Rust: Ask cargo what the package root is.

This patches a tiny edge case that might not show up in practice. The problem is that the Rust code is being generated into an existing project (i.e., there is already a Cargo.toml file in the src-gen directory), which means that sometimes srcGenPkgPath != the actual package root (according to cargo). The cost of collecting and parsing the metadata is probably not negligible, so it would be nice if we didn't have to do this.

See rust-lang/cargo#4933.

org.lflang/src/org/lflang/generator/rust/RustValidator.kt

@@ -1,6 +1,7 @@
 package org.lflang.generator.rust
 
 import com.fasterxml.jackson.annotation.JsonProperty
+import com.fasterxml.jackson.core.JsonProcessingException
 import com.fasterxml.jackson.databind.DeserializationFeature
 import com.fasterxml.jackson.databind.ObjectMapper
 import org.eclipse.lsp4j.DiagnosticSeverity
@@ -29,6 +30,14 @@ class RustValidator(
         private val mapper = ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
         private const val COMPILER_MESSAGE_REASON = "compiler-message"
     }
+    // See the following reference for details on cargo metadata: https://doc.rust-lang.org/cargo/commands/cargo-metadata.html
+    private data class RustMetadata(
+        // Other fields exist, but we don't need them. The mapper is configured not to fail on unknown properties.
+        @JsonProperty("workspace_root") private val _workspaceRoot: String
+    ) {
+        val workspaceRoot: Path
+            get() = Path.of(_workspaceRoot)
+    }
     // See the following references for details on these data classes:
     //  * https://doc.rust-lang.org/cargo/reference/external-tools.html#json-messages
     //  * https://doc.rust-lang.org/rustc/json.html
@@ -105,6 +114,23 @@ class RustValidator(
         @JsonProperty("highlight_end") val highlightEnd: Int
     )
 
+    private var _metadata: RustMetadata? = null
+
+    private fun getMetadata(): RustMetadata? {
+        val nullableCommand = LFCommand.get("cargo", listOf("metadata", "--format-version", "1"), fileConfig.srcGenPkgPath)
+        _metadata = _metadata ?: nullableCommand?.let { command ->
+            command.run({false}, false)
+            command.output.toString().lines().filter { it.startsWith("{") }.mapNotNull {
+                try {
+                    mapper.readValue(it, RustMetadata::class.java)
+                } catch (e: JsonProcessingException) {
+                    null
+                }
+            }.firstOrNull()
+        }
+        return _metadata
+    }
+
     override fun getPossibleStrategies(): Collection<ValidationStrategy> = listOf(object: ValidationStrategy {
         override fun getCommand(generatedFile: Path?): LFCommand {
             return LFCommand.get("cargo", listOf("clippy", "--message-format", "json"), fileConfig.srcGenPkgPath)
@@ -118,7 +144,7 @@ class RustValidator(
                     val message = mapper.readValue(messageLine, RustCompilerMessage::class.java).message
                     if (message.spans.isEmpty()) errorReporter.report(message.severity, message.message)
                     for (s: RustSpan in message.spans) {
-                        val p: Path = fileConfig.srcGenPkgPath.resolve(s.fileName)
+                        val p: Path? = getMetadata()?.workspaceRoot?.resolve(s.fileName)
                         map[p]?.let {
                             for (lfSourcePath: Path in it.lfSourcePaths()) {
                                 errorReporter.report(