Use llvm-args instead of RUST_TEST_THREADS

.github/.cspell/project-dictionary.txt

@@ -6,6 +6,7 @@ doctestbins
 easytime
 fcoverage
 fprofile
+instrprof
 libdir
 libhello
 microkernel

CHANGELOG.md

@@ -10,6 +10,8 @@ Note: In this file, do not use the hard wrap in the middle of a sentence for com
 
 ## [Unreleased]
 
+- cargo-llvm-cov no longer sets the `RUST_TEST_THREADS` environment variable and uses llvm-args instead for workaround [rust-lang/rust#91092](https://github.com/rust-lang/rust/issues/91092).
+
 ## [0.5.19] - 2023-04-28
 
 - Fix handling of `--cargo-profile` option for `cargo llvm-cov nextest`. ([#269](https://github.com/taiki-e/cargo-llvm-cov/pull/269))

README.md

@@ -622,7 +622,7 @@ pacman -S cargo-llvm-cov
 
 - Branch coverage is not supported yet. See [#8] and [rust-lang/rust#79649] for more.
 - Support for doc tests is unstable and has known issues. See [#2] and [rust-lang/rust#79417] for more.
-- All the tests are run with `RUST_TEST_THREADS=1` to work around [rust-lang/rust#91092]. You can pass `--test-threads` (e.g., `--test-threads=$(nproc)`) to override this behavior. (As for `nextest`, we also set `NEXTEST_TEST_THREADS=1`.)
+- `cargo llvm-cov nextest` is run with `NEXTEST_TEST_THREADS=1` to work around [rust-lang/rust#91092]. You can pass `--test-threads` (e.g., `--test-threads=$(nproc)`) to override this behavior.
 
 See also [the code-coverage-related issues reported in rust-lang/rust](https://github.com/rust-lang/rust/labels/A-code-coverage).
 

src/cli.rs

@@ -358,10 +358,10 @@ impl Args {
                 // build options
                 Short('r') | Long("release") => parse_flag_passthrough!(release),
                 Long("profile") if subcommand != Subcommand::Nextest => {
-                    parse_opt_passthrough!(profile)
+                    parse_opt_passthrough!(profile);
                 }
                 Long("cargo-profile") if subcommand == Subcommand::Nextest => {
-                    parse_opt_passthrough!(profile)
+                    parse_opt_passthrough!(profile);
                 }
                 Long("target") => parse_opt_passthrough!(target),
                 Long("coverage-target-only") => parse_flag!(coverage_target_only),

src/main.rs

@@ -190,6 +190,10 @@ fn set_env(cx: &Context, env: &mut dyn EnvTarget, IsNextest(is_nextest): IsNexte
                 flags.push("codegen-units=1");
             }
         }
+        // Workaround for https://github.com/rust-lang/rust/issues/91092
+        // TODO: skip passing this on newer nightly that includes https://github.com/rust-lang/rust/pull/111469.
+        flags.push("-C");
+        flags.push("llvm-args=--instrprof-atomic-counter-update-all");
         if !cx.args.cov.no_cfg_coverage {
             flags.push("--cfg=coverage");
         }
@@ -283,8 +287,6 @@ fn set_env(cx: &Context, env: &mut dyn EnvTarget, IsNextest(is_nextest): IsNexte
     }
     env.set("LLVM_PROFILE_FILE", llvm_profile_file.as_str())?;
     env.set("CARGO_INCREMENTAL", "0")?;
-    // Workaround for https://github.com/rust-lang/rust/issues/91092
-    env.set("RUST_TEST_THREADS", "1")?;
     if is_nextest {
         // Same as above
         env.set("NEXTEST_TEST_THREADS", "1")?;