Tests for bad --extern library path and file

compiler/rustc_metadata/src/locator.rs

@@ -1026,7 +1026,6 @@ impl CrateError {
                     None => String::new(),
                     Some(r) => format!(" which `{}` depends on", r.name),
                 };
-                // FIXME: There are no tests for CrateLocationUnknownType or LibFilenameForm
                 if !locator.crate_rejections.via_filename.is_empty() {
                     let mismatches = locator.crate_rejections.via_filename.iter();
                     for CrateMismatch { path, .. } in mismatches {

src/test/ui/errors/issue-104621-extern-bad-file.rs

@@ -0,0 +1,8 @@
+// compile-flags: --extern foo={{src-base}}/errors/issue-104621-extern-bad-file.rs
+// only-linux
+
+extern crate foo;
+//~^ ERROR extern location for foo is of an unknown type
+//~| ERROR file name should be lib*.rlib or lib*.so
+//~| ERROR can't find crate for `foo` [E0463]
+fn main() {}

src/test/ui/errors/issue-104621-extern-bad-file.stderr

@@ -0,0 +1,21 @@
+error: extern location for foo is of an unknown type: $DIR/issue-104621-extern-bad-file.rs
+  --> $DIR/issue-104621-extern-bad-file.rs:4:1
+   |
+LL | extern crate foo;
+   | ^^^^^^^^^^^^^^^^^
+
+error: file name should be lib*.rlib or lib*.so
+  --> $DIR/issue-104621-extern-bad-file.rs:4:1
+   |
+LL | extern crate foo;
+   | ^^^^^^^^^^^^^^^^^
+
+error[E0463]: can't find crate for `foo`
+  --> $DIR/issue-104621-extern-bad-file.rs:4:1
+   |
+LL | extern crate foo;
+   | ^^^^^^^^^^^^^^^^^ can't find crate
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0463`.

src/test/ui/errors/issue-104621-extern-not-file.rs

@@ -0,0 +1,4 @@
+// compile-flags: --extern foo=.
+
+extern crate foo; //~ ERROR extern location for foo is not a file: .
+fn main() {}

src/test/ui/errors/issue-104621-extern-not-file.stderr

@@ -0,0 +1,8 @@
+error: extern location for foo is not a file: .
+  --> $DIR/issue-104621-extern-not-file.rs:3:1
+   |
+LL | extern crate foo;
+   | ^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+