Emit error on [target.'cfg(debug_assertions)'.dependencies] and similar

crates/cargo-platform/src/lib.rs

@@ -61,6 +61,48 @@ impl Platform {
         }
         Ok(())
     }
+
+    pub fn check_cfg_attributes(&self, warnings: &mut Vec<String>) {
+        fn check_cfg_expr(expr: &CfgExpr, warnings: &mut Vec<String>) {
+            match *expr {
+                CfgExpr::Not(ref e) => check_cfg_expr(e, warnings),
+                CfgExpr::All(ref e) | CfgExpr::Any(ref e) => {
+                    for e in e {
+                        check_cfg_expr(e, warnings);
+                    }
+                }
+                CfgExpr::Value(ref e) => match e {
+                    Cfg::Name(name) => match name.as_str() {
+                        "test" | "debug_assertions" | "proc_macro" =>
+                            warnings.push(format!(
+                                "Found `{}` in `target.'cfg(...)'.dependencies`. \
+                                 This value is not supported for selecting dependencies \
+                                 and will not work as expected. \
+                                 To learn more visit \
+                                 https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#platform-specific-dependencies",
+                                 name
+                            )),
+                        _ => (),
+                    },
+                    Cfg::KeyPair(name, _) => match name.as_str() {
+                        "feature" =>
+                            warnings.push(String::from(
+                                "Found `feature = ...` in `target.'cfg(...)'.dependencies`. \
+                                 This key is not supported for selecting dependencies \
+                                 and will not work as expected. \
+                                 Use the [features] section instead: \
+                                 https://doc.rust-lang.org/cargo/reference/manifest.html#the-features-section"
+                            )),
+                        _ => (),
+                    },
+                }
+            }
+        }
+
+        if let Platform::Cfg(cfg) = self {
+            check_cfg_expr(cfg, warnings);
+        }
+    }
 }
 
 impl serde::Serialize for Platform {

crates/cargo-platform/tests/test_cfg.rs

@@ -176,3 +176,76 @@ fn round_trip_platform() {
          all(target_os = \"freebsd\", target_arch = \"x86_64\")))",
     );
 }
+
+#[test]
+fn check_cfg_attributes() {
+    fn ok(s: &str) {
+        let p = Platform::Cfg(s.parse().unwrap());
+        let mut warnings = Vec::new();
+        p.check_cfg_attributes(&mut warnings);
+        assert!(
+            warnings.is_empty(),
+            "Expected no warnings but got: {:?}",
+            warnings,
+        );
+    }
+
+    fn warn(s: &str, names: &[&str]) {
+        let p = Platform::Cfg(s.parse().unwrap());
+        let mut warnings = Vec::new();
+        p.check_cfg_attributes(&mut warnings);
+        assert_eq!(
+            warnings.len(),
+            names.len(),
+            "Expecter warnings about {:?} but got {:?}",
+            names,
+            warnings,
+        );
+        for (name, warning) in names.iter().zip(warnings.iter()) {
+            assert!(
+                warning.contains(name),
+                "Expected warning about '{}' but got: {}",
+                name,
+                warning,
+            );
+        }
+    }
+
+    ok("unix");
+    ok("windows");
+    ok("any(not(unix), windows)");
+    ok("foo");
+
+    ok("target_arch = \"abc\"");
+    ok("target_feature = \"abc\"");
+    ok("target_os = \"abc\"");
+    ok("target_family = \"abc\"");
+    ok("target_env = \"abc\"");
+    ok("target_endian = \"abc\"");
+    ok("target_pointer_width = \"abc\"");
+    ok("target_vendor = \"abc\"");
+    ok("bar = \"def\"");
+
+    warn("test", &["test"]);
+    warn("debug_assertions", &["debug_assertions"]);
+    warn("proc_macro", &["proc_macro"]);
+    warn("feature = \"abc\"", &["feature"]);
+
+    warn("any(not(debug_assertions), windows)", &["debug_assertions"]);
+    warn(
+        "any(not(feature = \"def\"), target_arch = \"abc\")",
+        &["feature"],
+    );
+    warn(
+        "any(not(target_os = \"windows\"), proc_macro)",
+        &["proc_macro"],
+    );
+    warn(
+        "any(not(feature = \"windows\"), proc_macro)",
+        &["feature", "proc_macro"],
+    );
+    warn(
+        "all(not(debug_assertions), any(windows, proc_macro))",
+        &["debug_assertions", "proc_macro"],
+    );
+}

src/cargo/util/toml/mod.rs

@@ -1085,7 +1085,11 @@ impl TomlManifest {
             process_dependencies(&mut cx, build_deps, Some(Kind::Build))?;
 
             for (name, platform) in me.target.iter().flatten() {
-                cx.platform = Some(name.parse()?);
+                cx.platform = {
+                    let platform: Platform = name.parse()?;
+                    platform.check_cfg_attributes(&mut cx.warnings);
+                    Some(platform)
+                };
                 process_dependencies(&mut cx, platform.dependencies.as_ref(), None)?;
                 let build_deps = platform
                     .build_dependencies

src/doc/src/reference/specifying-dependencies.md

@@ -476,6 +476,11 @@ dependencies based on optional crate features.
 Use [the `[features]` section](manifest.md#the-features-section)
 instead.
 
+The same applies to `cfg(debug_assertions)`, `cfg(test)` and `cfg(prog_macro)`.
+These values will not work as expected and will always have the default value
+returned by `rustc --print=cfg`.
+There is currently no way to add dependencies based on these configuration values.
+
 In addition to `#[cfg]` syntax, Cargo also supports listing out the full target
 the dependencies would apply to: