libtest: Print the names of failed tests eagerly

Previously, libtest would wait until all tests finished running to print the progress, which made it
annoying to run many tests at once (since you don't know which have failed). Change it to print the
names as soon as they fail.

This also adds a test for the terse output; previously it was untested.

library/test/src/formatters/terse.rs

@@ -23,6 +23,7 @@ pub(crate) struct TerseFormatter<T> {
     max_name_len: usize,
 
     test_count: usize,
+    test_column: usize,
     total_test_count: usize,
 }
 
@@ -39,6 +40,7 @@ pub fn new(
             max_name_len,
             is_multithreaded,
             test_count: 0,
+            test_column: 0,
             total_test_count: 0, // initialized later, when write_run_start is called
         }
     }
@@ -47,8 +49,20 @@ pub fn write_ok(&mut self) -> io::Result<()> {
         self.write_short_result(".", term::color::GREEN)
     }
 
-    pub fn write_failed(&mut self) -> io::Result<()> {
-        self.write_short_result("F", term::color::RED)
+    pub fn write_failed(&mut self, name: &str) -> io::Result<()> {
+        // Put failed tests on their own line and include the test name, so that it's faster
+        // to see which test failed without having to wait for them all to run.
+
+        // normally, we write the progress unconditionally, even if the previous line was cut short.
+        // but if this is the very first column, no short results will have been printed and we'll end up with *only* the progress on the line.
+        // avoid this.
+        if self.test_column != 0 {
+            self.write_progress()?;
+        }
+        self.test_count += 1;
+        self.write_plain(format!("{name} --- "))?;
+        self.write_pretty("FAILED", term::color::RED)?;
+        self.write_plain("\n")
     }
 
     pub fn write_ignored(&mut self) -> io::Result<()> {
@@ -65,15 +79,22 @@ pub fn write_short_result(
         color: term::color::Color,
     ) -> io::Result<()> {
         self.write_pretty(result, color)?;
-        if self.test_count % QUIET_MODE_MAX_COLUMN == QUIET_MODE_MAX_COLUMN - 1 {
+        self.test_count += 1;
+        self.test_column += 1;
+        if self.test_column % QUIET_MODE_MAX_COLUMN == QUIET_MODE_MAX_COLUMN - 1 {
             // We insert a new line regularly in order to flush the
             // screen when dealing with line-buffered output (e.g., piping to
             // `stamp` in the rust CI).
-            let out = format!(" {}/{}\n", self.test_count + 1, self.total_test_count);
-            self.write_plain(out)?;
+            self.write_progress()?;
         }
 
-        self.test_count += 1;
+        Ok(())
+    }
+
+    fn write_progress(&mut self) -> io::Result<()> {
+        let out = format!(" {}/{}\n", self.test_count, self.total_test_count);
+        self.write_plain(out)?;
+        self.test_column = 0;
         Ok(())
     }
 
@@ -213,7 +234,7 @@ fn write_result(
         match *result {
             TestResult::TrOk => self.write_ok(),
             TestResult::TrFailed | TestResult::TrFailedMsg(_) | TestResult::TrTimedFail => {
-                self.write_failed()
+                self.write_failed(desc.name.as_slice())
             }
             TestResult::TrIgnored => self.write_ignored(),
             TestResult::TrBench(ref bs) => {

tests/ui/test-attrs/terse.rs

@@ -0,0 +1,125 @@
+//@ compile-flags: --test
+//@ run-fail
+//@ run-flags: --test-threads=1 --quiet
+//@ check-run-results
+//@ exec-env:RUST_BACKTRACE=0
+//@ normalize-stdout-test "finished in \d+\.\d+s" -> "finished in $$TIME"
+//@ ignore-emscripten no threads support
+//@ needs-unwind
+
+#[test]
+fn abc() {
+    panic!();
+}
+
+#[test]
+fn foo() {
+    panic!();
+}
+
+#[test]
+fn foo2() {
+    panic!();
+}
+
+// run a whole bunch of tests so we can see what happens when we go over 88 columns
+#[test] fn f0() {}
+#[test] fn f1() {}
+#[test] fn f2() {}
+#[test] fn f3() {}
+#[test] fn f4() {}
+#[test] fn f5() {}
+#[test] fn f6() {}
+#[test] fn f7() {}
+#[test] fn f8() {}
+#[test] fn f9() {}
+#[test] fn f10() {}
+#[test] fn f11() {}
+#[test] fn f12() {}
+#[test] fn f13() {}
+#[test] fn f14() {}
+#[test] fn f15() {}
+#[test] fn f16() {}
+#[test] fn f17() {}
+#[test] fn f18() {}
+#[test] fn f19() {}
+#[test] fn f20() {}
+#[test] fn f21() {}
+#[test] fn f22() {}
+#[test] fn f23() {}
+#[test] fn f24() {}
+#[test] fn f25() {}
+#[test] fn f26() {}
+#[test] fn f27() {}
+#[test] fn f28() {}
+#[test] fn f29() {}
+#[test] fn f30() {}
+#[test] fn f31() {}
+#[test] fn f32() {}
+#[test] fn f33() {}
+#[test] fn f34() {}
+#[test] fn f35() {}
+#[test] fn f36() {}
+#[test] fn f37() {}
+#[test] fn f38() {}
+#[test] fn f39() {}
+#[test] fn f40() {}
+#[test] fn f41() {}
+#[test] fn f42() {}
+#[test] fn f43() {}
+#[test] fn f44() {}
+#[test] fn f45() {}
+#[test] fn f46() {}
+#[test] fn f47() {}
+#[test] fn f48() {}
+#[test] fn f49() {}
+#[test] fn f50() {}
+#[test] fn f51() {}
+#[test] fn f52() {}
+#[test] fn f53() {}
+#[test] fn f54() {}
+#[test] fn f55() {}
+#[test] fn f56() {}
+#[test] fn f57() {}
+#[test] fn f58() {}
+#[test] fn f59() {}
+#[test] fn f60() {}
+#[test] fn f61() {}
+#[test] fn f62() {}
+#[test] fn f63() {}
+#[test] fn f64() {}
+#[test] fn f65() {}
+#[test] fn f66() {}
+#[test] fn f67() {}
+#[test] fn f68() {}
+#[test] fn f69() {}
+#[test] fn f70() {}
+#[test] fn f71() {}
+#[test] fn f72() {}
+#[test] fn f73() {}
+#[test] fn f74() {}
+#[test] fn f75() {}
+#[test] fn f76() {}
+#[test] fn f77() {}
+#[test] fn f78() {}
+#[test] fn f79() {}
+#[test] fn f80() {}
+#[test] fn f81() {}
+#[test] fn f82() {}
+#[test] fn f83() {}
+#[test] fn f84() {}
+#[test] fn f85() {}
+#[test] fn f86() {}
+#[test] fn f87() {}
+#[test] fn f88() {}
+#[test] fn f89() {}
+#[test] fn f90() {}
+#[test] fn f91() {}
+#[test] fn f92() {}
+#[test] fn f93() {}
+#[test] fn f94() {}
+#[test] fn f95() {}
+#[test] fn f96() {}
+#[test] fn f97() {}
+#[test] fn f98() {}
+#[test] fn f99() {}

tests/ui/test-attrs/terse.run.stdout

@@ -0,0 +1,31 @@
+
+running 103 tests
+abc --- FAILED
+....................................................................................... 88/103
+............. 101/103
+foo --- FAILED
+foo2 --- FAILED
+
+failures:
+
+---- abc stdout ----
+thread 'abc' panicked at $DIR/terse.rs:12:5:
+explicit panic
+note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
+
+---- foo stdout ----
+thread 'foo' panicked at $DIR/terse.rs:17:5:
+explicit panic
+
+---- foo2 stdout ----
+thread 'foo2' panicked at $DIR/terse.rs:22:5:
+explicit panic
+
+
+failures:
+    abc
+    foo
+    foo2
+
+test result: FAILED. 100 passed; 3 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+