Use crate_name to also match crate_root. (#2824)

This allows targets to have special names that do not match their
sources while still ensuring a root is found. I've encountered this when
creating macros where I have a Rust target that is a side-effect and not
the named target itself. E.g.
```starlark
load("@rules_rust//rust:defs.bzl", "rust_binary")
load("//control/private:control.bzl", _control_binary = "control_binary")

def control_binary(
    name, 
    # ...
    **kwargs):

    rust_binary(
        name = name + "_bin",
        # ...
        **kwargs
    )

    _control_binary(
        name = name,
        bin = name + "_bin",
        # ...
        **kwargs
    )
```

Being able to specify `crate_name` will both ensure the binary is
compiled with a cleaner name for the runtime and be able to do the
nominal `crate_root` matching. The diff after this PR achieves the
desired behavior.

```diff
--- before/defs.bzl	2024-08-31 12:41:24
+++ after/defs.bzl	2024-08-31 12:42:09
@@ -8,6 +8,7 @@

     rust_binary(
         name = name + "_bin",
+        crate_name = name,
         # ...
         **kwargs
     )
```

rust/private/rust.bzl

@@ -148,7 +148,7 @@ def _rust_library_common(ctx, crate_type):
 
     crate_root = getattr(ctx.file, "crate_root", None)
     if not crate_root:
-        crate_root = crate_root_src(ctx.attr.name, ctx.files.srcs, crate_type)
+        crate_root = crate_root_src(ctx.attr.name, ctx.attr.crate_name, ctx.files.srcs, crate_type)
     srcs, crate_root = transform_sources(ctx, ctx.files.srcs, crate_root)
 
     # Determine unique hash for this rlib.
@@ -228,7 +228,7 @@ def _rust_binary_impl(ctx):
 
     crate_root = getattr(ctx.file, "crate_root", None)
     if not crate_root:
-        crate_root = crate_root_src(ctx.attr.name, ctx.files.srcs, ctx.attr.crate_type)
+        crate_root = crate_root_src(ctx.attr.name, ctx.attr.crate_name, ctx.files.srcs, ctx.attr.crate_type)
     srcs, crate_root = transform_sources(ctx, ctx.files.srcs, crate_root)
 
     providers = rustc_compile_action(
@@ -365,7 +365,7 @@ def _rust_test_impl(ctx):
 
         if not crate_root:
             crate_root_type = "lib" if ctx.attr.use_libtest_harness else "bin"
-            crate_root = crate_root_src(ctx.attr.name, ctx.files.srcs, crate_root_type)
+            crate_root = crate_root_src(ctx.attr.name, ctx.attr.crate_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)

rust/private/utils.bzl

@@ -708,11 +708,12 @@ def can_build_metadata(toolchain, ctx, crate_type):
            ctx.attr._process_wrapper and \
            crate_type in ("rlib", "lib")
 
-def crate_root_src(name, srcs, crate_type):
+def crate_root_src(name, crate_name, srcs, crate_type):
     """Determines the source file for the crate root, should it not be specified in `attr.crate_root`.
 
     Args:
         name (str): The name of the target.
+        crate_name (str): The target's `crate_name` attribute.
         srcs (list): A list of all sources for the target Crate.
         crate_type (str): The type of this crate ("bin", "lib", "rlib", "cdylib", etc).
 
@@ -723,10 +724,14 @@ def crate_root_src(name, srcs, crate_type):
     """
     default_crate_root_filename = "main.rs" if crate_type == "bin" else "lib.rs"
 
+    if not crate_name:
+        crate_name = name
+
     crate_root = (
         (srcs[0] if len(srcs) == 1 else None) or
         _shortest_src_with_basename(srcs, default_crate_root_filename) or
-        _shortest_src_with_basename(srcs, name + ".rs")
+        _shortest_src_with_basename(srcs, name + ".rs") or
+        _shortest_src_with_basename(srcs, crate_name + ".rs")
     )
     if not crate_root:
         file_names = [default_crate_root_filename, name + ".rs"]

test/unit/crate_name/add.rs

@@ -0,0 +1 @@
+

test/unit/crate_name/crate_name_test.bzl

@@ -126,6 +126,16 @@ def _crate_name_test():
         edition = "2018",
     )
 
+    rust_binary(
+        name = "custom_bin_target_name",
+        crate_name = "smain",
+        srcs = [
+            "smain.rs",
+            "add.rs",
+        ],
+        edition = "2018",
+    )
+
     rust_test(
         name = "default/crate-name-test",
         srcs = ["main.rs"],
@@ -139,12 +149,32 @@ def _crate_name_test():
         edition = "2018",
     )
 
+    rust_test(
+        name = "custom_named_test",
+        crate_name = "stest",
+        srcs = [
+            "stest.rs",
+            "add.rs",
+        ],
+        edition = "2018",
+    )
+
     rust_library(
         name = "slib",
         srcs = ["slib.rs"],
         edition = "2018",
     )
 
+    rust_library(
+        name = "custom_lib_target_name",
+        crate_name = "slib",
+        srcs = [
+            "slib.rs",
+            "add.rs",
+        ],
+        edition = "2018",
+    )
+
     rust_shared_library(
         name = "shared_lib",
         srcs = ["lib.rs"],

test/unit/crate_name/smain.rs

@@ -0,0 +1 @@
+fn main() {}

test/unit/crate_name/stest.rs

@@ -0,0 +1,16 @@
+//! https://doc.rust-lang.org/rust-by-example/testing/unit_testing.html
+
+pub fn add(a: i32, b: i32) -> i32 {
+    a + b
+}
+
+#[cfg(test)]
+mod tests {
+    // Note this useful idiom: importing names from outer (for mod tests) scope.
+    use super::*;
+
+    #[test]
+    fn test_add() {
+        assert_eq!(add(1, 2), 3);
+    }
+}