Suggest boxing or borrowing unsized fields

Author: estebank

src/librustc_middle/traits/mod.rs

@@ -229,6 +229,7 @@ pub enum ObligationCauseCode<'tcx> {
     /// Types of fields (other than the last, except for packed structs) in a struct must be sized.
     FieldSized {
         adt_kind: AdtKind,
+        span: Span,
         last: bool,
     },
 

src/librustc_middle/traits/structural_impls.rs

@@ -156,7 +156,9 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
             super::SizedYieldType => Some(super::SizedYieldType),
             super::InlineAsmSized => Some(super::InlineAsmSized),
             super::RepeatVec(suggest_flag) => Some(super::RepeatVec(suggest_flag)),
-            super::FieldSized { adt_kind, last } => Some(super::FieldSized { adt_kind, last }),
+            super::FieldSized { adt_kind, span, last } => {
+                Some(super::FieldSized { adt_kind, span, last })
+            }
             super::ConstSized => Some(super::ConstSized),
             super::ConstPatternStructural => Some(super::ConstPatternStructural),
             super::SharedStatic => Some(super::SharedStatic),

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

@@ -1856,26 +1856,43 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             ObligationCauseCode::StructInitializerSized => {
                 err.note("structs must have a statically known size to be initialized");
             }
-            ObligationCauseCode::FieldSized { adt_kind: ref item, last } => match *item {
-                AdtKind::Struct => {
-                    if last {
-                        err.note(
-                            "the last field of a packed struct may only have a \
-                             dynamically sized type if it does not need drop to be run",
-                        );
-                    } else {
-                        err.note(
-                            "only the last field of a struct may have a dynamically sized type",
-                        );
+            ObligationCauseCode::FieldSized { adt_kind: ref item, last, span } => {
+                match *item {
+                    AdtKind::Struct => {
+                        if last {
+                            err.note(
+                                "the last field of a packed struct may only have a \
+                                dynamically sized type if it does not need drop to be run",
+                            );
+                        } else {
+                            err.note(
+                                "only the last field of a struct may have a dynamically sized type",
+                            );
+                        }
+                    }
+                    AdtKind::Union => {
+                        err.note("no field of a union may have a dynamically sized type");
+                    }
+                    AdtKind::Enum => {
+                        err.note("no field of an enum variant may have a dynamically sized type");
                     }
                 }
-                AdtKind::Union => {
-                    err.note("no field of a union may have a dynamically sized type");
-                }
-                AdtKind::Enum => {
-                    err.note("no field of an enum variant may have a dynamically sized type");
-                }
-            },
+                err.help("change the field's type to have a statically known size");
+                err.span_suggestion(
+                    span.shrink_to_lo(),
+                    "borrowed types always have a statically known size",
+                    "&".to_string(),
+                    Applicability::MachineApplicable,
+                );
+                err.multipart_suggestion(
+                    "heap allocated types always have a statically known size",
+                    vec![
+                        (span.shrink_to_lo(), "Box<".to_string()),
+                        (span.shrink_to_hi(), ">".to_string()),
+                    ],
+                    Applicability::MachineApplicable,
+                );
+            }
             ObligationCauseCode::ConstSized => {
                 err.note("constant expressions must have a statically known size");
             }

src/librustc_typeck/check/wfcheck.rs

@@ -394,6 +394,7 @@ fn check_type_defn<'tcx, F>(
                                 Some(i) => i,
                                 None => bug!(),
                             },
+                            span: field.span,
                             last,
                         },
                     ),
@@ -1329,7 +1330,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 let field_ty = self.normalize_associated_types_in(field.ty.span, &field_ty);
                 let field_ty = self.resolve_vars_if_possible(&field_ty);
                 debug!("non_enum_variant: type of field {:?} is {:?}", field, field_ty);
-                AdtField { ty: field_ty, span: field.span }
+                AdtField { ty: field_ty, span: field.ty.span }
             })
             .collect();
         AdtVariant { fields, explicit_discr: None }

src/test/ui/const-generics/array-size-in-generic-struct-param.stderr

@@ -16,10 +16,10 @@ LL | struct ArithArrayLen<const N: usize>([u32; 0 + N]);
    = note: this may fail depending on what value the parameter takes
 
 error: constant expression depends on a generic parameter
-  --> $DIR/array-size-in-generic-struct-param.rs:14:5
+  --> $DIR/array-size-in-generic-struct-param.rs:14:10
    |
 LL |     arr: [u8; CFG.arr_size],
-   |     ^^^^^^^^^^^^^^^^^^^^^^^
+   |          ^^^^^^^^^^^^^^^^^^
    |
    = note: this may fail depending on what value the parameter takes
 

src/test/ui/error-codes/E0478.stderr

@@ -1,8 +1,8 @@
 error[E0478]: lifetime bound not satisfied
-  --> $DIR/E0478.rs:4:5
+  --> $DIR/E0478.rs:4:12
    |
 LL |     child: Box<dyn Wedding<'kiss> + 'SnowWhite>,
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: lifetime parameter instantiated with the lifetime `'SnowWhite` as defined on the struct at 3:22
   --> $DIR/E0478.rs:3:22

src/test/ui/feature-gates/feature-gate-infer_static_outlives_requirements.stderr

@@ -1,10 +1,10 @@
 error[E0310]: the parameter type `U` may not live long enough
-  --> $DIR/feature-gate-infer_static_outlives_requirements.rs:5:5
+  --> $DIR/feature-gate-infer_static_outlives_requirements.rs:5:10
    |
 LL | struct Foo<U> {
    |            - help: consider adding an explicit lifetime bound...: `U: 'static`
 LL |     bar: Bar<U>
-   |     ^^^^^^^^^^^ ...so that the type `U` will meet its required lifetime bounds
+   |          ^^^^^^ ...so that the type `U` will meet its required lifetime bounds
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-19380.stderr

@@ -1,13 +1,13 @@
 error[E0038]: the trait `Qiz` cannot be made into an object
-  --> $DIR/issue-19380.rs:11:3
+  --> $DIR/issue-19380.rs:11:9
    |
 LL | trait Qiz {
    |       --- this trait cannot be made into an object...
 LL |   fn qiz();
    |      --- ...because associated function `qiz` has no `self` parameter
 ...
 LL |   foos: &'static [&'static (dyn Qiz + 'static)]
-   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Qiz` cannot be made into an object
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Qiz` cannot be made into an object
    |
 help: consider turning `qiz` into a method by giving it a `&self` argument or constraining it so it does not apply to trait objects
    |

src/test/ui/issues/issue-22874.stderr

@@ -1,8 +1,8 @@
 error[E0277]: the size for values of type `[std::string::String]` cannot be known at compilation time
-  --> $DIR/issue-22874.rs:2:5
+  --> $DIR/issue-22874.rs:2:11
    |
 LL |     rows: [[String]],
-   |     ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |           ^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `[std::string::String]`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

src/test/ui/issues/issue-27060-2.stderr

@@ -1,14 +1,23 @@
 error[E0277]: the size for values of type `T` cannot be known at compilation time
-  --> $DIR/issue-27060-2.rs:3:5
+  --> $DIR/issue-27060-2.rs:3:11
    |
 LL | pub struct Bad<T: ?Sized> {
    |                - this type parameter needs to be `std::marker::Sized`
 LL |     data: T,
-   |     ^^^^^^^ doesn't have a size known at compile-time
+   |           ^ doesn't have a size known at compile-time
    |
    = help: the trait `std::marker::Sized` is not implemented for `T`
    = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: the last field of a packed struct may only have a dynamically sized type if it does not need drop to be run
+   = help: change the field's type to have a statically known size
+help: borrowed types always have a statically known size
+   |
+LL |     data: &T,
+   |           ^
+help: heap allocated types always have a statically known size
+   |
+LL |     data: Box<T>,
+   |           ^^^^ ^
 
 error: aborting due to previous error