Move dyn parser-disambiguation from ty::Dynamic to the using arms

Wrap multi-bound `impl`/`dyn` at the positions where the parser is
ambiguous: `&T`, `*const T`, `*mut T`, and the function-pointer /
`Fn(..) -> T` return slot. The trait method covering this is renamed
to `add_disambiguating_parens` since it now applies to both prefix
and return positions; `inner_needs_disambiguating_parens` for
`Dynamic` counts the printable region alongside the trait predicates
that the self-wrap used to handle.

Non-prefix positions where the parser does not need parens
(`Box<dyn A + 'a>`, `[dyn A + 'a; N]`, top-level dyn) drop them. The
bulk of the touched `.stderr` baselines reflect this cleanup; inline
`//~ ERROR` annotations are adjusted to match.

Author: onehr

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

@@ -353,11 +353,13 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
     fn should_print_optional_region(&self, region: ty::Region<'tcx>) -> bool;
 
     /// Whether `pretty_print_type` should wrap a multi-bound `impl` / `dyn`
-    /// inner in parens after a prefix type constructor (`&`, `&mut`, `*const`,
-    /// `*mut`), so the output is `&(impl A + B)` rather than the parser-
-    /// ambiguous `&impl A + B`. Byte-stable printers (mangling, `type_name`)
-    /// override this to `false`.
-    fn add_disambiguating_parens_in_prefix_position(&self) -> bool {
+    /// inner in parens at positions where the bare form would be parser-
+    /// ambiguous: after a prefix type constructor (`&`, `&mut`, `*const`,
+    /// `*mut`) where `&T + B` parses as `(&T) + B`, and in the function-
+    /// pointer / `Fn(..) -> T` return position where `-> T + B` parses as
+    /// the outer signature picking up an extra bound. Byte-stable printers
+    /// (mangling, etc.) override this to `false`.
+    fn add_disambiguating_parens(&self) -> bool {
         true
     }
 
@@ -817,16 +819,11 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
             }
             ty::Adt(def, args) => self.print_def_path(def.did(), args)?,
             ty::Dynamic(data, r) => {
-                let print_r = self.should_print_optional_region(r);
-                if print_r {
-                    write!(self, "(")?;
-                }
                 write!(self, "dyn ")?;
                 data.print(self)?;
-                if print_r {
+                if self.should_print_optional_region(r) {
                     write!(self, " + ")?;
                     r.print(self)?;
-                    write!(self, ")")?;
                 }
             }
             ty::Foreign(def_id) => self.print_def_path(def_id, &[])?,
@@ -1100,7 +1097,7 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
     /// print with a top-level `+`. Without the wrap, `&impl A + B` parses as
     /// the ambiguous `(&impl A) + B`.
     fn inner_needs_disambiguating_parens(&self, ty: Ty<'tcx>) -> bool {
-        if !self.add_disambiguating_parens_in_prefix_position() || self.should_print_verbose() {
+        if !self.add_disambiguating_parens() || self.should_print_verbose() {
             return false;
         }
         match ty.kind() {
@@ -1114,24 +1111,21 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
                 self.opaque_has_multiple_bounds(*def_id)
             }
             ty::Dynamic(predicates, region) => {
-                if self.should_print_optional_region(*region) {
-                    // `ty::Dynamic` self-wraps when it has an explicit region.
-                    false
-                } else {
-                    // Projections inline into the principal as `<Item = X>`;
-                    // only principal/auto traits produce a top-level `+`.
-                    predicates
-                        .iter()
-                        .filter(|pred| {
-                            matches!(
-                                pred.skip_binder(),
-                                ty::ExistentialPredicate::Trait(_)
-                                    | ty::ExistentialPredicate::AutoTrait(_)
-                            )
-                        })
-                        .count()
-                        > 1
-                }
+                // Projections inline into the principal as `<Item = X>`;
+                // only principal/auto traits produce a top-level `+`. An
+                // explicit (printable) region adds one more `+`-joined part.
+                let trait_count = predicates
+                    .iter()
+                    .filter(|pred| {
+                        matches!(
+                            pred.skip_binder(),
+                            ty::ExistentialPredicate::Trait(_)
+                                | ty::ExistentialPredicate::AutoTrait(_)
+                        )
+                    })
+                    .count();
+                let region_count = usize::from(self.should_print_optional_region(*region));
+                trait_count + region_count > 1
             }
             _ => false,
         }
@@ -1150,7 +1144,7 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
         // Mirror `pretty_print_opaque_impl_type`'s sized-bound handling:
         // positive `Sized` and `MetaSized` are absorbed into the synthetic
         // suffix below; negative `Sized` (`?Sized`) falls through and is
-        // printed inline. Only clause *kinds* are inspected here, so the
+        // printed inline. We only ever look at clause *kinds* here, so the
         // identity-instantiated bounds carry all the information we need.
         let mut trait_emits = 0usize;
         let mut lifetimes_count = 0usize;
@@ -1669,7 +1663,10 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
         write!(self, ")")?;
         if !output.is_unit() {
             write!(self, " -> ")?;
-            output.print(self)?;
+            // `Fn(..) -> X + B` parses as if `+ B` extended the outer signature,
+            // so wrap if `X` would print with `+`-joined bounds. Same machinery
+            // as the `&T`, `*const T`, `*mut T` prefix arms.
+            self.print_inner_with_disambiguating_parens(output)?;
         }
 
         Ok(())

compiler/rustc_symbol_mangling/src/legacy.rs

@@ -479,7 +479,7 @@ impl<'tcx> PrettyPrinter<'tcx> for LegacySymbolMangler<'tcx> {
 
     // Mangled symbols are ABI-stable; don't pick up the default printer's
     // parser-disambiguating parens.
-    fn add_disambiguating_parens_in_prefix_position(&self) -> bool {
+    fn add_disambiguating_parens(&self) -> bool {
         false
     }
 

src/tools/clippy/tests/ui/crashes/ice-3969.stderr

@@ -7,7 +7,7 @@ LL |     str: Sized;
    = note: `-D trivial-bounds` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(trivial_bounds)]`
 
-error: trait bound for<'a> Dst<(dyn A + 'a)>: std::marker::Sized does not depend on any type or lifetime parameters
+error: trait bound for<'a> Dst<dyn A + 'a>: std::marker::Sized does not depend on any type or lifetime parameters
   --> tests/ui/crashes/ice-3969.rs:25:30
    |
 LL |     for<'a> Dst<dyn A + 'a>: Sized,

tests/ui/abi/invalid-self-parameter-type-56806.stderr

@@ -1,4 +1,4 @@
-error[E0307]: invalid `self` parameter type: `Box<(dyn Trait + 'static)>`
+error[E0307]: invalid `self` parameter type: `Box<dyn Trait + 'static>`
   --> $DIR/invalid-self-parameter-type-56806.rs:3:34
    |
 LL |     fn dyn_instead_of_self(self: Box<dyn Trait>);

tests/ui/associated-types/issue-43924.stderr

@@ -1,20 +1,20 @@
-error[E0277]: the trait bound `(dyn ToString + 'static): Default` is not satisfied
+error[E0277]: the trait bound `dyn ToString + 'static: Default` is not satisfied
   --> $DIR/issue-43924.rs:8:45
    |
 LL |     type Out: Default + ToString + ?Sized = dyn ToString;
-   |                                             ^^^^^^^^^^^^ the trait `Default` is not implemented for `(dyn ToString + 'static)`
+   |                                             ^^^^^^^^^^^^ the trait `Default` is not implemented for `dyn ToString + 'static`
    |
 note: required by a bound in `Foo::Out`
   --> $DIR/issue-43924.rs:8:15
    |
 LL |     type Out: Default + ToString + ?Sized = dyn ToString;
    |               ^^^^^^^ required by this bound in `Foo::Out`
 
-error[E0599]: no associated function or constant named `default` found for trait object `(dyn ToString + 'static)` in the current scope
+error[E0599]: no associated function or constant named `default` found for trait object `dyn ToString + 'static` in the current scope
   --> $DIR/issue-43924.rs:15:39
    |
 LL |     assert_eq!(<() as Foo<u32>>::Out::default().to_string(), "false");
-   |                                       ^^^^^^^ associated function or constant not found in `(dyn ToString + 'static)`
+   |                                       ^^^^^^^ associated function or constant not found in `dyn ToString + 'static`
 
 error: aborting due to 2 previous errors
 

tests/ui/associated-types/issue-59324.stderr

@@ -83,13 +83,13 @@ LL | pub trait Foo: NotFoo {
    | ^^^^^^^^^^^^^^^^^^^^^
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error[E0277]: the size for values of type `(dyn ThriftService<(), AssocType = _> + 'static)` cannot be known at compilation time
+error[E0277]: the size for values of type `dyn ThriftService<(), AssocType = _> + 'static` cannot be known at compilation time
   --> $DIR/issue-59324.rs:24:29
    |
 LL | fn with_factory<H>(factory: dyn ThriftService<()>) {}
    |                             ^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-   = help: the trait `Sized` is not implemented for `(dyn ThriftService<(), AssocType = _> + 'static)`
+   = help: the trait `Sized` is not implemented for `dyn ThriftService<(), AssocType = _> + 'static`
    = help: unsized fn params are gated as an unstable feature
 help: you can use `impl Trait` as the argument type
    |

tests/ui/associated-types/projection-dyn-associated-type.rs

@@ -21,8 +21,8 @@ impl<T: ?Sized> Mirror for A {
 pub fn foo<'a>(
     x: &'a <dyn A + 'static as Mirror>::Assoc
 ) -> &'a <dyn B + 'static as Mirror>::Assoc {
-    //~^ ERROR the trait bound `(dyn B + 'static): Mirror` is not satisfied [E0277]
-    //~| ERROR the trait bound `(dyn B + 'static): Mirror` is not satisfied [E0277]
+    //~^ ERROR the trait bound `dyn B + 'static: Mirror` is not satisfied [E0277]
+    //~| ERROR the trait bound `dyn B + 'static: Mirror` is not satisfied [E0277]
     static
 } //~ ERROR expected identifier, found `}`
 

tests/ui/associated-types/projection-dyn-associated-type.stderr

@@ -29,23 +29,23 @@ error[E0207]: the type parameter `T` is not constrained by the impl trait, self
 LL | impl<T: ?Sized> Mirror for A {
    |      ^ unconstrained type parameter
 
-error[E0277]: the trait bound `(dyn B + 'static): Mirror` is not satisfied
+error[E0277]: the trait bound `dyn B + 'static: Mirror` is not satisfied
   --> $DIR/projection-dyn-associated-type.rs:23:6
    |
 LL | ) -> &'a <dyn B + 'static as Mirror>::Assoc {
-   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Mirror` is not implemented for `(dyn B + 'static)`
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Mirror` is not implemented for `dyn B + 'static`
    |
 help: the trait `Mirror` is implemented for `dyn A`
   --> $DIR/projection-dyn-associated-type.rs:14:1
    |
 LL | impl<T: ?Sized> Mirror for A {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0277]: the trait bound `(dyn B + 'static): Mirror` is not satisfied
+error[E0277]: the trait bound `dyn B + 'static: Mirror` is not satisfied
   --> $DIR/projection-dyn-associated-type.rs:23:6
    |
 LL | ) -> &'a <dyn B + 'static as Mirror>::Assoc {
-   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Mirror` is not implemented for `(dyn B + 'static)`
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Mirror` is not implemented for `dyn B + 'static`
    |
 help: the trait `Mirror` is implemented for `dyn A`
   --> $DIR/projection-dyn-associated-type.rs:14:1

tests/ui/async-await/async-closures/fn-exception.stderr

@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `unsafe fn() -> Pin<Box<(dyn Future<Output = ()> + 'static)>> {unsafety}: AsyncFn()` is not satisfied
+error[E0277]: the trait bound `unsafe fn() -> Pin<Box<dyn Future<Output = ()> + 'static>> {unsafety}: AsyncFn()` is not satisfied
   --> $DIR/fn-exception.rs:17:10
    |
 LL |     test(unsafety);
-   |     ---- ^^^^^^^^ the trait `AsyncFn()` is not implemented for fn item `unsafe fn() -> Pin<Box<(dyn Future<Output = ()> + 'static)>> {unsafety}`
+   |     ---- ^^^^^^^^ the trait `AsyncFn()` is not implemented for fn item `unsafe fn() -> Pin<Box<dyn Future<Output = ()> + 'static>> {unsafety}`
    |     |
    |     required by a bound introduced by this call
    |
@@ -12,11 +12,11 @@ note: required by a bound in `test`
 LL | fn test(f: impl AsyncFn()) {}
    |                 ^^^^^^^^^ required by this bound in `test`
 
-error[E0277]: the trait bound `extern "C" fn() -> Pin<Box<(dyn Future<Output = ()> + 'static)>> {abi}: AsyncFn()` is not satisfied
+error[E0277]: the trait bound `extern "C" fn() -> Pin<Box<dyn Future<Output = ()> + 'static>> {abi}: AsyncFn()` is not satisfied
   --> $DIR/fn-exception.rs:18:10
    |
 LL |     test(abi);
-   |     ---- ^^^ the trait `AsyncFn()` is not implemented for fn item `extern "C" fn() -> Pin<Box<(dyn Future<Output = ()> + 'static)>> {abi}`
+   |     ---- ^^^ the trait `AsyncFn()` is not implemented for fn item `extern "C" fn() -> Pin<Box<dyn Future<Output = ()> + 'static>> {abi}`
    |     |
    |     required by a bound introduced by this call
    |

tests/ui/async-await/higher-ranked-auto-trait-12.no_assumptions.stderr

@@ -11,8 +11,8 @@ LL | |         yield_now().await;
 LL | |     });
    | |______^ implementation of `Robot` is not general enough
    |
-   = note: `Box<(dyn Robot<Id = u32> + Send + '0)>` must implement `Robot`, for any lifetime `'0`...
-   = note: ...but `Robot` is actually implemented for the type `Box<(dyn Robot<Id = u32> + Send + 'static)>`
+   = note: `Box<dyn Robot<Id = u32> + Send + '0>` must implement `Robot`, for any lifetime `'0`...
+   = note: ...but `Robot` is actually implemented for the type `Box<dyn Robot<Id = u32> + Send + 'static>`
 
 error: aborting due to 1 previous error