Add more --extern tests.

src/test/run-make-fulldeps/extern-flag-pathless/Makefile

@@ -0,0 +1,18 @@
+-include ../tools.mk
+
+# Test mixing pathless --extern with paths.
+
+all:
+	$(RUSTC) bar-static.rs --crate-name=bar --crate-type=rlib
+	$(RUSTC) bar-dynamic.rs --crate-name=bar --crate-type=dylib -C prefer-dynamic
+	# rlib preferred over dylib
+	$(RUSTC) foo.rs --extern bar
+	$(call RUN,foo) | $(CGREP) 'static'
+	$(RUSTC) foo.rs --extern bar=$(TMPDIR)/libbar.rlib --extern bar
+	$(call RUN,foo) | $(CGREP) 'static'
+	# explicit --extern overrides pathless
+	$(RUSTC) foo.rs --extern bar=$(call DYLIB,bar) --extern bar
+	$(call RUN,foo) | $(CGREP) 'dynamic'
+	# prefer-dynamic does what it says
+	$(RUSTC) foo.rs --extern bar -C prefer-dynamic
+	$(call RUN,foo) | $(CGREP) 'dynamic'

src/test/run-make-fulldeps/extern-flag-pathless/bar-dynamic.rs

@@ -0,0 +1,3 @@
+pub fn f() {
+    println!("dynamic");
+}

src/test/run-make-fulldeps/extern-flag-pathless/bar-static.rs

@@ -0,0 +1,3 @@
+pub fn f() {
+    println!("static");
+}

src/test/run-make-fulldeps/extern-flag-pathless/foo.rs

@@ -0,0 +1,3 @@
+fn main() {
+    bar::f();
+}

src/test/rustdoc/inline_cross/use_crate.rs

@@ -2,7 +2,7 @@
 // aux-build:use_crate_2.rs
 // build-aux-docs
 // edition:2018
-// compile-flags:--extern use_crate --extern use_crate_2 -Z unstable-options
+// compile-flags:--extern use_crate --extern use_crate_2
 
 // During the buildup to Rust 2018, rustdoc would eagerly inline `pub use some_crate;` as if it
 // were a module, so we changed it to make `pub use`ing crate roots remain as a `pub use` statement

src/test/ui-fulldeps/pathless-extern-unstable.rs

@@ -0,0 +1,10 @@
+// ignore-stage1
+// edition:2018
+// compile-flags:--extern rustc
+
+// Test that `--extern rustc` fails with `rustc_private`.
+
+pub use rustc;
+//~^ use of unstable library feature 'rustc_private'
+
+fn main() {}

src/test/ui-fulldeps/pathless-extern-unstable.stderr

@@ -0,0 +1,12 @@
+error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
+  --> $DIR/pathless-extern-unstable.rs:7:9
+   |
+LL | pub use rustc;
+   |         ^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/27812
+   = help: add `#![feature(rustc_private)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/pathless-extern-ok.rs

@@ -0,0 +1,9 @@
+// edition:2018
+// compile-flags:--extern alloc
+// build-pass
+
+// Test that `--extern alloc` will load from the sysroot without error.
+
+fn main() {
+    let _: Vec<i32> = alloc::vec::Vec::new();
+}