Fix exclusions in gen_rust_project target pattern

Signed-off-by: Patrick Steadman <ptsteadman@gmail.com>

Author: ptsteadman

tools/rust_analyzer/lib.rs

@@ -112,6 +112,7 @@ fn generate_crate_info(
             "--aspects={rules_rust}//rust:defs.bzl%rust_analyzer_aspect"
         ))
         .arg("--output_groups=rust_analyzer_crate_spec,rust_generated_srcs,rust_analyzer_proc_macro_dylib,rust_analyzer_src")
+        .arg("--")
         .args(targets)
         .output()?;
 
@@ -166,6 +167,53 @@ where
     serde_json::from_str(&content).with_context(|| format!("failed to deserialize file: {path}"))
 }
 
+/// Returns the bazel build args that `generate_crate_info` would pass, for testing.
+#[cfg(test)]
+fn build_crate_info_args(rules_rust: &str, targets: &[String]) -> Vec<String> {
+    let mut args = vec![
+        "build".to_owned(),
+        "--norun_validations".to_owned(),
+        "--remote_download_all".to_owned(),
+        format!("--aspects={rules_rust}//rust:defs.bzl%rust_analyzer_aspect"),
+        "--output_groups=rust_analyzer_crate_spec,rust_generated_srcs,rust_analyzer_proc_macro_dylib,rust_analyzer_src".to_owned(),
+        "--".to_owned(),
+    ];
+    args.extend_from_slice(targets);
+    args
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Without `--` before targets, Bazel treats exclusion patterns like `-//foo:bar`
+    /// as unknown flags and exits with "unrecognized option". This test replicates
+    /// that failure mode and verifies the separator is in the correct position.
+    #[test]
+    fn exclusion_targets_preceded_by_double_dash() {
+        let targets = vec![
+            "//...".to_owned(),
+            "-//some/excluded:target".to_owned(),
+        ];
+        let args = build_crate_info_args("rules_rust", &targets);
+
+        let dash_pos = args.iter().position(|a| a == "--").expect("-- must be present");
+        let targets_start = dash_pos + 1;
+        assert_eq!(
+            &args[targets_start..],
+            targets.as_slice(),
+            "all targets must appear after --"
+        );
+        // Exclusion pattern must not appear before --
+        for arg in &args[..dash_pos] {
+            assert!(
+                !arg.starts_with('-') || arg.starts_with("--"),
+                "target exclusion {arg:?} appeared before -- and would be parsed as a flag"
+            );
+        }
+    }
+}
+
 /// `rust-analyzer` associates workspaces with buildfiles. Therefore, when it passes in a
 /// source file path, we use this function to identify the buildfile the file belongs to.
 fn source_file_to_buildfile(file: &Utf8Path) -> anyhow::Result<Utf8PathBuf> {