fix(manifest): Point out when a key belongs to config

Inspired by #16251

Author: epage

src/cargo/util/context/mod.rs

@@ -158,6 +158,29 @@ macro_rules! get_value_typed {
     };
 }
 
+pub const TOP_LEVEL_CONFIG_KEYS: &[&str] = &[
+    "paths",
+    "alias",
+    "build",
+    "credential-alias",
+    "doc",
+    "env",
+    "future-incompat-report",
+    "cache",
+    "cargo-new",
+    "http",
+    "install",
+    "net",
+    "patch",
+    "profile",
+    "resolver",
+    "registries",
+    "registry",
+    "source",
+    "target",
+    "term",
+];
+
 /// Indicates why a config value is being loaded.
 #[derive(Clone, Copy, Debug)]
 enum WhyLoad {

src/cargo/util/toml/mod.rs

@@ -35,6 +35,7 @@ use crate::util::interning::InternedString;
 use crate::util::lints::{get_key_value_span, rel_cwd_manifest_path};
 use crate::util::{
     self, GlobalContext, IntoUrl, OnceExt, OptVersionReq, context::ConfigRelativePath,
+    context::TOP_LEVEL_CONFIG_KEYS,
 };
 
 mod embedded;
@@ -2887,8 +2888,18 @@ fn deprecated_underscore<T>(
 }
 
 fn warn_on_unused(unused: &BTreeSet<String>, warnings: &mut Vec<String>) {
+    use std::fmt::Write as _;
+
     for key in unused {
-        warnings.push(format!("unused manifest key: {}", key));
+        let mut message = format!("unused manifest key: {}", key);
+        if TOP_LEVEL_CONFIG_KEYS.iter().any(|c| c == key) {
+            write!(
+                &mut message,
+                "\nhelp: {key} is a valid .cargo/config.toml key"
+            )
+            .unwrap();
+        }
+        warnings.push(message);
     }
 }
 

tests/testsuite/bad_config.rs

@@ -853,26 +853,43 @@ quiet = false
     p.cargo("check")
         .with_stderr_data(str![[r#"
 [WARNING] unused manifest key: alias
+[HELP] alias is a valid .cargo/config.toml key
 [WARNING] unused manifest key: build
+[HELP] build is a valid .cargo/config.toml key
 [WARNING] unused manifest key: cache
+[HELP] cache is a valid .cargo/config.toml key
 [WARNING] unused manifest key: cargo-new
+[HELP] cargo-new is a valid .cargo/config.toml key
 [WARNING] unused manifest key: credential-alias
+[HELP] credential-alias is a valid .cargo/config.toml key
 [WARNING] unused manifest key: doc
+[HELP] doc is a valid .cargo/config.toml key
 [WARNING] unused manifest key: env
+[HELP] env is a valid .cargo/config.toml key
 [WARNING] unused manifest key: future-incompat-report
+[HELP] future-incompat-report is a valid .cargo/config.toml key
 [WARNING] unused manifest key: http
+[HELP] http is a valid .cargo/config.toml key
 [WARNING] unused manifest key: install
+[HELP] install is a valid .cargo/config.toml key
 [WARNING] unused manifest key: lib.build
 [WARNING] unused manifest key: net
+[HELP] net is a valid .cargo/config.toml key
 [WARNING] unused manifest key: package.unused
 [WARNING] unused manifest key: paths
+[HELP] paths is a valid .cargo/config.toml key
 [WARNING] unused manifest key: registries
+[HELP] registries is a valid .cargo/config.toml key
 [WARNING] unused manifest key: registry
+[HELP] registry is a valid .cargo/config.toml key
 [WARNING] unused manifest key: resolver
+[HELP] resolver is a valid .cargo/config.toml key
 [WARNING] unused manifest key: source
+[HELP] source is a valid .cargo/config.toml key
 [WARNING] unused manifest key: target.cfg(unix).linker
 [WARNING] unused manifest key: target.foo.bar
 [WARNING] unused manifest key: term
+[HELP] term is a valid .cargo/config.toml key
 [CHECKING] foo v0.1.0 ([ROOT]/foo)
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s