Detect missing fields with default values and suggest ..

When a struct definition has default field values, and the use struct ctor has missing field, if all those missing fields have defaults suggest `..`:

```
error[E0063]: missing fields `field1` and `field2` in initializer of `S`
  --> $DIR/non-exhaustive-ctor.rs:16:13
   |
LL |     let _ = S { field: () };
   |             ^ missing `field1` and `field2`
   |
help: all remaining fields have defaults, use `..`
   |
LL |     let _ = S { field: (), .. };
   |                          ++++
```

compiler/rustc_hir_typeck/src/expr.rs

@@ -2349,6 +2349,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 self.report_missing_fields(
                     adt_ty,
                     path_span,
+                    expr.span,
                     remaining_fields,
                     variant,
                     hir_fields,
@@ -2386,6 +2387,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         adt_ty: Ty<'tcx>,
         span: Span,
+        full_span: Span,
         remaining_fields: UnordMap<Ident, (FieldIdx, &ty::FieldDef)>,
         variant: &'tcx ty::VariantDef,
         hir_fields: &'tcx [hir::ExprField<'tcx>],
@@ -2425,6 +2427,35 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         );
         err.span_label(span, format!("missing {remaining_fields_names}{truncated_fields_error}"));
 
+        if remaining_fields.items().all(|(_, (_, field))| field.value.is_some()) {
+            let msg = format!(
+                "all remaining fields have default values, {you_can} use those values with `..`",
+                you_can = if self.tcx.features().default_field_values() {
+                    "you can"
+                } else if self.tcx.sess.is_nightly_build() {
+                    "if you added `#![feature(default_field_values)]` to your crate you could"
+                } else {
+                    "if your crate were nightly-only, you could add \
+                     `#![feature(default_field_values)]` to your crate and"
+                },
+            );
+            if let Some(hir_field) = hir_fields.last() {
+                err.span_suggestion_verbose(
+                    hir_field.span.shrink_to_hi(),
+                    msg,
+                    ", ..".to_string(),
+                    Applicability::MachineApplicable,
+                );
+            } else if hir_fields.is_empty() {
+                err.span_suggestion_verbose(
+                    span.shrink_to_hi().with_hi(full_span.hi()),
+                    msg,
+                    " { .. }".to_string(),
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+
         if let Some(hir_field) = hir_fields.last() {
             self.suggest_fru_from_range_and_emit(hir_field, variant, args, err);
         } else {

tests/ui/structs/default-field-values/non-exhaustive-ctor.rs

@@ -0,0 +1,26 @@
+#![feature(default_field_values)]
+use m::S;
+
+mod m {
+    pub struct S {
+        pub field: () = (),
+        pub field1: Priv = Priv,
+        pub field2: Priv = Priv,
+    }
+    struct Priv;
+}
+
+fn main() {
+    let _ = S { .. }; // ok
+    let _ = S { field: (), .. }; // ok
+    let _ = S { };
+    //~^ ERROR missing fields `field`, `field1` and `field2`
+    let _ = S { field: () };
+    //~^ ERROR missing fields `field1` and `field2`
+    let _ = S { field: (), field1: m::Priv };
+    //~^ ERROR missing field `field2`
+    //~| ERROR unit struct `Priv` is private
+    let _ = S { field: (), field1: m::Priv, field2: m::Priv };
+    //~^ ERROR unit struct `Priv` is private
+    //~| ERROR unit struct `Priv` is private
+}

tests/ui/structs/default-field-values/non-exhaustive-ctor.stderr

@@ -0,0 +1,73 @@
+error[E0603]: unit struct `Priv` is private
+  --> $DIR/non-exhaustive-ctor.rs:20:39
+   |
+LL |     let _ = S { field: (), field1: m::Priv };
+   |                                       ^^^^ private unit struct
+   |
+note: the unit struct `Priv` is defined here
+  --> $DIR/non-exhaustive-ctor.rs:10:5
+   |
+LL |     struct Priv;
+   |     ^^^^^^^^^^^^
+
+error[E0603]: unit struct `Priv` is private
+  --> $DIR/non-exhaustive-ctor.rs:23:39
+   |
+LL |     let _ = S { field: (), field1: m::Priv, field2: m::Priv };
+   |                                       ^^^^ private unit struct
+   |
+note: the unit struct `Priv` is defined here
+  --> $DIR/non-exhaustive-ctor.rs:10:5
+   |
+LL |     struct Priv;
+   |     ^^^^^^^^^^^^
+
+error[E0603]: unit struct `Priv` is private
+  --> $DIR/non-exhaustive-ctor.rs:23:56
+   |
+LL |     let _ = S { field: (), field1: m::Priv, field2: m::Priv };
+   |                                                        ^^^^ private unit struct
+   |
+note: the unit struct `Priv` is defined here
+  --> $DIR/non-exhaustive-ctor.rs:10:5
+   |
+LL |     struct Priv;
+   |     ^^^^^^^^^^^^
+
+error[E0063]: missing fields `field`, `field1` and `field2` in initializer of `S`
+  --> $DIR/non-exhaustive-ctor.rs:16:13
+   |
+LL |     let _ = S { };
+   |             ^ missing `field`, `field1` and `field2`
+   |
+help: all remaining fields have default values, you can use those values with `..`
+   |
+LL |     let _ = S { .. };
+   |               ~~~~~~
+
+error[E0063]: missing fields `field1` and `field2` in initializer of `S`
+  --> $DIR/non-exhaustive-ctor.rs:18:13
+   |
+LL |     let _ = S { field: () };
+   |             ^ missing `field1` and `field2`
+   |
+help: all remaining fields have default values, you can use those values with `..`
+   |
+LL |     let _ = S { field: (), .. };
+   |                          ++++
+
+error[E0063]: missing field `field2` in initializer of `S`
+  --> $DIR/non-exhaustive-ctor.rs:20:13
+   |
+LL |     let _ = S { field: (), field1: m::Priv };
+   |             ^ missing `field2`
+   |
+help: all remaining fields have default values, you can use those values with `..`
+   |
+LL |     let _ = S { field: (), field1: m::Priv, .. };
+   |                                           ++++
+
+error: aborting due to 6 previous errors
+
+Some errors have detailed explanations: E0063, E0603.
+For more information about an error, try `rustc --explain E0063`.