middle: print {Meta,Pointee}Sized in opaques

When `sized_hierarchy` is enabled, rustc should print `MetaSized` or
`PointeeSized` instead of `?Sized` in opaques.

Author: davidtwco

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1018,16 +1018,20 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
 
         let mut traits = FxIndexMap::default();
         let mut fn_traits = FxIndexMap::default();
+        let mut lifetimes = SmallVec::<[ty::Region<'tcx>; 1]>::new();
+
         let mut has_sized_bound = false;
         let mut has_negative_sized_bound = false;
-        let mut lifetimes = SmallVec::<[ty::Region<'tcx>; 1]>::new();
+        let mut has_metasized_bound = false;
+        let mut has_pointeesized_bound = false;
 
         for (predicate, _) in bounds.iter_instantiated_copied(tcx, args) {
             let bound_predicate = predicate.kind();
 
             match bound_predicate.skip_binder() {
                 ty::ClauseKind::Trait(pred) => {
-                    // Don't print `+ Sized`, but rather `+ ?Sized` if absent.
+                    // With `feature(sized_hierarchy)`, don't print `?Sized` as an alias for
+                    // `MetaSized`, and skip sizedness bounds to be added at the end.
                     if tcx.is_lang_item(pred.def_id(), LangItem::Sized) {
                         match pred.polarity {
                             ty::PredicatePolarity::Positive => {
@@ -1036,6 +1040,13 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
                             }
                             ty::PredicatePolarity::Negative => has_negative_sized_bound = true,
                         }
+                    } else if tcx.is_lang_item(pred.def_id(), LangItem::MetaSized) {
+                        has_metasized_bound = true;
+                        continue;
+                    } else if tcx.is_lang_item(pred.def_id(), LangItem::PointeeSized) {
+                        // Unexpected - `PointeeSized` is the absence of bounds.
+                        has_pointeesized_bound = true;
+                        continue;
                     }
 
                     self.insert_trait_and_projection(
@@ -1204,8 +1215,11 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
             })?;
         }
 
+        let using_sized_hierarchy = self.tcx().features().sized_hierarchy();
         let add_sized = has_sized_bound && (first || has_negative_sized_bound);
-        let add_maybe_sized = !has_sized_bound && !has_negative_sized_bound;
+        let add_maybe_sized = has_metasized_bound && !has_negative_sized_bound && !using_sized_hierarchy;
+        // Set `has_pointeesized_bound` if there were no `Sized` or `MetaSized` bounds.
+        has_pointeesized_bound = has_pointeesized_bound || (!has_sized_bound && !has_metasized_bound && !has_negative_sized_bound);
         if add_sized || add_maybe_sized {
             if !first {
                 write!(self, " + ")?;
@@ -1214,6 +1228,16 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
                 write!(self, "?")?;
             }
             write!(self, "Sized")?;
+        } else if has_metasized_bound && using_sized_hierarchy {
+            if !first {
+                write!(self, " + ")?;
+            }
+            write!(self, "MetaSized")?;
+        } else if has_pointeesized_bound && using_sized_hierarchy {
+            if !first {
+                write!(self, " + ")?;
+            }
+            write!(self, "PointeeSized")?;
         }
 
         if !with_forced_trimmed_paths() {

tests/ui/sized-hierarchy/pretty-print-opaque-no-feat.rs

@@ -0,0 +1,13 @@
+//@ compile-flags: --crate-type=lib
+
+pub trait Tr {}
+impl Tr for u32 {}
+
+pub fn foo() -> Box<impl Tr + ?Sized> {
+    if true {
+        let x = foo();
+        let y: Box<dyn Tr> = x;
+//~^ ERROR: the size for values of type `impl Tr + ?Sized` cannot be known
+    }
+    Box::new(1u32)
+}

tests/ui/sized-hierarchy/pretty-print-opaque-no-feat.stderr

@@ -0,0 +1,12 @@
+error[E0277]: the size for values of type `impl Tr + ?Sized` cannot be known at compilation time
+  --> $DIR/pretty-print-opaque-no-feat.rs:9:30
+   |
+LL |         let y: Box<dyn Tr> = x;
+   |                              ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `impl Tr + ?Sized`
+   = note: required for the cast from `Box<impl Tr + ?Sized>` to `Box<dyn Tr>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/sized-hierarchy/pretty-print-opaque.rs

@@ -0,0 +1,45 @@
+//@ compile-flags: --crate-type=lib
+#![feature(sized_hierarchy)]
+
+use std::marker::{MetaSized, PointeeSized};
+
+pub trait Tr: PointeeSized {}
+impl Tr for u32 {}
+
+pub fn sized() -> Box<impl Tr + Sized> {
+    if true {
+        let x = sized();
+        let y: Box<dyn Tr> = x;
+    }
+    Box::new(1u32)
+}
+
+pub fn neg_sized() -> Box<impl Tr + ?Sized> {
+    if true {
+        let x = neg_sized();
+        let y: Box<dyn Tr> = x;
+//~^ ERROR: the size for values of type `impl Tr + MetaSized` cannot be known
+    }
+    Box::new(1u32)
+}
+
+pub fn metasized() -> Box<impl Tr + MetaSized> {
+    if true {
+        let x = metasized();
+        let y: Box<dyn Tr> = x;
+//~^ ERROR: the size for values of type `impl Tr + MetaSized` cannot be known
+    }
+    Box::new(1u32)
+}
+
+pub fn pointeesized() -> Box<impl Tr + PointeeSized> {
+//~^ ERROR: the size for values of type `impl Tr + PointeeSized` cannot be known
+    if true {
+        let x = pointeesized();
+//~^ ERROR: the size for values of type `impl Tr + PointeeSized` cannot be known
+        let y: Box<dyn Tr> = x;
+//~^ ERROR: the size for values of type `impl Tr + PointeeSized` cannot be known
+    }
+    Box::new(1u32)
+}
+

tests/ui/sized-hierarchy/pretty-print-opaque.stderr

@@ -0,0 +1,50 @@
+error[E0277]: the size for values of type `impl Tr + PointeeSized` cannot be known
+  --> $DIR/pretty-print-opaque.rs:35:26
+   |
+LL | pub fn pointeesized() -> Box<impl Tr + PointeeSized> {
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a known size
+   |
+   = help: the trait `MetaSized` is not implemented for `impl Tr + PointeeSized`
+note: required by a bound in `Box`
+  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
+
+error[E0277]: the size for values of type `impl Tr + MetaSized` cannot be known at compilation time
+  --> $DIR/pretty-print-opaque.rs:20:30
+   |
+LL |         let y: Box<dyn Tr> = x;
+   |                              ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `impl Tr + MetaSized`
+   = note: required for the cast from `Box<impl Tr + MetaSized>` to `Box<dyn Tr>`
+
+error[E0277]: the size for values of type `impl Tr + MetaSized` cannot be known at compilation time
+  --> $DIR/pretty-print-opaque.rs:29:30
+   |
+LL |         let y: Box<dyn Tr> = x;
+   |                              ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `impl Tr + MetaSized`
+   = note: required for the cast from `Box<impl Tr + MetaSized>` to `Box<dyn Tr>`
+
+error[E0277]: the size for values of type `impl Tr + PointeeSized` cannot be known
+  --> $DIR/pretty-print-opaque.rs:38:17
+   |
+LL |         let x = pointeesized();
+   |                 ^^^^^^^^^^^^^^ doesn't have a known size
+   |
+   = help: the trait `MetaSized` is not implemented for `impl Tr + PointeeSized`
+note: required by a bound in `Box`
+  --> $SRC_DIR/alloc/src/boxed.rs:LL:COL
+
+error[E0277]: the size for values of type `impl Tr + PointeeSized` cannot be known at compilation time
+  --> $DIR/pretty-print-opaque.rs:40:30
+   |
+LL |         let y: Box<dyn Tr> = x;
+   |                              ^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `impl Tr + PointeeSized`
+   = note: required for the cast from `Box<impl Tr + PointeeSized>` to `Box<dyn Tr>`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0277`.