Parse hints permissively to allow for future expansion

Make it only a warning, not an error, to have a hint value of the wrong
type.

Author: joshtriplett

crates/cargo-util-schemas/manifest.schema.json

@@ -1012,9 +1012,13 @@
       "type": "object",
       "properties": {
         "mostly-unused": {
-          "type": [
-            "boolean",
-            "null"
+          "anyOf": [
+            {
+              "$ref": "#/$defs/TomlValue"
+            },
+            {
+              "type": "null"
+            }
           ]
         }
       }

crates/cargo-util-schemas/src/manifest/mod.rs

@@ -1647,7 +1647,11 @@ pub enum TomlLintLevel {
 #[serde(rename_all = "kebab-case")]
 #[cfg_attr(feature = "unstable-schema", derive(schemars::JsonSchema))]
 pub struct Hints {
-    pub mostly_unused: Option<bool>,
+    #[cfg_attr(
+        feature = "unstable-schema",
+        schemars(with = "Option<TomlValueWrapper>")
+    )]
+    pub mostly_unused: Option<toml::Value>,
 }
 
 #[derive(Copy, Clone, Debug)]

src/cargo/core/compiler/mod.rs

@@ -1130,7 +1130,7 @@ fn build_base_args(
         strip,
         rustflags: profile_rustflags,
         trim_paths,
-        hint_mostly_unused,
+        hint_mostly_unused: profile_hint_mostly_unused,
         ..
     } = unit.profile.clone();
     let hints = unit.pkg.hints().cloned().unwrap_or_default();
@@ -1322,15 +1322,29 @@ fn build_base_args(
         opt(cmd, "-C", "incremental=", Some(dir));
     }
 
-    if hint_mostly_unused.or(hints.mostly_unused).unwrap_or(false) {
+    let pkg_hint_mostly_unused = match hints.mostly_unused {
+        None => None,
+        Some(toml::Value::Boolean(b)) => Some(b),
+        Some(v) => {
+            bcx.gctx.shell().warn(format!(
+                "ignoring unknown value type ({}) for 'hints.mostly-unused'",
+                v.type_str()
+            ))?;
+            None
+        }
+    };
+    if profile_hint_mostly_unused
+        .or(pkg_hint_mostly_unused)
+        .unwrap_or(false)
+    {
         if bcx.gctx.cli_unstable().profile_hint_mostly_unused {
             cmd.arg("-Zhint-mostly-unused");
         } else {
-            if hint_mostly_unused.is_some() {
+            if profile_hint_mostly_unused.is_some() {
                 bcx.gctx
                     .shell()
                     .warn("ignoring 'hint-mostly-unused' profile option, pass `-Zprofile-hint-mostly-unused` to enable it")?;
-            } else if hints.mostly_unused.is_some() {
+            } else if pkg_hint_mostly_unused.is_some() {
                 bcx.gctx
                     .shell()
                     .warn("ignoring 'hints.mostly-unused', pass `-Zprofile-hint-mostly-unused` to enable it")?;

tests/testsuite/hints.rs

@@ -58,6 +58,56 @@ fn unknown_hints_warn() {
         .run();
 }
 
+#[cargo_test]
+fn hint_unknown_type_warn() {
+    Package::new("bar", "1.0.0")
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "bar"
+            version = "1.0.0"
+            edition = "2015"
+
+            [hints]
+            mostly-unused = 1
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .publish();
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            version = "0.0.1"
+            edition = "2015"
+
+            [dependencies]
+            bar = "1.0"
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+    p.cargo("check -v")
+        .with_stderr_data(str![[r#"
+[UPDATING] `dummy-registry` index
+[LOCKING] 1 package to latest compatible version
+[DOWNLOADING] crates ...
+[DOWNLOADED] bar v1.0.0 (registry `dummy-registry`)
+[WARNING] ignoring unknown value type (integer) for 'hints.mostly-unused'
+[CHECKING] bar v1.0.0
+[RUNNING] `rustc --crate-name bar [..]`
+[CHECKING] foo v0.0.1 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name foo [..]`
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .with_stderr_does_not_contain("-Zhint-mostly-unused")
+        .run();
+}
+
 #[cargo_test]
 fn hints_mostly_unused_warn_without_gate() {
     Package::new("bar", "1.0.0")