Add a UI test for stdlib-private dependencies

Introduce a test that shows stdlib-private dependencies leaking into
diagnostics. This is resolved by a later commit.

Author: tgross35

tests/ui/std/sysroot-private.default.stderr

@@ -0,0 +1,39 @@
+error[E0405]: cannot find trait `Equivalent` in this scope
+  --> $DIR/sysroot-private.rs:24:18
+   |
+LL | trait Trait2<K>: Equivalent<K> {}
+   |                  ^^^^^^^^^^ not found in this scope
+
+error[E0412]: cannot find type `K` in this scope
+  --> $DIR/sysroot-private.rs:29:35
+   |
+LL | fn trait_member<T>(val: &T, key: &K) -> bool {
+   |                 -                 ^
+   |                 |
+   |                 similarly named type parameter `T` defined here
+   |
+help: a type parameter with a similar name exists
+   |
+LL | fn trait_member<T>(val: &T, key: &T) -> bool {
+   |                                   ~
+help: you might be missing a type parameter
+   |
+LL | fn trait_member<T, K>(val: &T, key: &K) -> bool {
+   |                  +++
+
+error[E0220]: associated type `Buf` not found for `Trait`
+  --> $DIR/sysroot-private.rs:19:31
+   |
+LL | type AssociatedTy = dyn Trait<Buf = i32, Bar = i32>;
+   |                               ^^^ there is an associated type `Buf` in the trait `addr2line::lookup::LookupContinuation`
+
+error[E0425]: cannot find function `memchr2` in this scope
+  --> $DIR/sysroot-private.rs:37:5
+   |
+LL |     memchr2(b'a', b'b', buf)
+   |     ^^^^^^^ not found in this scope
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0220, E0405, E0412, E0425.
+For more information about an error, try `rustc --explain E0220`.

tests/ui/std/sysroot-private.rs

@@ -0,0 +1,40 @@
+//! Test that private dependencies of `std` that live in the sysroot do not reach through to
+//! diagnostics.
+//!
+//! This test would be more robust if we could patch the sysroot with an "evil" crate that
+//! provided known types that we control; however, this would effectively require rebuilding
+//! `std` (or patching crate metadata). So, this test relies on what is currently public API
+//! of `std`'s dependencies, but may not be robust against dependency upgrades/changes.
+
+//@ revisions: default rustc_private_enabled
+
+// Enabling `rustc_private` should `std`'s dependencies accessible, so they should show up
+// in diagnostics. NB: not all diagnostics are affected by this.
+#![cfg_attr(rustc_private_enabled, feature(rustc_private))]
+#![crate_type = "lib"]
+
+trait Trait { type Bar; }
+
+// Attempt to get a suggestion for `addr2line::LookupContinuation`, which has member `Buf`
+type AssociatedTy = dyn Trait<Buf = i32, Bar = i32>;
+//~^ ERROR associated type `Buf` not found
+//~| NOTE there is an associated type `Buf` in the trait `addr2line::lookup::LookupContinuation`
+
+// Attempt to get a suggestion for `hashbrown::Equivalent`
+trait Trait2<K>: Equivalent<K> {}
+//~^ ERROR cannot find trait
+//~| NOTE not found
+
+// Attempt to get a suggestion for `hashbrown::Equivalent::equivalent`
+fn trait_member<T>(val: &T, key: &K) -> bool {
+    //~^ ERROR cannot find type `K`
+    //~| NOTE similarly named
+    val.equivalent(key)
+}
+
+// Attempt to get a suggestion for `memchr::memchr2`
+fn free_function(buf: &[u8]) -> Option<usize> {
+    memchr2(b'a', b'b', buf)
+    //~^ ERROR cannot find function
+    //~| NOTE not found
+}

tests/ui/std/sysroot-private.rustc_private_enabled.stderr

@@ -0,0 +1,39 @@
+error[E0405]: cannot find trait `Equivalent` in this scope
+  --> $DIR/sysroot-private.rs:24:18
+   |
+LL | trait Trait2<K>: Equivalent<K> {}
+   |                  ^^^^^^^^^^ not found in this scope
+
+error[E0412]: cannot find type `K` in this scope
+  --> $DIR/sysroot-private.rs:29:35
+   |
+LL | fn trait_member<T>(val: &T, key: &K) -> bool {
+   |                 -                 ^
+   |                 |
+   |                 similarly named type parameter `T` defined here
+   |
+help: a type parameter with a similar name exists
+   |
+LL | fn trait_member<T>(val: &T, key: &T) -> bool {
+   |                                   ~
+help: you might be missing a type parameter
+   |
+LL | fn trait_member<T, K>(val: &T, key: &K) -> bool {
+   |                  +++
+
+error[E0220]: associated type `Buf` not found for `Trait`
+  --> $DIR/sysroot-private.rs:19:31
+   |
+LL | type AssociatedTy = dyn Trait<Buf = i32, Bar = i32>;
+   |                               ^^^ there is an associated type `Buf` in the trait `addr2line::lookup::LookupContinuation`
+
+error[E0425]: cannot find function `memchr2` in this scope
+  --> $DIR/sysroot-private.rs:37:5
+   |
+LL |     memchr2(b'a', b'b', buf)
+   |     ^^^^^^^ not found in this scope
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0220, E0405, E0412, E0425.
+For more information about an error, try `rustc --explain E0220`.