fix(manifest): Point out when a key belongs to config (#16256)

### What does this PR try to resolve?

Inspired by #16251

### How to test and review this PR?

Author: weihanglo

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;
@@ -2887,11 +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));
-        if key == "profiles.debug" {
-            warnings.push("use `[profile.dev]` to configure debug builds".to_string());
+        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

@@ -772,74 +772,125 @@ fn unused_keys() {
         .file(
             "Cargo.toml",
             r#"
-               [package]
-               name = "foo"
-               version = "0.1.0"
-               edition = "2015"
-               authors = []
+paths = ["/path/to/override"]
 
-               [target.foo]
-               bar = "3"
-            "#,
-        )
-        .file("src/lib.rs", "")
-        .build();
+[package]
+name = "foo"
+version = "0.1.0"
+edition = "2015"
+authors = []
+unused = "foo"
 
-    p.cargo("check")
-        .with_stderr_data(str![[r#"
-[WARNING] unused manifest key: target.foo.bar
-[CHECKING] foo v0.1.0 ([ROOT]/foo)
-[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+[target.foo]
+bar = "3"
 
-"#]])
-        .run();
+[lib]
+build = "foo"
 
-    let p = project()
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
+## Config fields
 
-                name = "foo"
-                version = "0.5.0"
-                edition = "2015"
-                authors = ["[email protected]"]
-                unused = "foo"
-            "#,
-        )
-        .file("src/lib.rs", "pub fn foo() {}")
-        .build();
-    p.cargo("check")
-        .with_stderr_data(str![[r#"
-[WARNING] unused manifest key: package.unused
-[CHECKING] foo v0.5.0 ([ROOT]/foo)
-[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+[alias]
+b = "build"
 
-"#]])
-        .run();
+[build]
+jobs = 1
 
-    let p = project()
-        .at("bar")
-        .file(
-            "Cargo.toml",
-            r#"
-                [package]
+[credential-alias]
+my-alias = ["/usr/bin/cargo-credential-example", "--argument", "value", "--flag"]
 
-                name = "foo"
-                version = "0.5.0"
-                edition = "2015"
-                authors = ["[email protected]"]
+[doc]
+browser = "chromium"
 
-                [lib]
-                build = "foo"
-            "#,
+[env]
+ENV_VAR_NAME = "value"
+
+[future-incompat-report]
+frequency = 'always'
+
+[cache]
+auto-clean-frequency = "1 day"
+
+[cargo-new]
+vcs = "none"
+
+[http]
+debug = false
+
+[install]
+root = "/some/path"
+
+[net]
+retry = 3
+
+[net.ssh]
+known-hosts = ["..."]
+
+[resolver]
+incompatible-rust-versions = "allow"
+
+[registries.alternative]
+index = "…"
+
+[registries.crates-io]
+protocol = "sparse"
+
+[registry]
+default = "…"
+
+[source.alternative]
+replace-with = "…"
+
+[target.'cfg(unix)']
+linker = "…"
+
+[term]
+quiet = false
+"#,
         )
-        .file("src/lib.rs", "pub fn foo() {}")
+        .file("src/lib.rs", "")
         .build();
+
     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
-[CHECKING] foo v0.5.0 ([ROOT]/bar)
+[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
 
 "#]])