Auto merge of #13262 - epage:unused, r=ehuss

fix(manifest): Provide unused key warnings for lints table

### What does this PR try to resolve?

The use of `flatten` was getting in the way of `serde_ignored`.
A common workaround is to add our own `unused` tracking but that would
cause duplicates with `workspace.lints` (or we'd just ignore it).

Since the manual deserializer was relatively simple, I went that route.

Fixes #12917

### How should we test and review this PR?

Per commit

A test was added for the issue.  I then was worried about regressions in `workspace = false` errors (and I was right) so I added a test for that.  To get `workspace = false` to work nicely, I made it share code with other `workspace: bool` fields.

### Additional information

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

@@ -410,11 +410,7 @@ impl<'de> de::Deserialize<'de> for InheritableBtreeMap {
         if let Ok(w) = TomlInheritedField::deserialize(
             serde_value::ValueDeserializer::<D::Error>::new(value.clone()),
         ) {
-            return if w.workspace {
-                Ok(InheritableField::Inherit(w))
-            } else {
-                Err(de::Error::custom("`workspace` cannot be false"))
-            };
+            return Ok(InheritableField::Inherit(w));
         }
         BTreeMap::deserialize(serde_value::ValueDeserializer::<D::Error>::new(value))
             .map(InheritableField::Value)
@@ -424,13 +420,14 @@ impl<'de> de::Deserialize<'de> for InheritableBtreeMap {
 #[derive(Deserialize, Serialize, Copy, Clone, Debug)]
 #[serde(rename_all = "kebab-case")]
 pub struct TomlInheritedField {
-    #[serde(deserialize_with = "bool_no_false")]
-    workspace: bool,
+    workspace: WorkspaceValue,
 }
 
 impl TomlInheritedField {
     pub fn new() -> Self {
-        TomlInheritedField { workspace: true }
+        TomlInheritedField {
+            workspace: WorkspaceValue,
+        }
     }
 }
 
@@ -440,12 +437,25 @@ impl Default for TomlInheritedField {
     }
 }
 
-fn bool_no_false<'de, D: de::Deserializer<'de>>(deserializer: D) -> Result<bool, D::Error> {
-    let b: bool = Deserialize::deserialize(deserializer)?;
-    if b {
-        Ok(b)
-    } else {
-        Err(de::Error::custom("`workspace` cannot be false"))
+#[derive(Deserialize, Serialize, Copy, Clone, Debug)]
+#[serde(try_from = "bool")]
+#[serde(into = "bool")]
+struct WorkspaceValue;
+
+impl TryFrom<bool> for WorkspaceValue {
+    type Error = String;
+    fn try_from(other: bool) -> Result<WorkspaceValue, Self::Error> {
+        if other {
+            Ok(WorkspaceValue)
+        } else {
+            Err("`workspace` cannot be false".to_owned())
+        }
+    }
+}
+
+impl From<WorkspaceValue> for bool {
+    fn from(_: WorkspaceValue) -> bool {
+        true
     }
 }
 
@@ -1250,12 +1260,9 @@ impl TomlPlatform {
     }
 }
 
-#[derive(Deserialize, Serialize, Debug, Clone)]
-#[serde(expecting = "a lints table")]
-#[serde(rename_all = "kebab-case")]
+#[derive(Serialize, Debug, Clone)]
 pub struct InheritableLints {
     #[serde(skip_serializing_if = "is_false")]
-    #[serde(deserialize_with = "bool_no_false", default)]
     pub workspace: bool,
     #[serde(flatten)]
     pub lints: TomlLints,
@@ -1265,6 +1272,54 @@ fn is_false(b: &bool) -> bool {
     !b
 }
 
+impl<'de> Deserialize<'de> for InheritableLints {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: de::Deserializer<'de>,
+    {
+        struct InheritableLintsVisitor;
+
+        impl<'de> de::Visitor<'de> for InheritableLintsVisitor {
+            // The type that our Visitor is going to produce.
+            type Value = InheritableLints;
+
+            // Format a message stating what data this Visitor expects to receive.
+            fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+                formatter.write_str("a lints table")
+            }
+
+            // Deserialize MyMap from an abstract "map" provided by the
+            // Deserializer. The MapAccess input is a callback provided by
+            // the Deserializer to let us see each entry in the map.
+            fn visit_map<M>(self, mut access: M) -> Result<Self::Value, M::Error>
+            where
+                M: de::MapAccess<'de>,
+            {
+                let mut lints = TomlLints::new();
+                let mut workspace = false;
+
+                // While there are entries remaining in the input, add them
+                // into our map.
+                while let Some(key) = access.next_key()? {
+                    if key == "workspace" {
+                        workspace = match access.next_value()? {
+                            Some(WorkspaceValue) => true,
+                            None => false,
+                        };
+                    } else {
+                        let value = access.next_value()?;
+                        lints.insert(key, value);
+                    }
+                }
+
+                Ok(InheritableLints { workspace, lints })
+            }
+        }
+
+        deserializer.deserialize_map(InheritableLintsVisitor)
+    }
+}
+
 pub type TomlLints = BTreeMap<String, TomlToolLints>;
 
 pub type TomlToolLints = BTreeMap<String, TomlLint>;

tests/testsuite/lints.rs

@@ -148,6 +148,37 @@ Caused by:
         .run();
 }
 
+#[cargo_test]
+fn warn_on_unused_key() {
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+
+                [workspace.lints.rust]
+                rust-2018-idioms = { level = "allow", unused = true }
+                [lints.rust]
+                rust-2018-idioms = { level = "allow", unused = true }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    foo.cargo("check")
+        .with_stderr(
+            "\
+[WARNING] [CWD]/Cargo.toml: unused manifest key: lints.rust.rust-2018-idioms.unused
+[WARNING] [CWD]/Cargo.toml: unused manifest key: workspace.lints.rust.rust-2018-idioms.unused
+[CHECKING] foo v0.0.1 ([CWD])
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]s
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn fail_on_tool_injection() {
     let foo = project()
@@ -276,6 +307,51 @@ error: usage of an `unsafe` block
         .run();
 }
 
+#[cargo_test]
+fn workspace_cant_be_false() {
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+                authors = []
+
+                [lints]
+                workspace = false
+
+                [workspace.lints.rust]
+                "unsafe_code" = "deny"
+            "#,
+        )
+        .file(
+            "src/lib.rs",
+            "
+pub fn foo(num: i32) -> u32 {
+    unsafe { std::mem::transmute(num) }
+}
+",
+        )
+        .build();
+
+    foo.cargo("check")
+        .with_status(101)
+        .with_stderr_contains(
+            "\
+error: failed to parse manifest at `[CWD]/Cargo.toml`
+
+Caused by:
+  TOML parse error at line 8, column 29
+    |
+  8 |                 workspace = false
+    |                             ^^^^^
+  `workspace` cannot be false
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn workspace_lint_deny() {
     let foo = project()