libtest harness: avoid 'extern crate test' with custom runner

compiler/rustc_builtin_macros/src/test_harness.rs

@@ -364,7 +364,12 @@ fn add_main(cx: &mut TestCtxt<'_>, c: &mut ast::Crate) {
     });
 
     // Integrate the new item into existing module structures.
-    let items = AstFragment::Items(smallvec![test_extern_stmt, main]);
+    // `extern crate test;` is only needed with the default runner.
+    let items = AstFragment::Items(if cx.test_runner.is_none() {
+        smallvec![test_extern_stmt, main]
+    } else {
+        smallvec![main]
+    });
     c.items.extend(cx.ext_cx.monotonic_expander().fully_expand_fragment(items).make_items());
 }
 

tests/pretty/custom-test-runner.pp

@@ -0,0 +1,32 @@
+#![feature(prelude_import)]
+#![no_std]
+//@ compile-flags: --crate-type=lib --test --remap-path-prefix={{src-base}}/=/the/src/ --remap-path-prefix={{src-base}}\=/the/src/
+//@ pretty-compare-only
+//@ pretty-mode:expanded
+//@ pp-exact:custom-test-runner.pp
+
+// Example taken from the unstable book.
+
+#![feature(custom_test_frameworks)]
+#![test_runner(my_runner)]
+extern crate std;
+#[prelude_import]
+use ::std::prelude::rust_2015::*;
+
+fn my_runner(tests: &[&i32]) {
+    for t in tests {
+        if **t == 0 {
+
+
+            { ::std::io::_print(format_args!("PASSED\n")); };
+        } else { { ::std::io::_print(format_args!("FAILED\n")); }; }
+    }
+}
+#[rustc_test_marker = "WILL_PASS"]
+pub const WILL_PASS: i32 = 0;
+#[rustc_test_marker = "WILL_FAIL"]
+pub const WILL_FAIL: i32 = 4;
+#[rustc_main]
+#[coverage(off)]
+#[doc(hidden)]
+pub fn main() -> () { my_runner(&[&WILL_FAIL, &WILL_PASS]) }

tests/pretty/custom-test-runner.rs

@@ -0,0 +1,25 @@
+//@ compile-flags: --crate-type=lib --test --remap-path-prefix={{src-base}}/=/the/src/ --remap-path-prefix={{src-base}}\=/the/src/
+//@ pretty-compare-only
+//@ pretty-mode:expanded
+//@ pp-exact:custom-test-runner.pp
+
+// Example taken from the unstable book.
+
+#![feature(custom_test_frameworks)]
+#![test_runner(my_runner)]
+
+fn my_runner(tests: &[&i32]) {
+    for t in tests {
+        if **t == 0 {
+            println!("PASSED");
+        } else {
+            println!("FAILED");
+        }
+    }
+}
+
+#[test_case]
+const WILL_PASS: i32 = 0;
+
+#[test_case]
+const WILL_FAIL: i32 = 4;