Auto merge of #6181 - cgm616:undropped-manually-drops, r=flip1995

Add new lint for undropped ManuallyDrop values

Adds a new lint for the following code:

```rust
struct S;

impl Drop for S {
    fn drop(&mut self) {
        println!("drip drop");
    }
}

fn main() {
    // This will not drop the `S`!!!
    drop(std::mem::ManuallyDrop::new(S));
    unsafe {
        // This will.
        std::mem::ManuallyDrop::drop(&mut std::mem::ManuallyDrop::new(S));
    }
}
```

The inner value of a `ManuallyDrop` will not be dropped unless the proper, unsafe drop function is called on it. This lint makes sure that a user does not accidently use the wrong function and forget to drop a `ManuallyDrop` value.

Fixes #5581.

---

*Please keep the line below*
changelog: none

clippy_lints/src/lib.rs

@@ -314,6 +314,7 @@ mod transmute;
 mod transmuting_null;
 mod try_err;
 mod types;
+mod undropped_manually_drops;
 mod unicode;
 mod unit_return_expecting_ord;
 mod unnamed_address;
@@ -865,6 +866,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &types::UNIT_CMP,
         &types::UNNECESSARY_CAST,
         &types::VEC_BOX,
+        &undropped_manually_drops::UNDROPPED_MANUALLY_DROPS,
         &unicode::INVISIBLE_CHARACTERS,
         &unicode::NON_ASCII_LITERAL,
         &unicode::UNICODE_NOT_NFC,
@@ -1141,6 +1143,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move || box disallowed_method::DisallowedMethod::new(&disallowed_methods));
     store.register_early_pass(|| box asm_syntax::InlineAsmX86AttSyntax);
     store.register_early_pass(|| box asm_syntax::InlineAsmX86IntelSyntax);
+    store.register_late_pass(|| box undropped_manually_drops::UndroppedManuallyDrops);
 
 
     store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
@@ -1528,6 +1531,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&types::UNIT_CMP),
         LintId::of(&types::UNNECESSARY_CAST),
         LintId::of(&types::VEC_BOX),
+        LintId::of(&undropped_manually_drops::UNDROPPED_MANUALLY_DROPS),
         LintId::of(&unicode::INVISIBLE_CHARACTERS),
         LintId::of(&unit_return_expecting_ord::UNIT_RETURN_EXPECTING_ORD),
         LintId::of(&unnamed_address::FN_ADDRESS_COMPARISONS),
@@ -1800,6 +1804,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&types::ABSURD_EXTREME_COMPARISONS),
         LintId::of(&types::CAST_REF_TO_MUT),
         LintId::of(&types::UNIT_CMP),
+        LintId::of(&undropped_manually_drops::UNDROPPED_MANUALLY_DROPS),
         LintId::of(&unicode::INVISIBLE_CHARACTERS),
         LintId::of(&unit_return_expecting_ord::UNIT_RETURN_EXPECTING_ORD),
         LintId::of(&unnamed_address::FN_ADDRESS_COMPARISONS),

clippy_lints/src/undropped_manually_drops.rs

@@ -0,0 +1,50 @@
+use crate::utils::{is_type_lang_item, match_function_call, paths, span_lint_and_help};
+use rustc_hir::{lang_items, Expr};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+
+declare_clippy_lint! {
+    /// **What it does:** Prevents the safe `std::mem::drop` function from being called on `std::mem::ManuallyDrop`.
+    ///
+    /// **Why is this bad?** The safe `drop` function does not drop the inner value of a `ManuallyDrop`.
+    ///
+    /// **Known problems:** Does not catch cases if the user binds `std::mem::drop`
+    /// to a different name and calls it that way.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// struct S;
+    /// drop(std::mem::ManuallyDrop::new(S));
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// struct S;
+    /// unsafe {
+    ///     std::mem::ManuallyDrop::drop(&mut std::mem::ManuallyDrop::new(S));
+    /// }
+    /// ```
+    pub UNDROPPED_MANUALLY_DROPS,
+    correctness,
+    "use of safe `std::mem::drop` function to drop a std::mem::ManuallyDrop, which will not drop the inner value"
+}
+
+declare_lint_pass!(UndroppedManuallyDrops => [UNDROPPED_MANUALLY_DROPS]);
+
+impl LateLintPass<'tcx> for UndroppedManuallyDrops {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        if let Some(ref args) = match_function_call(cx, expr, &paths::DROP) {
+            let ty = cx.typeck_results().expr_ty(&args[0]);
+            if is_type_lang_item(cx, ty, lang_items::LangItem::ManuallyDrop) {
+                span_lint_and_help(
+                    cx,
+                    UNDROPPED_MANUALLY_DROPS,
+                    expr.span,
+                    "the inner value of this ManuallyDrop will not be dropped",
+                    None,
+                    "to drop a `ManuallyDrop<T>`, use std::mem::ManuallyDrop::drop",
+                );
+            }
+        }
+    }
+}