Auto merge of rust-lang#12584 - epage:lints, r=arlosi

fix(lints): Fail when overriding inherited lints

### What does this PR try to resolve?

Overriding of inherited lints was reserved for the future but as pointed out in rust-lang#12115 (comment), we aren't failing on these when we should but silently ignoring the overrides.

This turns it into a hard error.

In fixing this, I had to add a `#[serde(expecting)]` attribute to maintain behavior on an error case (otherwise it would say "expecting struct WorkspaceLints").  Since this drew the error message to my attention, I also tweaked it to make it more specific.

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

Commits are broken down by the relevant tests and fixes to make the intended behavior changes obvious.

src/cargo/util/toml/mod.rs

@@ -1475,29 +1475,6 @@ impl<'de> de::Deserialize<'de> for MaybeWorkspaceBtreeMap {
     }
 }
 
-type MaybeWorkspaceLints = MaybeWorkspace<TomlLints, TomlWorkspaceField>;
-
-impl<'de> de::Deserialize<'de> for MaybeWorkspaceLints {
-    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
-    where
-        D: de::Deserializer<'de>,
-    {
-        let value = serde_value::Value::deserialize(deserializer)?;
-
-        if let Ok(w) = TomlWorkspaceField::deserialize(
-            serde_value::ValueDeserializer::<D::Error>::new(value.clone()),
-        ) {
-            return if w.workspace() {
-                Ok(MaybeWorkspace::Workspace(w))
-            } else {
-                Err(de::Error::custom("`workspace` cannot be false"))
-            };
-        }
-        TomlLints::deserialize(serde_value::ValueDeserializer::<D::Error>::new(value))
-            .map(MaybeWorkspace::Defined)
-    }
-}
-
 #[derive(Deserialize, Serialize, Copy, Clone, Debug)]
 pub struct TomlWorkspaceField {
     #[serde(deserialize_with = "bool_no_false")]
@@ -2277,7 +2254,7 @@ impl TomlManifest {
 
         let lints =
             parse_unstable_lints::<MaybeWorkspaceLints>(me.lints.clone(), config, cx.warnings)?
-                .map(|mw| mw.resolve("lints", || inherit()?.lints()))
+                .map(|mw| mw.resolve(|| inherit()?.lints()))
                 .transpose()?;
         let lints = verify_lints(lints)?;
         let default = TomlLints::default();
@@ -2579,8 +2556,7 @@ impl TomlManifest {
                 .badges
                 .as_ref()
                 .map(|_| MaybeWorkspace::Defined(metadata.badges.clone())),
-            lints: lints
-                .map(|lints| toml::Value::try_from(MaybeWorkspaceLints::Defined(lints)).unwrap()),
+            lints: lints.map(|lints| toml::Value::try_from(lints).unwrap()),
         };
         let mut manifest = Manifest::new(
             summary,
@@ -3522,6 +3498,38 @@ impl fmt::Debug for PathValue {
     }
 }
 
+#[derive(Deserialize, Serialize, Debug, Clone)]
+#[serde(expecting = "a lints table")]
+pub struct MaybeWorkspaceLints {
+    #[serde(skip_serializing_if = "is_false")]
+    #[serde(deserialize_with = "bool_no_false", default)]
+    workspace: bool,
+    #[serde(flatten)]
+    lints: TomlLints,
+}
+
+fn is_false(b: &bool) -> bool {
+    !b
+}
+
+impl MaybeWorkspaceLints {
+    fn resolve<'a>(
+        self,
+        get_ws_inheritable: impl FnOnce() -> CargoResult<TomlLints>,
+    ) -> CargoResult<TomlLints> {
+        if self.workspace {
+            if !self.lints.is_empty() {
+                anyhow::bail!("cannot override `workspace.lints` in `lints`, either remove the overrides or `lints.workspace = true` and manually specify the lints");
+            }
+            get_ws_inheritable().with_context(|| {
+                "error inheriting `lints` from workspace root manifest's `workspace.lints`"
+            })
+        } else {
+            Ok(self.lints)
+        }
+    }
+}
+
 pub type TomlLints = BTreeMap<String, TomlToolLints>;
 
 pub type TomlToolLints = BTreeMap<String, TomlLint>;

tests/testsuite/lints.rs

@@ -172,7 +172,7 @@ fn malformed_on_nightly() {
 error: failed to parse manifest[..]
 
 Caused by:
-  invalid type: integer `20`, expected a map
+  invalid type: integer `20`, expected a lints table
 ",
         )
         .run();
@@ -413,6 +413,50 @@ error: usage of an `unsafe` block
         .run();
 }
 
+#[cargo_test]
+fn workspace_and_package_lints() {
+    let foo = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+                authors = []
+
+                [lints]
+                workspace = true
+                [lints.rust]
+                "unsafe_code" = "allow"
+
+                [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 -Zlints")
+        .masquerade_as_nightly_cargo(&["lints"])
+        .with_status(101)
+        .with_stderr(
+            "\
+[ERROR] failed to parse manifest at `[CWD]/Cargo.toml`
+
+Caused by:
+  cannot override `workspace.lints` in `lints`, either remove the overrides or `lints.workspace = true` and manually specify the lints
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn attribute_has_precedence() {
     let foo = project()