Warning on conflicting dev_dependencies keys of platform

Signed-off-by: hi-rustin <rustin.liu@gmail.com>

Author: 0xPoe

src/cargo/util/toml/mod.rs

@@ -1308,6 +1308,13 @@ impl TomlManifest {
                     .as_ref()
                     .or_else(|| platform.build_dependencies2.as_ref());
                 process_dependencies(&mut cx, build_deps, Some(DepKind::Build))?;
+                if platform.dev_dependencies.is_some() && platform.dev_dependencies2.is_some() {
+                    cx.warnings.push(format!(
+                        "found both `dev-dependencies` and `dev_dependencies` are set \
+                         in the `{}` platform target",
+                        name
+                    ));
+                }
                 let dev_deps = platform
                     .dev_dependencies
                     .as_ref()

tests/testsuite/build.rs

@@ -3023,6 +3023,46 @@ fn cargo_platform_specific_dependency_build_dependencies_conflicting_warning() {
     assert!(p.bin("foo").is_file());
 }
 
+#[cargo_test]
+fn cargo_platform_specific_dependency_dev_dependencies_conflicting_warning() {
+    let host = rustc_host();
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [project]
+                    name = "foo"
+                    version = "0.5.0"
+                    authors = ["wycats@example.com"]
+
+                    [target.{host}.dev-dependencies]
+                    dev = {{ path = "dev" }}
+                    [target.{host}.dev_dependencies]
+                    dev = {{ path = "dev" }}
+                "#,
+                host = host
+            ),
+        )
+        .file("src/main.rs", "fn main() { }")
+        .file(
+            "tests/foo.rs",
+            "extern crate dev; #[test] fn foo() { dev::dev() }",
+        )
+        .file("dev/Cargo.toml", &basic_manifest("dev", "0.5.0"))
+        .file("dev/src/lib.rs", "pub fn dev() {}")
+        .build();
+
+    p.cargo("build")
+        .with_stderr_contains(
+        format!("[WARNING] found both `dev-dependencies` and `dev_dependencies` are set in the `{}` platform target", host),
+        )
+            .run();
+
+    assert!(p.bin("foo").is_file());
+    p.cargo("test").run();
+}
+
 #[cargo_test]
 fn bad_platform_specific_dependency() {
     let p = project()