Skip doctests of proc-macro crates

cargo-miri/bin.rs

@@ -903,6 +903,13 @@ fn phase_rustdoc(fst_arg: &str, mut args: env::Args) {
         show_error(format!("cross-interpreting doc-tests is not currently supported by Miri."));
     }
 
+    // Doc-tests of `proc-macro` crates (and their dependencies) are always built for the host,
+    // so we are not able to run them in Miri.
+    if ArgFlagValueIter::new("--crate-type").any(|crate_type| crate_type == "proc-macro") {
+        eprintln!("Running doc-tests of `proc-macro` crates is not currently supported by Miri.");
+        return;
+    }
+
     // For each doc-test, rustdoc starts two child processes: first the test is compiled,
     // then the produced executable is invoked. We want to reroute both of these to cargo-miri,
     // such that the first time we'll enter phase_cargo_rustc, and phase_cargo_runner second.

test-cargo-miri/run-test.py

@@ -140,6 +140,10 @@ def test_cargo_miri_test():
         "test.subcrate.stdout.ref", "test.stderr-proc-macro.ref",
         env={'MIRIFLAGS': "-Zmiri-disable-isolation"},
     )
+    test("`cargo miri test` (subcrate, doctests)",
+        cargo_miri("test") + ["-p", "subcrate", "--doc"],
+        "test.stdout-empty.ref", "test.stderr-proc-macro-doctest.ref",
+    )
 
 os.chdir(os.path.dirname(os.path.realpath(__file__)))
 os.environ["RUST_TEST_NOCAPTURE"] = "0" # this affects test output, so make sure it is not set

test-cargo-miri/subcrate/src/lib.rs

@@ -1,2 +1,5 @@
+#[cfg(doctest)]
+use num_cpus as _;
+
 #[cfg(test)]
 compile_error!("Miri should not touch me");

test-cargo-miri/test.stderr-proc-macro-doctest.ref

@@ -0,0 +1 @@
+Running doc-tests of `proc-macro` crates is not currently supported by Miri.