add failing test, bless incorrect output

Author: Qelxiros

tests/ui/typeck/auxiliary/private-dep.rs

@@ -0,0 +1,3 @@
+pub trait A {
+    fn foo() {}
+}

tests/ui/typeck/auxiliary/public-dep.rs

@@ -0,0 +1,11 @@
+//@ aux-crate:priv:private_dep=private-dep.rs
+//@ compile-flags: -Zunstable-options
+
+extern crate private_dep;
+use private_dep::A;
+
+pub struct B;
+
+impl A for B {
+    fn foo() {}
+}

tests/ui/typeck/dont-suggest-private-dependencies.rs

@@ -0,0 +1,30 @@
+// Don't suggest importing a function from a private dependency.
+// Issues: #138191, #142676
+
+// Avoid suggesting traits from std-private deps
+//@ forbid-output: compiler_builtins
+//@ forbid-output: object
+
+// Check a custom trait to withstand changes in above crates
+//@ aux-crate:public_dep=public-dep.rs
+//@ compile-flags: -Zunstable-options
+//@ forbid-output: private_dep
+
+struct VecReader(Vec<u8>);
+
+impl std::io::Read for VecReader {
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        self.0.read(buf)
+        //~^ ERROR no method named `read` found for struct `Vec<u8>`
+    }
+}
+
+extern crate public_dep;
+use public_dep::B;
+
+fn main() {
+    let _ = u8::cast_from_lossy(9);
+    //~^ ERROR no function or associated item named `cast_from_lossy` found for type `u8`
+    let _ = B::foo();
+    //~^ ERROR no function or associated item named `foo` found for struct `B`
+}

tests/ui/typeck/dont-suggest-private-dependencies.stderr

@@ -0,0 +1,43 @@
+error[E0599]: no method named `read` found for struct `Vec<u8>` in the current scope
+  --> $DIR/dont-suggest-private-dependencies.rs:17:16
+   |
+LL |         self.0.read(buf)
+   |                ^^^^
+   |
+   = help: items from traits can only be used if the trait is in scope
+help: trait `ReadRef` which provides `read` is implemented but not in scope; perhaps you want to import it
+   |
+LL + use object::read::read_ref::ReadRef;
+   |
+help: there is a method `read_at` with a similar name
+   |
+LL |         self.0.read_at(buf)
+   |                    +++
+
+error[E0599]: no function or associated item named `cast_from_lossy` found for type `u8` in the current scope
+  --> $DIR/dont-suggest-private-dependencies.rs:26:17
+   |
+LL |     let _ = u8::cast_from_lossy(9);
+   |                 ^^^^^^^^^^^^^^^ function or associated item not found in `u8`
+   |
+   = help: items from traits can only be used if the trait is in scope
+help: trait `CastFrom` which provides `cast_from_lossy` is implemented but not in scope; perhaps you want to import it
+   |
+LL + use compiler_builtins::math::libm_math::support::int_traits::CastFrom;
+   |
+
+error[E0599]: no function or associated item named `foo` found for struct `B` in the current scope
+  --> $DIR/dont-suggest-private-dependencies.rs:28:16
+   |
+LL |     let _ = B::foo();
+   |                ^^^ function or associated item not found in `B`
+   |
+   = help: items from traits can only be used if the trait is in scope
+help: trait `A` which provides `foo` is implemented but not in scope; perhaps you want to import it
+   |
+LL + use private_dep::A;
+   |
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0599`.