Add internal lint `derive_deserialize_allowing_unknown` (#14360)

Adds an internal lint to check for `#[derive(serde::Deserialize)]`
without
[`#[serde(deny_unknown_fields)]`](https://serde.rs/container-attrs.html#deny_unknown_fields).

Today, if you run Clippy with the following clippy.toml, Clippy will
produce a warning, but there will be no accompanying note:
```toml
# In the following configuration, "recommendation" should be "reason" or "replacement".
disallowed-macros = [
    { path = "std::panic", recommendation = "return a `std::result::Result::Error` instead" },
]
```
```sh
$ cargo clippy
    Checking a v0.1.0 (/home/smoelius/tmp/a)
warning: use of a disallowed macro `std::panic`
 --> src/lib.rs:2:5
  |
2 |     panic!();
  |     ^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#disallowed_macros
  = note: `#[warn(clippy::disallowed_macros)]` on by default
```
The underlying problem is: the enum that derives `serde::Deserialize`
([`DisallowedPathEnum`](https://github.com/rust-lang/rust-clippy/blob/81643e297cf44ce3c7648b8443fc4d6592fa81eb/clippy_config/src/types.rs#L47))
does not have the attribute `#[serde(deny_unknown_fields)]`.

This lint identifies such problems by checking trait `impl`s. An
alternative I considered was to walk `clippy_config::conf::Conf`
directly. However, that would not catch the `DisallowedPathEnum` case
because it [is not used in `Conf`
directly](https://github.com/rust-lang/rust-clippy/blob/81643e297cf44ce3c7648b8443fc4d6592fa81eb/clippy_config/src/types.rs#L31).

Just to be clear, no one asked for this. So I hope the maintainers do
not mind.

changelog: none

5 files changed, 100 insertions, 0 deletions

diff --git a/tests/ui-internal/derive_deserialize_allowing_unknown.rs b/tests/ui-internal/derive_deserialize_allowing_unknown.rs
new file mode 100644
index 00000000000..9dc8e9e8f4c
--- /dev/null
+++ b/tests/ui-internal/derive_deserialize_allowing_unknown.rs
@@ -0,0 +1,60 @@
+#![deny(clippy::derive_deserialize_allowing_unknown)]
+
+use serde::{Deserialize, Deserializer};
+
+#[derive(Deserialize)] //~ derive_deserialize_allowing_unknown
+struct Struct {
+    flag: bool,
+    limit: u64,
+}
+
+#[derive(Deserialize)] //~ derive_deserialize_allowing_unknown
+enum Enum {
+    A(bool),
+    B { limit: u64 },
+}
+
+// negative tests
+
+#[derive(Deserialize)]
+#[serde(deny_unknown_fields)]
+struct StructWithDenyUnknownFields {
+    flag: bool,
+    limit: u64,
+}
+
+#[derive(Deserialize)]
+#[serde(deny_unknown_fields)]
+enum EnumWithDenyUnknownFields {
+    A(bool),
+    B { limit: u64 },
+}
+
+#[derive(Deserialize)]
+#[serde(untagged, deny_unknown_fields)]
+enum MultipleSerdeAttributes {
+    A(bool),
+    B { limit: u64 },
+}
+
+#[derive(Deserialize)]
+struct TupleStruct(u64, bool);
+
+#[derive(Deserialize)]
+#[serde(deny_unknown_fields)]
+enum EnumWithOnlyTupleVariants {
+    A(bool),
+    B(u64),
+}
+
+struct ManualSerdeImplementation;
+
+impl<'de> Deserialize<'de> for ManualSerdeImplementation {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let () = <() as Deserialize>::deserialize(deserializer)?;
+        Ok(ManualSerdeImplementation)
+    }
+}
diff --git a/tests/ui-internal/derive_deserialize_allowing_unknown.stderr b/tests/ui-internal/derive_deserialize_allowing_unknown.stderr
new file mode 100644
index 00000000000..93d64826c99
--- /dev/null
+++ b/tests/ui-internal/derive_deserialize_allowing_unknown.stderr
@@ -0,0 +1,23 @@
+error: `#[derive(serde::Deserialize)]` without `#[serde(deny_unknown_fields)]`
+  --> tests/ui-internal/derive_deserialize_allowing_unknown.rs:5:10
+   |
+LL | #[derive(Deserialize)]
+   |          ^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> tests/ui-internal/derive_deserialize_allowing_unknown.rs:1:9
+   |
+LL | #![deny(clippy::derive_deserialize_allowing_unknown)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: `#[derive(serde::Deserialize)]` without `#[serde(deny_unknown_fields)]`
+  --> tests/ui-internal/derive_deserialize_allowing_unknown.rs:11:10
+   |
+LL | #[derive(Deserialize)]
+   |          ^^^^^^^^^^^
+   |
+   = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
+
diff --git a/tests/ui-toml/toml_unknown_config_struct_field/clippy.toml b/tests/ui-toml/toml_unknown_config_struct_field/clippy.toml
new file mode 100644
index 00000000000..82560cfd5e2
--- /dev/null
+++ b/tests/ui-toml/toml_unknown_config_struct_field/clippy.toml
@@ -0,0 +1,4 @@
+# In the following configuration, "recommendation" should be "reason" or "replacement".
+disallowed-macros = [
+    { path = "std::panic", recommendation = "return a `std::result::Result::Error` instead" },
+]
diff --git a/tests/ui-toml/toml_unknown_config_struct_field/toml_unknown_config_struct_field.rs b/tests/ui-toml/toml_unknown_config_struct_field/toml_unknown_config_struct_field.rs
new file mode 100644
index 00000000000..9c770c31f6f
--- /dev/null
+++ b/tests/ui-toml/toml_unknown_config_struct_field/toml_unknown_config_struct_field.rs
@@ -0,0 +1,5 @@
+#[rustfmt::skip]
+//@error-in-other-file: error reading Clippy's configuration file: data did not match any variant of untagged enum DisallowedPathEnum
+fn main() {
+    panic!();
+}
diff --git a/tests/ui-toml/toml_unknown_config_struct_field/toml_unknown_config_struct_field.stderr b/tests/ui-toml/toml_unknown_config_struct_field/toml_unknown_config_struct_field.stderr
new file mode 100644
index 00000000000..b564709721d
--- /dev/null
+++ b/tests/ui-toml/toml_unknown_config_struct_field/toml_unknown_config_struct_field.stderr
@@ -0,0 +1,8 @@
+error: error reading Clippy's configuration file: data did not match any variant of untagged enum DisallowedPathEnum
+  --> $DIR/tests/ui-toml/toml_unknown_config_struct_field/clippy.toml:3:5
+   |
+LL |     { path = "std::panic", recommendation = "return a `std::result::Result::Error` instead" },
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+