middle: print {Meta,Pointee}Sized in opaques

davidtwco · davidtwco · commit 118d4e62c380 · 2025-06-16T23:04:34.000Z
When `sized_hierarchy` is enabled, rustc should print `MetaSized` or
`PointeeSized` instead of `?Sized` in opaques.
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -1069,24 +1069,35 @@ 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_meta_sized_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.
-                    if tcx.is_lang_item(pred.def_id(), LangItem::Sized) {
-                        match pred.polarity {
+                    // With `feature(sized_hierarchy)`, don't print `?Sized` as an alias for
+                    // `MetaSized`, and skip sizedness bounds to be added at the end.
+                    match tcx.as_lang_item(pred.def_id()) {
+                        Some(LangItem::Sized) => match pred.polarity {
                             ty::PredicatePolarity::Positive => {
                                 has_sized_bound = true;
                                 continue;
                             }
                             ty::PredicatePolarity::Negative => has_negative_sized_bound = true,
+                        },
+                        Some(LangItem::MetaSized) => {
+                            has_meta_sized_bound = true;
+                            continue;
+                        }
+                        Some(LangItem::PointeeSized) => {
+                            bug!("`PointeeSized` is removed during lowering");
                         }
+                        _ => (),
                     }
 
                     self.insert_trait_and_projection(
@@ -1255,8 +1266,13 @@ 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_meta_sized_bound && !has_negative_sized_bound && !using_sized_hierarchy;
+        // Set `has_pointee_sized_bound` if there were no `Sized` or `MetaSized` bounds.
+        let has_pointee_sized_bound =
+            !has_sized_bound && !has_meta_sized_bound && !has_negative_sized_bound;
         if add_sized || add_maybe_sized {
             if !first {
                 write!(self, " + ")?;
@@ -1265,6 +1281,16 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
                 write!(self, "?")?;
             }
             write!(self, "Sized")?;
+        } else if has_meta_sized_bound && using_sized_hierarchy {
+            if !first {
+                write!(self, " + ")?;
+            }
+            write!(self, "MetaSized")?;
+        } else if has_pointee_sized_bound && using_sized_hierarchy {
+            if !first {
+                write!(self, " + ")?;
+            }
+            write!(self, "PointeeSized")?;
         }
 
         if !with_forced_trimmed_paths() {
diff --git a/tests/ui/sized-hierarchy/pretty-print-opaque-no-feat.rs b/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)
+}
diff --git a/tests/ui/sized-hierarchy/pretty-print-opaque-no-feat.stderr b/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`.
diff --git a/tests/ui/sized-hierarchy/pretty-print-opaque.rs b/tests/ui/sized-hierarchy/pretty-print-opaque.rs
@@ -0,0 +1,44 @@
+//@ 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)
+}
diff --git a/tests/ui/sized-hierarchy/pretty-print-opaque.stderr b/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`.
