Add new lint unneeded_struct_pattern

clippy_lints/src/unneeded_struct_pattern.rs

@@ -0,0 +1,76 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::is_from_proc_macro;
+use rustc_errors::Applicability;
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::{Pat, PatKind, QPath};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for struct patterns that match against unit variant.
+    ///
+    /// ### Why is this bad?
+    /// Struct pattern `{ }` or `{ .. }` is not needed for unit variant.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// match Some(42) {
+    ///     Some(v) => v,
+    ///     None { .. } => 0,
+    /// };
+    /// // Or
+    /// match Some(42) {
+    ///     Some(v) => v,
+    ///     None { } => 0,
+    /// };
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// match Some(42) {
+    ///     Some(v) => v,
+    ///     None => 0,
+    /// };
+    /// ```
+    #[clippy::version = "1.83.0"]
+    pub UNNEEDED_STRUCT_PATTERN,
+    style,
+    "using struct pattern to match against unit variant"
+}
+
+declare_lint_pass!(UnneededStructPattern => [UNNEEDED_STRUCT_PATTERN]);
+
+impl LateLintPass<'_> for UnneededStructPattern {
+    fn check_pat(&mut self, cx: &LateContext<'_>, pat: &Pat<'_>) {
+        if !pat.span.from_expansion()
+            && let PatKind::Struct(path, [], _) = &pat.kind
+            && let QPath::Resolved(_, path) = path
+            && let Res::Def(DefKind::Variant, did) = path.res
+        {
+            let enum_did = cx.tcx.parent(did);
+            let variant = cx.tcx.adt_def(enum_did).variant_with_id(did);
+
+            let has_only_fields_brackets = variant.ctor.is_some() && variant.fields.is_empty();
+            let non_exhaustive_activated = !variant.def_id.is_local() && variant.is_field_list_non_exhaustive();
+            if !has_only_fields_brackets || non_exhaustive_activated {
+                return;
+            }
+
+            if is_from_proc_macro(cx, *path) {
+                return;
+            }
+
+            if let Some(brackets_span) = pat.span.trim_start(path.span) {
+                span_lint_and_sugg(
+                    cx,
+                    UNNEEDED_STRUCT_PATTERN,
+                    brackets_span,
+                    "struct pattern is not needed for a unit variant",
+                    "remove the struct pattern",
+                    String::new(),
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+    }
+}