Reword diagnostic about relaxing non-Sized bound

* The phrasing "only does something for" made sense back when this
  diagnostic was a (hard) warning. Now however, it's simply a hard
  error and thus completely rules out.
* The primary message was way too long
* The new wording more closely mirrors the wording we use for applying
  other bound modifiers (like `const` and `async`) to incompatible
  traits.
* "all other traits are not bound by default" is no longer accurate
  under Sized Hierarchy. E.g., traits and assoc tys are (currently)
  bounded by `MetaSized` by default but can't be relaxed using
  `?MetaSized` (instead, you relax it by adding `PointeeSized`).
* I've decided against adding any diagnositic notes or suggestions
  for now like "trait `Trait` can't be relaxed as it's not bound by
  default" /which would be incorrect for `MetaSized` and assoc tys
  as mentioned above) or "consider changing `?MetaSized` to
  `PointeeSized`" as the Sized Hierarchy impl is still WIP)

Author: fmease

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -85,17 +85,17 @@ fn search_bounds_for<'tcx>(
     }
 }
 
-fn collect_unbounds<'tcx>(
+fn collect_relaxed_bounds<'tcx>(
     hir_bounds: &'tcx [hir::GenericBound<'tcx>],
     self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
 ) -> SmallVec<[&'tcx PolyTraitRef<'tcx>; 1]> {
-    let mut unbounds: SmallVec<[_; 1]> = SmallVec::new();
+    let mut relaxed_bounds: SmallVec<[_; 1]> = SmallVec::new();
     search_bounds_for(hir_bounds, self_ty_where_predicates, |ptr| {
         if matches!(ptr.modifiers.polarity, hir::BoundPolarity::Maybe(_)) {
-            unbounds.push(ptr);
+            relaxed_bounds.push(ptr);
         }
     });
-    unbounds
+    relaxed_bounds
 }
 
 fn collect_bounds<'a, 'tcx>(
@@ -204,9 +204,13 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 return;
             }
         } else {
-            // Report invalid unbounds on sizedness-bounded generic parameters.
-            let unbounds = collect_unbounds(hir_bounds, self_ty_where_predicates);
-            self.check_and_report_invalid_unbounds_on_param(unbounds);
+            // Report invalid relaxed bounds.
+            // FIXME(more_maybe_bounds): This validation function is only called in contexts where
+            // we perform "sized elaboration". This means, it doesn't get called for e.g.,
+            // trait object types or supertrait bounds!
+            // Ideally, we would do these checks in `Self::lower_poly_trait_ref`!
+            let bounds = collect_relaxed_bounds(hir_bounds, self_ty_where_predicates);
+            self.check_and_report_invalid_relaxed_bounds(bounds);
         }
 
         let collected = collect_sizedness_bounds(tcx, hir_bounds, self_ty_where_predicates, span);

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -8,7 +8,7 @@ use rustc_errors::{
 };
 use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::def_id::DefId;
-use rustc_hir::{self as hir, HirId, LangItem, PolyTraitRef};
+use rustc_hir::{self as hir, HirId, PolyTraitRef};
 use rustc_middle::bug;
 use rustc_middle::ty::fast_reject::{TreatParams, simplify_type};
 use rustc_middle::ty::print::{PrintPolyTraitRefExt as _, PrintTraitRefExt as _};
@@ -34,26 +34,24 @@ use crate::fluent_generated as fluent;
 use crate::hir_ty_lowering::{AssocItemQSelf, HirTyLowerer};
 
 impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
-    /// Check for multiple relaxed default bounds and relaxed bounds of non-sizedness traits.
-    pub(crate) fn check_and_report_invalid_unbounds_on_param(
+    /// Check for duplicate relaxed bounds and relaxed bounds of non-default traits.
+    pub(crate) fn check_and_report_invalid_relaxed_bounds(
         &self,
-        unbounds: SmallVec<[&PolyTraitRef<'_>; 1]>,
+        relaxed_bounds: SmallVec<[&PolyTraitRef<'_>; 1]>,
     ) {
         let tcx = self.tcx();
 
-        let sized_did = tcx.require_lang_item(LangItem::Sized, DUMMY_SP);
-
         let mut unique_bounds = FxIndexSet::default();
         let mut seen_repeat = false;
-        for unbound in &unbounds {
-            if let Res::Def(DefKind::Trait, unbound_def_id) = unbound.trait_ref.path.res {
-                seen_repeat |= !unique_bounds.insert(unbound_def_id);
+        for bound in &relaxed_bounds {
+            if let Res::Def(DefKind::Trait, trait_def_id) = bound.trait_ref.path.res {
+                seen_repeat |= !unique_bounds.insert(trait_def_id);
             }
         }
 
-        if unbounds.len() > 1 {
+        if relaxed_bounds.len() > 1 {
             let err = errors::MultipleRelaxedDefaultBounds {
-                spans: unbounds.iter().map(|ptr| ptr.span).collect(),
+                spans: relaxed_bounds.iter().map(|ptr| ptr.span).collect(),
             };
 
             if seen_repeat {
@@ -63,24 +61,23 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             };
         }
 
-        for unbound in unbounds {
-            if let Res::Def(DefKind::Trait, did) = unbound.trait_ref.path.res
-                && ((did == sized_did) || tcx.is_default_trait(did))
+        let sized_def_id = tcx.require_lang_item(hir::LangItem::Sized, DUMMY_SP);
+
+        for bound in relaxed_bounds {
+            if let Res::Def(DefKind::Trait, def_id) = bound.trait_ref.path.res
+                && (def_id == sized_def_id || tcx.is_default_trait(def_id))
             {
                 continue;
             }
-
-            let unbound_traits = match tcx.sess.opts.unstable_opts.experimental_default_bounds {
-                true => "`?Sized` and `experimental_default_bounds`",
-                false => "`?Sized`",
-            };
             self.dcx().span_err(
-                unbound.span,
-                format!(
-                    "relaxing a default bound only does something for {}; all other traits are \
-                     not bound by default",
-                    unbound_traits
-                ),
+                bound.span,
+                if tcx.sess.opts.unstable_opts.experimental_default_bounds
+                    || tcx.features().more_maybe_bounds()
+                {
+                    "bound modifier `?` can only be applied to default traits like `Sized`"
+                } else {
+                    "bound modifier `?` can only be applied to `Sized`"
+                },
             );
         }
     }

tests/ui/feature-gates/feature-gate-more-maybe-bounds.rs

@@ -9,14 +9,14 @@ fn foo(_: Box<dyn Trait1 + ?Trait2>) {}
 //~^  ERROR relaxed bounds are not permitted in trait object types
 fn bar<T: ?Trait1 + ?Trait2>(_: T) {}
 //~^ ERROR type parameter has more than one relaxed default bound, only one is supported
-//~| ERROR relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
-//~| ERROR relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+//~| ERROR bound modifier `?` can only be applied to `Sized`
+//~| ERROR bound modifier `?` can only be applied to `Sized`
 
 trait Trait {}
 // Do not suggest `#![feature(more_maybe_bounds)]` for repetitions
 fn baz<T: ?Trait + ?Trait>(_ : T) {}
 //~^ ERROR type parameter has more than one relaxed default bound, only one is supported
-//~| ERROR relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
-//~| ERROR relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+//~| ERROR bound modifier `?` can only be applied to `Sized`
+//~| ERROR bound modifier `?` can only be applied to `Sized`
 
 fn main() {}

tests/ui/feature-gates/feature-gate-more-maybe-bounds.stderr

@@ -35,13 +35,13 @@ LL | fn bar<T: ?Trait1 + ?Trait2>(_: T) {}
    = help: add `#![feature(more_maybe_bounds)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/feature-gate-more-maybe-bounds.rs:10:11
    |
 LL | fn bar<T: ?Trait1 + ?Trait2>(_: T) {}
    |           ^^^^^^^
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/feature-gate-more-maybe-bounds.rs:10:21
    |
 LL | fn bar<T: ?Trait1 + ?Trait2>(_: T) {}
@@ -53,13 +53,13 @@ error[E0203]: type parameter has more than one relaxed default bound, only one i
 LL | fn baz<T: ?Trait + ?Trait>(_ : T) {}
    |           ^^^^^^   ^^^^^^
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/feature-gate-more-maybe-bounds.rs:17:11
    |
 LL | fn baz<T: ?Trait + ?Trait>(_ : T) {}
    |           ^^^^^^
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/feature-gate-more-maybe-bounds.rs:17:20
    |
 LL | fn baz<T: ?Trait + ?Trait>(_ : T) {}

tests/ui/impl-trait/opt-out-bound-not-satisfied.rs

@@ -3,8 +3,8 @@
 
 use std::future::Future;
 fn foo() -> impl ?Future<Output = impl Send> {
-    //~^ ERROR: relaxing a default bound only does something for `?Sized`
-    //~| ERROR: relaxing a default bound only does something for `?Sized`
+    //~^ ERROR: bound modifier `?` can only be applied to `Sized`
+    //~| ERROR: bound modifier `?` can only be applied to `Sized`
     ()
 }
 

tests/ui/impl-trait/opt-out-bound-not-satisfied.stderr

@@ -1,10 +1,10 @@
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/opt-out-bound-not-satisfied.rs:5:18
    |
 LL | fn foo() -> impl ?Future<Output = impl Send> {
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/opt-out-bound-not-satisfied.rs:5:18
    |
 LL | fn foo() -> impl ?Future<Output = impl Send> {

tests/ui/issues/issue-37534.rs

@@ -1,5 +1,5 @@
 struct Foo<T: ?Hash> {}
 //~^ ERROR expected trait, found derive macro `Hash`
-//~| ERROR relaxing a default bound only does something for `?Sized`
+//~| ERROR bound modifier `?` can only be applied to `Sized`
 
 fn main() {}

tests/ui/issues/issue-37534.stderr

@@ -9,7 +9,7 @@ help: consider importing this trait instead
 LL + use std::hash::Hash;
    |
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/issue-37534.rs:1:15
    |
 LL | struct Foo<T: ?Hash> {}

tests/ui/issues/issue-87199.rs

@@ -6,12 +6,12 @@
 
 // Check that these function definitions only emit warnings, not errors
 fn arg<T: ?Send>(_: T) {}
-//~^ ERROR: relaxing a default bound only does something for `?Sized`
+//~^ ERROR: bound modifier `?` can only be applied to `Sized`
 fn ref_arg<T: ?Send>(_: &T) {}
-//~^ ERROR: relaxing a default bound only does something for `?Sized`
+//~^ ERROR: bound modifier `?` can only be applied to `Sized`
 fn ret() -> impl Iterator<Item = ()> + ?Send { std::iter::empty() }
-//~^ ERROR: relaxing a default bound only does something for `?Sized`
-//~| ERROR: relaxing a default bound only does something for `?Sized`
+//~^ ERROR: bound modifier `?` can only be applied to `Sized`
+//~| ERROR: bound modifier `?` can only be applied to `Sized`
 
 // Check that there's no `?Sized` relaxation!
 fn main() {

tests/ui/issues/issue-87199.stderr

@@ -1,22 +1,22 @@
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/issue-87199.rs:8:11
    |
 LL | fn arg<T: ?Send>(_: T) {}
    |           ^^^^^
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/issue-87199.rs:10:15
    |
 LL | fn ref_arg<T: ?Send>(_: &T) {}
    |               ^^^^^
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/issue-87199.rs:12:40
    |
 LL | fn ret() -> impl Iterator<Item = ()> + ?Send { std::iter::empty() }
    |                                        ^^^^^
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/issue-87199.rs:12:40
    |
 LL | fn ret() -> impl Iterator<Item = ()> + ?Send { std::iter::empty() }

tests/ui/sized-hierarchy/default-bound.rs

@@ -14,13 +14,13 @@ fn neg_sized<T: ?Sized>() {}
 fn metasized<T: MetaSized>() {}
 
 fn neg_metasized<T: ?MetaSized>() {}
-//~^ ERROR relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+//~^ ERROR bound modifier `?` can only be applied to `Sized`
 
 
 fn pointeesized<T: PointeeSized>() { }
 
 fn neg_pointeesized<T: ?PointeeSized>() { }
-//~^ ERROR relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+//~^ ERROR bound modifier `?` can only be applied to `Sized`
 
 
 fn main() {

tests/ui/sized-hierarchy/default-bound.stderr

@@ -1,10 +1,10 @@
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/default-bound.rs:16:21
    |
 LL | fn neg_metasized<T: ?MetaSized>() {}
    |                     ^^^^^^^^^^
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/default-bound.rs:22:24
    |
 LL | fn neg_pointeesized<T: ?PointeeSized>() { }

tests/ui/trait-bounds/maybe-bound-has-path-args.rs

@@ -2,6 +2,6 @@ trait Trait {}
 
 fn test<T: ?self::<i32>::Trait>() {}
 //~^ ERROR type arguments are not allowed on module `maybe_bound_has_path_args`
-//~| ERROR relaxing a default bound only does something for `?Sized`
+//~| ERROR bound modifier `?` can only be applied to `Sized`
 
 fn main() {}

tests/ui/trait-bounds/maybe-bound-has-path-args.stderr

@@ -1,4 +1,4 @@
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/maybe-bound-has-path-args.rs:3:12
    |
 LL | fn test<T: ?self::<i32>::Trait>() {}

tests/ui/trait-bounds/maybe-bound-with-assoc.rs

@@ -2,11 +2,11 @@ trait HasAssoc {
     type Assoc;
 }
 fn hasassoc<T: ?HasAssoc<Assoc = ()>>() {}
-//~^ ERROR relaxing a default bound
+//~^ ERROR bound modifier `?` can only be applied to `Sized`
 
 trait NoAssoc {}
 fn noassoc<T: ?NoAssoc<Missing = ()>>() {}
-//~^ ERROR relaxing a default bound
+//~^ ERROR bound modifier `?` can only be applied to `Sized`
 //~| ERROR associated type `Missing` not found for `NoAssoc`
 
 fn main() {}

tests/ui/trait-bounds/maybe-bound-with-assoc.stderr

@@ -1,10 +1,10 @@
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/maybe-bound-with-assoc.rs:4:16
    |
 LL | fn hasassoc<T: ?HasAssoc<Assoc = ()>>() {}
    |                ^^^^^^^^^^^^^^^^^^^^^
 
-error: relaxing a default bound only does something for `?Sized`; all other traits are not bound by default
+error: bound modifier `?` can only be applied to `Sized`
   --> $DIR/maybe-bound-with-assoc.rs:8:15
    |
 LL | fn noassoc<T: ?NoAssoc<Missing = ()>>() {}

tests/ui/trait-bounds/more_maybe_bounds.rs

@@ -0,0 +1,25 @@
+// FIXME(more_maybe_bounds): This used to be a check-pass test before rustc started to reject
+// relaxed bounds of *non-default* traits. Thefore, the original test intention has been lost.
+#![feature(auto_traits, more_maybe_bounds, negative_impls)]
+
+trait Trait1 {}
+auto trait Trait2 {}
+
+trait Trait3 : ?Trait1 {}
+trait Trait4 where Self: Trait1 {}
+
+fn foo(_: Box<(dyn Trait3 + ?Trait2)>) {}
+fn bar<T: ?Sized + ?Trait2 + ?Trait1 + ?Trait4>(_: &T) {}
+//~^ ERROR bound modifier `?` can only be applied to default traits like `Sized`
+//~| ERROR bound modifier `?` can only be applied to default traits like `Sized`
+//~| ERROR bound modifier `?` can only be applied to default traits like `Sized`
+
+struct S;
+impl !Trait2 for S {}
+impl Trait1 for S {}
+impl Trait3 for S {}
+
+fn main() {
+    foo(Box::new(S));
+    bar(&S);
+}

tests/ui/trait-bounds/more_maybe_bounds.stderr

@@ -0,0 +1,20 @@
+error: bound modifier `?` can only be applied to default traits like `Sized`
+  --> $DIR/more_maybe_bounds.rs:12:20
+   |
+LL | fn bar<T: ?Sized + ?Trait2 + ?Trait1 + ?Trait4>(_: &T) {}
+   |                    ^^^^^^^
+
+error: bound modifier `?` can only be applied to default traits like `Sized`
+  --> $DIR/more_maybe_bounds.rs:12:30
+   |
+LL | fn bar<T: ?Sized + ?Trait2 + ?Trait1 + ?Trait4>(_: &T) {}
+   |                              ^^^^^^^
+
+error: bound modifier `?` can only be applied to default traits like `Sized`
+  --> $DIR/more_maybe_bounds.rs:12:40
+   |
+LL | fn bar<T: ?Sized + ?Trait2 + ?Trait1 + ?Trait4>(_: &T) {}
+   |                                        ^^^^^^^
+
+error: aborting due to 3 previous errors
+