 'static> {
+    pub field: K,
+}
+
+// Partial fix for #31260, doesn't work without {...}.
+static STRUCT: Struct<&'static [u8]> = Struct {
+    field: {&[1]}
+};
+
+fn main() {}