Add support for shared test-only modules (#2734)

Currently, `rust_test_suite` is not capable of supporting integration
tests with shared modules. A very basic example is:
```
[crate]/
    src/
        lib.rs
    tests/
        integration_test_a.rs
        integration_test_b.rs
        util/
            mod.rs
```
With `integration_test_a.rs` and `integration_test_b.rs` importing code
from `util`.

This PR adds a `shared_srcs` argument to `rust_test_suite`. That way, we
can finally compile and run those tests with:
```python
rust_test_suite(
    name = "integrated_tests_suite",
    srcs = glob(["tests/**"]),
    shared_srcs = ["tests/util/mod.rs"],
    deps = [":example_dep"],
)
```

docs/defs.md

@@ -650,7 +650,7 @@ Run the test with `bazel test //hello_lib:greeting_test`.
 ## rust_test_suite
 
 <pre>
-rust_test_suite(<a href="#rust_test_suite-name">name</a>, <a href="#rust_test_suite-srcs">srcs</a>, <a href="#rust_test_suite-kwargs">kwargs</a>)
+rust_test_suite(<a href="#rust_test_suite-name">name</a>, <a href="#rust_test_suite-srcs">srcs</a>, <a href="#rust_test_suite-shared_srcs">shared_srcs</a>, <a href="#rust_test_suite-kwargs">kwargs</a>)
 </pre>
 
 A rule for creating a test suite for a set of `rust_test` targets.
@@ -670,6 +670,8 @@ directory structure:
         integrated_test_c.rs
         patterns/
             fibonacci_test.rs
+        helpers/
+            mod.rs
 ```
 
 The rule can be used to generate [rust_test](#rust_test) targets for each source file under `tests`
@@ -691,6 +693,7 @@ rust_binary(
 rust_test_suite(
     name = "integrated_tests_suite",
     srcs = glob(["tests/**"]),
+    shared_srcs=glob(["tests/helpers/**"]),
     deps = [":math_lib"],
 )
 ```
@@ -706,6 +709,7 @@ rust_test_suite(
 | :------------- | :------------- | :------------- |
 | <a id="rust_test_suite-name"></a>name |  The name of the `test_suite`.   |  none |
 | <a id="rust_test_suite-srcs"></a>srcs |  All test sources, typically `glob(["tests/**/*.rs"])`.   |  none |
+| <a id="rust_test_suite-shared_srcs"></a>shared_srcs |  Optional argument for sources shared among tests, typically helper functions.   |  `[]` |
 | <a id="rust_test_suite-kwargs"></a>kwargs |  Additional keyword arguments for the underyling [rust_test](#rust_test) targets. The `tags` argument is also passed to the generated `test_suite` target.   |  none |
 
 

docs/flatten.md

@@ -1934,7 +1934,7 @@ Assembles a remote repository for the given toolchain params, produces a proxy r
 ## rust_test_suite
 
 <pre>
-rust_test_suite(<a href="#rust_test_suite-name">name</a>, <a href="#rust_test_suite-srcs">srcs</a>, <a href="#rust_test_suite-kwargs">kwargs</a>)
+rust_test_suite(<a href="#rust_test_suite-name">name</a>, <a href="#rust_test_suite-srcs">srcs</a>, <a href="#rust_test_suite-shared_srcs">shared_srcs</a>, <a href="#rust_test_suite-kwargs">kwargs</a>)
 </pre>
 
 A rule for creating a test suite for a set of `rust_test` targets.
@@ -1954,6 +1954,8 @@ directory structure:
         integrated_test_c.rs
         patterns/
             fibonacci_test.rs
+        helpers/
+            mod.rs
 ```
 
 The rule can be used to generate [rust_test](#rust_test) targets for each source file under `tests`
@@ -1975,6 +1977,7 @@ rust_binary(
 rust_test_suite(
     name = "integrated_tests_suite",
     srcs = glob(["tests/**"]),
+    shared_srcs=glob(["tests/helpers/**"]),
     deps = [":math_lib"],
 )
 ```
@@ -1990,6 +1993,7 @@ rust_test_suite(
 | :------------- | :------------- | :------------- |
 | <a id="rust_test_suite-name"></a>name |  The name of the `test_suite`.   |  none |
 | <a id="rust_test_suite-srcs"></a>srcs |  All test sources, typically `glob(["tests/**/*.rs"])`.   |  none |
+| <a id="rust_test_suite-shared_srcs"></a>shared_srcs |  Optional argument for sources shared among tests, typically helper functions.   |  `[]` |
 | <a id="rust_test_suite-kwargs"></a>kwargs |  Additional keyword arguments for the underyling [rust_test](#rust_test) targets. The `tags` argument is also passed to the generated `test_suite` target.   |  none |
 
 

rust/private/rust.bzl

@@ -1393,7 +1393,7 @@ rust_test = rule(
 """),
 )
 
-def rust_test_suite(name, srcs, **kwargs):
+def rust_test_suite(name, srcs, shared_srcs = [], **kwargs):
     """A rule for creating a test suite for a set of `rust_test` targets.
 
     This rule can be used for setting up typical rust [integration tests][it]. Given the following
@@ -1411,6 +1411,8 @@ def rust_test_suite(name, srcs, **kwargs):
             integrated_test_c.rs
             patterns/
                 fibonacci_test.rs
+            helpers/
+                mod.rs
     ```
 
     The rule can be used to generate [rust_test](#rust_test) targets for each source file under `tests`
@@ -1432,6 +1434,7 @@ def rust_test_suite(name, srcs, **kwargs):
     rust_test_suite(
         name = "integrated_tests_suite",
         srcs = glob(["tests/**"]),
+        shared_srcs=glob(["tests/helpers/**"]),
         deps = [":math_lib"],
     )
     ```
@@ -1442,6 +1445,7 @@ def rust_test_suite(name, srcs, **kwargs):
     Args:
         name (str): The name of the `test_suite`.
         srcs (list): All test sources, typically `glob(["tests/**/*.rs"])`.
+        shared_srcs (list): Optional argument for sources shared among tests, typically helper functions.
         **kwargs (dict): Additional keyword arguments for the underyling [rust_test](#rust_test) targets. The
             `tags` argument is also passed to the generated `test_suite` target.
     """
@@ -1451,12 +1455,16 @@ def rust_test_suite(name, srcs, **kwargs):
         if not src.endswith(".rs"):
             fail("srcs should have `.rs` extensions")
 
+        if src in shared_srcs:
+            continue
+
         # Prefixed with `name` to allow parameterization with macros
         # The test name should not end with `.rs`
         test_name = name + "_" + src[:-3]
         rust_test(
             name = test_name,
-            srcs = [src],
+            crate_root = src,
+            srcs = [src] + shared_srcs,
             **kwargs
         )
         tests.append(test_name)

test/rust_test_suite_shared/BUILD.bazel

@@ -0,0 +1,15 @@
+load("//rust:defs.bzl", "rust_library", "rust_test_suite")
+
+rust_library(
+    name = "math_lib",
+    srcs = ["src/lib.rs"],
+    edition = "2018",
+)
+
+rust_test_suite(
+    name = "tests_suite",
+    srcs = glob(["tests/**"]),
+    edition = "2018",
+    shared_srcs = glob(["tests/helpers/**"]),
+    deps = [":math_lib"],
+)

test/rust_test_suite_shared/src/lib.rs

@@ -0,0 +1,8 @@
+/// Calculate the n'th fibonacci number
+pub fn fibonacci(n: i32) -> i32 {
+    match n {
+        0 => 1,
+        1 => 1,
+        _ => fibonacci(n - 1) + fibonacci(n - 2),
+    }
+}

test/rust_test_suite_shared/tests/helpers/mod.rs

@@ -0,0 +1,12 @@
+// Binet's formula for checking Fibonacci numbers
+pub fn is_fibonacci(x: i32) -> bool {
+    is_perfect_square(5 * x * x + 4) || is_perfect_square(5 * x * x - 4)
+}
+
+fn is_perfect_square(x: i32) -> bool {
+    if x < 0 {
+        return false;
+    }
+    let y = (x as f64).sqrt() as i32;
+    y * y == x
+}

test/rust_test_suite_shared/tests/integrated_test_a.rs

@@ -0,0 +1,11 @@
+mod helpers;
+
+use helpers::is_fibonacci;
+use math_lib::fibonacci;
+
+#[test]
+fn fibonacci_test() {
+    let fib6 = fibonacci(6);
+    assert_eq!(fib6, fibonacci(5) + fibonacci(4));
+    assert!(is_fibonacci(fib6));
+}

test/rust_test_suite_shared/tests/integrated_test_b.rs

@@ -0,0 +1,11 @@
+mod helpers;
+
+use helpers::is_fibonacci;
+use math_lib::fibonacci;
+
+#[test]
+fn fibonacci_test() {
+    let fib7 = fibonacci(7);
+    assert_eq!(fib7, fibonacci(6) + fibonacci(5));
+    assert!(is_fibonacci(fib7));
+}