Auto merge of #83908 - Flying-Toast:master, r=davidtwco

Add enum_intrinsics_non_enums lint

There is a clippy lint to prevent calling [`mem::discriminant`](https://doc.rust-lang.org/std/mem/fn.discriminant.html) with a non-enum type. I think the lint is worthy of being included in rustc, given that `discriminant::<T>()` where `T` is a non-enum has an unspecified return value, and there are no valid use cases where you'd actually want this.

I've also made the lint check [variant_count](https://doc.rust-lang.org/core/mem/fn.variant_count.html) (#73662).

closes #83899

5 files changed, 1 insertions, 87 deletions

diff --git a/src/tools/clippy/clippy_lints/src/lib.register_all.rs b/src/tools/clippy/clippy_lints/src/lib.register_all.rs
index 3e6e0244754..6a3ee35b41a 100644
--- a/src/tools/clippy/clippy_lints/src/lib.register_all.rs
+++ b/src/tools/clippy/clippy_lints/src/lib.register_all.rs
@@ -127,7 +127,6 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(matches::REDUNDANT_PATTERN_MATCHING),
     LintId::of(matches::SINGLE_MATCH),
     LintId::of(matches::WILDCARD_IN_OR_PATTERNS),
-    LintId::of(mem_discriminant::MEM_DISCRIMINANT_NON_ENUM),
     LintId::of(mem_replace::MEM_REPLACE_OPTION_WITH_NONE),
     LintId::of(mem_replace::MEM_REPLACE_WITH_DEFAULT),
     LintId::of(mem_replace::MEM_REPLACE_WITH_UNINIT),
diff --git a/src/tools/clippy/clippy_lints/src/lib.register_correctness.rs b/src/tools/clippy/clippy_lints/src/lib.register_correctness.rs
index e0ef7b3b8af..bbe47a0e772 100644
--- a/src/tools/clippy/clippy_lints/src/lib.register_correctness.rs
+++ b/src/tools/clippy/clippy_lints/src/lib.register_correctness.rs
@@ -36,7 +36,6 @@ store.register_group(true, "clippy::correctness", Some("clippy_correctness"), ve
     LintId::of(loops::ITER_NEXT_LOOP),
     LintId::of(loops::NEVER_LOOP),
     LintId::of(loops::WHILE_IMMUTABLE_CONDITION),
-    LintId::of(mem_discriminant::MEM_DISCRIMINANT_NON_ENUM),
     LintId::of(mem_replace::MEM_REPLACE_WITH_UNINIT),
     LintId::of(methods::CLONE_DOUBLE_REF),
     LintId::of(methods::ITERATOR_STEP_BY_ZERO),
diff --git a/src/tools/clippy/clippy_lints/src/lib.register_lints.rs b/src/tools/clippy/clippy_lints/src/lib.register_lints.rs
index 2ba2b3da55c..b0be3b65366 100644
--- a/src/tools/clippy/clippy_lints/src/lib.register_lints.rs
+++ b/src/tools/clippy/clippy_lints/src/lib.register_lints.rs
@@ -241,7 +241,6 @@ store.register_lints(&[
     matches::SINGLE_MATCH_ELSE,
     matches::WILDCARD_ENUM_MATCH_ARM,
     matches::WILDCARD_IN_OR_PATTERNS,
-    mem_discriminant::MEM_DISCRIMINANT_NON_ENUM,
     mem_forget::MEM_FORGET,
     mem_replace::MEM_REPLACE_OPTION_WITH_NONE,
     mem_replace::MEM_REPLACE_WITH_DEFAULT,
diff --git a/src/tools/clippy/clippy_lints/src/lib.rs b/src/tools/clippy/clippy_lints/src/lib.rs
index 9fc6a9e0ccc..5534f9c94f3 100644
--- a/src/tools/clippy/clippy_lints/src/lib.rs
+++ b/src/tools/clippy/clippy_lints/src/lib.rs
@@ -266,7 +266,6 @@ mod map_unit_fn;
 mod match_on_vec_items;
 mod match_result_ok;
 mod matches;
-mod mem_discriminant;
 mod mem_forget;
 mod mem_replace;
 mod methods;
@@ -600,7 +599,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     let doc_valid_idents = conf.doc_valid_idents.iter().cloned().collect::<FxHashSet<_>>();
     store.register_late_pass(move || Box::new(doc::DocMarkdown::new(doc_valid_idents.clone())));
     store.register_late_pass(|| Box::new(neg_multiply::NegMultiply));
-    store.register_late_pass(|| Box::new(mem_discriminant::MemDiscriminant));
     store.register_late_pass(|| Box::new(mem_forget::MemForget));
     store.register_late_pass(|| Box::new(arithmetic::Arithmetic::default()));
     store.register_late_pass(|| Box::new(assign_ops::AssignOps));
@@ -850,6 +848,7 @@ pub fn register_renamed(ls: &mut rustc_lint::LintStore) {
     ls.register_renamed("clippy::panic_params", "non_fmt_panics");
     ls.register_renamed("clippy::unknown_clippy_lints", "unknown_lints");
     ls.register_renamed("clippy::invalid_atomic_ordering", "invalid_atomic_ordering");
+    ls.register_renamed("clippy::mem_discriminant_non_enum", "enum_intrinsics_non_enums");
 }
 
 // only exists to let the dogfood integration test works.
diff --git a/src/tools/clippy/clippy_lints/src/mem_discriminant.rs b/src/tools/clippy/clippy_lints/src/mem_discriminant.rs
deleted file mode 100644
index 59176c4b846..00000000000
--- a/src/tools/clippy/clippy_lints/src/mem_discriminant.rs
+++ /dev/null
@@ -1,82 +0,0 @@
-use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::source::snippet;
-use clippy_utils::ty::walk_ptrs_ty_depth;
-use clippy_utils::{match_def_path, paths};
-use if_chain::if_chain;
-use rustc_errors::Applicability;
-use rustc_hir::{BorrowKind, Expr, ExprKind};
-use rustc_lint::{LateContext, LateLintPass};
-use rustc_session::{declare_lint_pass, declare_tool_lint};
-
-declare_clippy_lint! {
-    /// ### What it does
-    /// Checks for calls of `mem::discriminant()` on a non-enum type.
-    ///
-    /// ### Why is this bad?
-    /// The value of `mem::discriminant()` on non-enum types
-    /// is unspecified.
-    ///
-    /// ### Example
-    /// ```rust
-    /// use std::mem;
-    ///
-    /// mem::discriminant(&"hello");
-    /// mem::discriminant(&&Some(2));
-    /// ```
-    pub MEM_DISCRIMINANT_NON_ENUM,
-    correctness,
-    "calling `mem::descriminant` on non-enum type"
-}
-
-declare_lint_pass!(MemDiscriminant => [MEM_DISCRIMINANT_NON_ENUM]);
-
-impl<'tcx> LateLintPass<'tcx> for MemDiscriminant {
-    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
-        if_chain! {
-            if let ExprKind::Call(func, func_args) = expr.kind;
-            // is `mem::discriminant`
-            if let ExprKind::Path(ref func_qpath) = func.kind;
-            if let Some(def_id) = cx.qpath_res(func_qpath, func.hir_id).opt_def_id();
-            if match_def_path(cx, def_id, &paths::MEM_DISCRIMINANT);
-            // type is non-enum
-            let ty_param = cx.typeck_results().node_substs(func.hir_id).type_at(0);
-            if !ty_param.is_enum();
-
-            then {
-                span_lint_and_then(
-                    cx,
-                    MEM_DISCRIMINANT_NON_ENUM,
-                    expr.span,
-                    &format!("calling `mem::discriminant` on non-enum type `{}`", ty_param),
-                    |diag| {
-                        // if this is a reference to an enum, suggest dereferencing
-                        let (base_ty, ptr_depth) = walk_ptrs_ty_depth(ty_param);
-                        if ptr_depth >= 1 && base_ty.is_enum() {
-                            let param = &func_args[0];
-
-                            // cancel out '&'s first
-                            let mut derefs_needed = ptr_depth;
-                            let mut cur_expr = param;
-                            while derefs_needed > 0  {
-                                if let ExprKind::AddrOf(BorrowKind::Ref, _, inner_expr) = cur_expr.kind {
-                                    derefs_needed -= 1;
-                                    cur_expr = inner_expr;
-                                } else {
-                                    break;
-                                }
-                            }
-
-                            let derefs = "*".repeat(derefs_needed);
-                            diag.span_suggestion(
-                                param.span,
-                                "try dereferencing",
-                                format!("{}{}", derefs, snippet(cx, cur_expr.span, "<param>")),
-                                Applicability::MachineApplicable,
-                            );
-                        }
-                    },
-                )
-            }
-        }
-    }
-}