Add support for using only one output base

Example/HelloWorld/BUILD

@@ -263,4 +263,5 @@ setup_sourcekit_bsp(
     targets = [
         "//HelloWorld/...",
     ],
+    no_extra_output_base = True,
 )

Sources/SourceKitBazelBSP/RequestHandlers/InitializeHandler.swift

@@ -85,11 +85,18 @@ final class InitializeHandler {
         logger.debug("regularOutputBase: \(regularOutputBase, privacy: .public)")
 
         // Setup the special output base path where we will run indexing commands from.
-        let regularOutputBaseLastPath = regularOutputBase.lastPathComponent
-        let outputBase = regularOutputBase.deletingLastPathComponent().appendingPathComponent(
-            "\(regularOutputBaseLastPath)-sourcekit-bazel-bsp"
-        ).path
-        logger.debug("outputBase: \(outputBase, privacy: .public)")
+        let outputBase: String
+        if baseConfig.noExtraOutputBase {
+            outputBase = regularOutputBase.path
+            logger.debug("Will use the regular output base for all actions")
+        } else {
+            let regularOutputBaseLastPath = regularOutputBase.lastPathComponent
+            outputBase =
+                regularOutputBase.deletingLastPathComponent().appendingPathComponent(
+                    "\(regularOutputBaseLastPath)-sourcekit-bazel-bsp"
+                ).path
+            logger.debug("outputBase: \(outputBase, privacy: .public)")
+        }
 
         // Now, get the full output path based on the above output base.
         let outputPath: String = try commandRunner.bazelIndexAction(

Sources/SourceKitBazelBSP/Server/BaseServerConfig.swift

@@ -34,6 +34,7 @@ package struct BaseServerConfig: Equatable {
     let dependencyRulesToDiscover: [DependencyRuleType]
     let topLevelTargetsToExclude: [String]
     let dependencyTargetsToExclude: [String]
+    let noExtraOutputBase: Bool
 
     package init(
         bazelWrapper: String,
@@ -44,7 +45,8 @@ package struct BaseServerConfig: Equatable {
         topLevelRulesToDiscover: [TopLevelRuleType] = TopLevelRuleType.allCases,
         dependencyRulesToDiscover: [DependencyRuleType] = DependencyRuleType.allCases,
         topLevelTargetsToExclude: [String] = [],
-        dependencyTargetsToExclude: [String] = []
+        dependencyTargetsToExclude: [String] = [],
+        noExtraOutputBase: Bool = false
     ) {
         self.bazelWrapper = bazelWrapper
         self.targets = targets
@@ -55,5 +57,6 @@ package struct BaseServerConfig: Equatable {
         self.dependencyRulesToDiscover = dependencyRulesToDiscover
         self.topLevelTargetsToExclude = topLevelTargetsToExclude
         self.dependencyTargetsToExclude = dependencyTargetsToExclude
+        self.noExtraOutputBase = noExtraOutputBase
     }
 }

Sources/SourceKitBazelBSP/Server/InitializedServerConfig.swift

@@ -38,6 +38,8 @@ struct InitializedServerConfig: Equatable {
     }
 
     var indexStorePath: String {
+        // This is the same path hardcoded by rules_swift
+        // when the global index store feature is enabled.
         outputPath + "/_global_index_store"
     }
 }

Sources/SourceKitBazelBSP/SharedUtils/Shell/CommandRunner.swift

@@ -85,20 +85,21 @@ extension CommandRunner {
         additionalStartupFlags: [String] = []
     ) -> String {
         let indexFlags = baseConfig.indexFlags
-        let additionalFlags = additionalFlags.map { " \($0)" }
-        let flagsString: String
-        if indexFlags.isEmpty {
-            flagsString = additionalFlags.joined(separator: "")
-        } else {
-            flagsString = (additionalFlags + indexFlags.map { " \($0)" }).joined(separator: "")
+        var components: [String] = []
+        if !baseConfig.noExtraOutputBase {
+            components.append("--output_base=\(outputBase)")
         }
-        let startupFlagsString: String
-        if additionalStartupFlags.isEmpty {
-            startupFlagsString = ""
-        } else {
-            startupFlagsString = " " + additionalStartupFlags.joined(separator: " ")
+        if !additionalStartupFlags.isEmpty {
+            components.append(contentsOf: additionalStartupFlags)
         }
-        return "--output_base=\(outputBase)\(startupFlagsString) \(cmd)\(flagsString)"
+        components.append(cmd)
+        if !additionalFlags.isEmpty {
+            components.append(contentsOf: additionalFlags)
+        }
+        if !indexFlags.isEmpty {
+            components.append(contentsOf: indexFlags)
+        }
+        return components.joined(separator: " ")
     }
 
     /// A regular bazel command, but at this BSP's special output base and taking into account the special index flags.

Sources/sourcekit-bazel-bsp/Commands/Serve.swift

@@ -79,6 +79,12 @@ struct Serve: ParsableCommand {
     )
     var dependencyTargetToExclude: [String] = []
 
+    @Flag(
+        help:
+            "If enabled, the BSP will not create a separate output base for its indexing actions. You can use this in conjunction with rules_swift's `index_while_building` and `use_global_module_cache` to improve indexing performance and reduce disk usage at the cost of potentially slower builds."
+    )
+    var noExtraOutputBase: Bool = false
+
     func run() throws {
         logger.info("`serve` invoked, initializing BSP server...")
 
@@ -98,7 +104,8 @@ struct Serve: ParsableCommand {
             topLevelRulesToDiscover: topLevelRulesToDiscover,
             dependencyRulesToDiscover: dependencyRulesToDiscover,
             topLevelTargetsToExclude: topLevelTargetToExclude,
-            dependencyTargetsToExclude: dependencyTargetToExclude
+            dependencyTargetsToExclude: dependencyTargetToExclude,
+            noExtraOutputBase: noExtraOutputBase
         )
 
         logger.debug("Initializing BSP with targets: \(targets)")

rules/setup_sourcekit_bsp.bzl

@@ -35,6 +35,8 @@ def _setup_sourcekit_bsp_impl(ctx):
     if files_to_watch:
         bsp_config_argv.append("--files-to-watch")
         bsp_config_argv.append(files_to_watch)
+    if ctx.attr.no_extra_output_base:
+        bsp_config_argv.append("--no-extra-output-base")
     ctx.actions.expand_template(
         template = ctx.file._bsp_config_template,
         output = rendered_bsp_config,
@@ -146,5 +148,9 @@ setup_sourcekit_bsp = rule(
             doc = "Instead of attempting to build targets individually, build the top-level parent. If your project contains build_test targets for your individual libraries and you're passing them as the top-level targets for the BSP, you can use this flag to build those targets directly for better predictability and caching.",
             default = False,
         ),
+        "no_extra_output_base": attr.bool(
+            doc = "If enabled, the BSP will not create a separate output base for its indexing actions. You can use this in conjunction with rules_swift's `index_while_building` and `use_global_module_cache` to improve indexing performance and reduce disk usage at the cost of potentially slower builds.",
+            default = False,
+        ),
     },
 )