fix(cargo): Add a warning on [project] table being used in a manifest

src/cargo/util/toml/mod.rs

@@ -1588,7 +1588,16 @@ impl TomlManifest {
                 project.clone()
             }
             (Some(package), None) => package.clone(),
-            (None, Some(project)) => project.clone(),
+            (None, Some(project)) => {
+                if source_id.is_path() {
+                    config.shell().warn(format!(
+                        "manifest at `{}` contains `[project]` instead of `[package]`, \
+                                this could become a hard error in the future",
+                        package_root.display()
+                    ))?;
+                }
+                project.clone()
+            }
             (None, None) => bail!("no `package` section found"),
         };
 

tests/testsuite/check.rs

@@ -1054,3 +1054,29 @@ fn warn_manifest_package_and_project() {
         )
         .run();
 }
+
+#[cargo_test]
+fn warn_manifest_with_project() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [project]
+                name = "foo"
+                version = "0.0.1"
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("check")
+        .with_status(0)
+        .with_stderr(
+            "\
+[WARNING] manifest at `[CWD]` contains `[project]` instead of `[package]`, this could become a hard error in the future
+[CHECKING] foo v0.0.1 ([CWD])
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]
+",
+        )
+        .run();
+}