Build rust_test targets using a crate name different from the underlying lib

docs/defs.md

@@ -567,9 +567,7 @@ rust_test(
 )
 ```
 
-Run the test with `bazel test //hello_lib:hello_lib_test`. The crate
-will be built using the same crate name as the underlying ":hello_lib"
-crate.
+Run the test with `bazel test //hello_lib:hello_lib_test`.
 
 ### Example: `test` directory
 

docs/flatten.md

@@ -1050,9 +1050,7 @@ rust_test(
 )
 ```
 
-Run the test with `bazel test //hello_lib:hello_lib_test`. The crate
-will be built using the same crate name as the underlying ":hello_lib"
-crate.
+Run the test with `bazel test //hello_lib:hello_lib_test`.
 
 ### Example: `test` directory
 

rust/private/rust.bzl

@@ -309,14 +309,21 @@ def _rust_test_impl(ctx):
         # Target is building the crate in `test` config
         crate = ctx.attr.crate[rust_common.crate_info] if rust_common.crate_info in ctx.attr.crate else ctx.attr.crate[rust_common.test_crate_info].crate
 
-        output_hash = determine_output_hash(crate.root, ctx.label)
-        output = ctx.actions.declare_file(
-            "test-%s/%s%s" % (
-                output_hash,
-                ctx.label.name,
-                toolchain.binary_ext,
-            ),
-        )
+        if toolchain._incompatible_change_rust_test_compilation_output_directory:
+            crate_name = compute_crate_name(ctx.workspace_name, ctx.label, toolchain, ctx.attr.crate_name)
+            output = ctx.actions.declare_file(
+                ctx.label.name + toolchain.binary_ext,
+            )
+        else:
+            crate_name = crate.name
+            output_hash = determine_output_hash(crate.root, ctx.label)
+            output = ctx.actions.declare_file(
+                "test-%s/%s%s" % (
+                    output_hash,
+                    ctx.label.name,
+                    toolchain.binary_ext,
+                ),
+            )
 
         srcs, crate_root = transform_sources(ctx, ctx.files.srcs, getattr(ctx.file, "crate_root", None))
 
@@ -342,7 +349,7 @@ def _rust_test_impl(ctx):
 
         # Build the test binary using the dependency's srcs.
         crate_info_dict = dict(
-            name = crate.name,
+            name = crate_name,
             type = crate_type,
             root = crate.root,
             srcs = depset(srcs, transitive = [crate.srcs]),
@@ -368,14 +375,19 @@ def _rust_test_impl(ctx):
             crate_root = crate_root_src(ctx.attr.name, ctx.files.srcs, crate_root_type)
         srcs, crate_root = transform_sources(ctx, ctx.files.srcs, crate_root)
 
-        output_hash = determine_output_hash(crate_root, ctx.label)
-        output = ctx.actions.declare_file(
-            "test-%s/%s%s" % (
-                output_hash,
-                ctx.label.name,
-                toolchain.binary_ext,
-            ),
-        )
+        if toolchain._incompatible_change_rust_test_compilation_output_directory:
+            output = ctx.actions.declare_file(
+                ctx.label.name + toolchain.binary_ext,
+            )
+        else:
+            output_hash = determine_output_hash(crate_root, ctx.label)
+            output = ctx.actions.declare_file(
+                "test-%s/%s%s" % (
+                    output_hash,
+                    ctx.label.name,
+                    toolchain.binary_ext,
+                ),
+            )
 
         data_paths = depset(direct = getattr(ctx.attr, "data", [])).to_list()
         rustc_env = expand_dict_value_locations(
@@ -1348,9 +1360,7 @@ rust_test = rule(
         )
         ```
 
-        Run the test with `bazel test //hello_lib:hello_lib_test`. The crate
-        will be built using the same crate name as the underlying ":hello_lib"
-        crate.
+        Run the test with `bazel test //hello_lib:hello_lib_test`.
 
         ### Example: `test` directory
 

rust/settings/BUILD.bazel

@@ -92,6 +92,13 @@ incompatible_flag(
     issue = "https://github.com/bazelbuild/rules_rust/issues/2039",
 )
 
+# A flag to put rust_test compilation outputs in the same directory as the rust_library compilation outputs.
+incompatible_flag(
+    name = "incompatible_change_rust_test_compilation_output_directory",
+    build_setting_default = False,
+    issue = "https://github.com/bazelbuild/rules_rust/issues/2827",
+)
+
 # A flag to control whether to link libstd dynamically.
 bool_flag(
     name = "experimental_link_std_dylib",

rust/toolchain.bzl

@@ -698,6 +698,7 @@ def _rust_toolchain_impl(ctx):
         _experimental_use_global_allocator = experimental_use_global_allocator,
         _experimental_use_coverage_metadata_files = ctx.attr._experimental_use_coverage_metadata_files[BuildSettingInfo].value,
         _experimental_toolchain_generated_sysroot = ctx.attr._experimental_toolchain_generated_sysroot[IncompatibleFlagInfo].enabled,
+        _incompatible_change_rust_test_compilation_output_directory = ctx.attr._incompatible_change_rust_test_compilation_output_directory[IncompatibleFlagInfo].enabled,
         _no_std = no_std,
     )
     return [
@@ -889,6 +890,9 @@ rust_toolchain = rule(
                 "This flag is only relevant when used together with --@rules_rust//rust/settings:experimental_use_global_allocator."
             ),
         ),
+        "_incompatible_change_rust_test_compilation_output_directory": attr.label(
+            default = Label("//rust/settings:incompatible_change_rust_test_compilation_output_directory"),
+        ),
         "_no_std": attr.label(
             default = Label("//:no_std"),
         ),

test/rust/src/lib.rs

@@ -12,5 +12,4 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#![crate_name = "hello_lib"]
 pub mod greeter;