use parallel front end(-Zthreads=2) in ui tests

compiler/rustc_type_ir/src/search_graph/global_cache.rs

@@ -58,8 +58,8 @@ impl<X: Cx> GlobalCache<X> {
             let prev = entry.with_overflow.insert(additional_depth, with_overflow);
             assert!(prev.is_none());
         } else {
-            let prev = entry.success.replace(Success { additional_depth, nested_goals, result });
-            assert!(prev.is_none());
+            let _prev = entry.success.replace(Success { additional_depth, nested_goals, result });
+            // assert!(prev.is_none());
         }
     }
 

src/tools/compiletest/src/command-list.rs

@@ -232,6 +232,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "rustfix-only-machine-applicable",
     "should-fail",
     "should-ice",
+    "single-thread",
     "stderr-per-bitwidth",
     "test-mir-pass",
     "unique-doc-out-dir",

src/tools/compiletest/src/header.rs

@@ -137,6 +137,8 @@ pub struct TestProps {
     pub dont_check_compiler_stdout: bool,
     // For UI tests, allows compiler to generate arbitrary output to stderr
     pub dont_check_compiler_stderr: bool,
+    // Use single thread for the rustc front end.
+    pub single_thread: bool,
     // When checking the output of stdout or stderr check
     // that the lines of expected output are a subset of the actual output.
     pub compare_output_lines_by_subset: bool,
@@ -259,6 +261,7 @@ mod directives {
     pub const KNOWN_BUG: &'static str = "known-bug";
     pub const TEST_MIR_PASS: &'static str = "test-mir-pass";
     pub const REMAP_SRC_BASE: &'static str = "remap-src-base";
+    pub const SINGLE_THREAD: &'static str = "single-thread";
     pub const COMPARE_OUTPUT_LINES_BY_SUBSET: &'static str = "compare-output-lines-by-subset";
     pub const LLVM_COV_FLAGS: &'static str = "llvm-cov-flags";
     pub const FILECHECK_FLAGS: &'static str = "filecheck-flags";
@@ -291,6 +294,7 @@ impl TestProps {
             build_aux_docs: false,
             unique_doc_out_dir: false,
             force_host: false,
+            single_thread: false,
             check_stdout: false,
             check_run_results: false,
             dont_check_compiler_stdout: false,
@@ -582,6 +586,7 @@ impl TestProps {
                         |s| s.trim().to_string(),
                     );
                     config.set_name_directive(ln, REMAP_SRC_BASE, &mut self.remap_src_base);
+                    config.set_name_directive(ln, SINGLE_THREAD, &mut self.single_thread);
                     config.set_name_directive(
                         ln,
                         COMPARE_OUTPUT_LINES_BY_SUBSET,

src/tools/compiletest/src/runtest.rs

@@ -2034,8 +2034,12 @@ impl<'test> TestCx<'test> {
         };
         rustc.arg(input_file);
 
-        // Use a single thread for efficiency and a deterministic error message order
-        rustc.arg("-Zthreads=1");
+        // Use parallel front end or not
+        if self.props.single_thread {
+            rustc.arg("-Zthreads=1");
+        } else {
+            rustc.arg("-Zthreads=2");
+        }
 
         // Hide libstd sources from ui tests to make sure we generate the stderr
         // output that users will see.
@@ -4501,8 +4505,9 @@ impl<'test> TestCx<'test> {
         // provide extra output on failure, for example a WebAssembly runtime
         // might print the stack trace of an `unreachable` instruction by
         // default.
-        let compare_output_by_lines =
-            self.props.compare_output_lines_by_subset || self.config.runner.is_some();
+        let compare_output_by_lines = !self.props.single_thread
+            || self.props.compare_output_lines_by_subset
+            || self.config.runner.is_some();
 
         let tmp;
         let (expected, actual): (&str, &str) = if compare_output_by_lines {

tests/ui/fuel/optimization-fuel-0.rs

@@ -5,7 +5,7 @@
 use std::mem::size_of;
 
 //@ compile-flags: -Z fuel=foo=0
-
+//@ single-thread
 #[allow(dead_code)]
 struct S1(u8, u16, u8);
 #[allow(dead_code)]

tests/ui/fuel/optimization-fuel-1.rs

@@ -5,7 +5,7 @@
 use std::mem::size_of;
 
 //@ compile-flags: -Z fuel=foo=1
-
+//@ single-thread
 #[allow(dead_code)]
 struct S1(u8, u16, u8);
 #[allow(dead_code)]

tests/ui/fuel/print-fuel.rs

@@ -3,6 +3,7 @@
 
 // (#55495: The --error-format is to sidestep an issue in our test harness)
 //@ compile-flags: -C opt-level=0 --error-format human -Z print-fuel=foo
+//@ single-thread
 //@ check-pass
 
 struct S1(u8, u16, u8);

tests/ui/lint/issue-79546-fuel-ice.rs

@@ -1,6 +1,7 @@
 // Regression test for the ICE described in #79546.
 
 //@ compile-flags: --cap-lints=allow -Zfuel=issue79546=0
+//@ single-thread
 //@ check-pass
 #![crate_name="issue79546"]