Enhance suggestions of dropping_* lints

compiler/rustc_lint/messages.ftl

@@ -233,6 +233,9 @@ lint_drop_trait_constraints =
 lint_dropping_copy_types = calls to `std::mem::drop` with a value that implements `Copy` does nothing
     .label = argument has type `{$arg_ty}`
 
+lint_dropping_mutable_references = calls to `std::mem::drop` with a mutable reference instead of an owned value only makes the reference inaccessible, it does not drop the underlying value
+    .label = argument has type `{$arg_ty}`
+
 lint_dropping_references = calls to `std::mem::drop` with a reference instead of an owned value does nothing
     .label = argument has type `{$arg_ty}`
 
@@ -270,6 +273,9 @@ lint_for_loops_over_fallibles =
 lint_forgetting_copy_types = calls to `std::mem::forget` with a value that implements `Copy` does nothing
     .label = argument has type `{$arg_ty}`
 
+lint_forgetting_mutable_references = calls to `std::mem::forget` with a mutable reference instead of an owned value only makes the reference inaccessible, it does not drop the underlying value
+    .label = argument has type `{$arg_ty}`
+
 lint_forgetting_references = calls to `std::mem::forget` with a reference instead of an owned value does nothing
     .label = argument has type `{$arg_ty}`
 
@@ -683,6 +689,10 @@ lint_redundant_semicolons =
         *[false] this semicolon
     }
 
+lint_remove_entirely_suggestion = remove the whole expression
+
+lint_remove_function_call_suggestion = remove the function call
+
 lint_remove_mut_from_pattern = remove `mut` from the parameter
 
 lint_removed_lint = lint `{$name}` has been removed: {$reason}

compiler/rustc_lint/src/drop_forget_useless.rs

@@ -1,12 +1,12 @@
-use rustc_hir::{Arm, Expr, ExprKind, Node, StmtKind};
-use rustc_middle::ty;
+use rustc_hir::{Arm, Expr, ExprKind, Mutability, Node, StmtKind};
+use rustc_middle::ty::{self, Ty};
 use rustc_session::{declare_lint, declare_lint_pass};
 use rustc_span::sym;
 
 use crate::{
     lints::{
-        DropCopyDiag, DropRefDiag, ForgetCopyDiag, ForgetRefDiag, UndroppedManuallyDropsDiag,
-        UndroppedManuallyDropsSuggestion, UseLetUnderscoreIgnoreSuggestion,
+        DropCopyDiag, DropMutRefDiag, DropRefDiag, ForgetCopyDiag, ForgetMutRefDiag, ForgetRefDiag,
+        IgnoreSuggestion, UndroppedManuallyDropsDiag, UndroppedManuallyDropsSuggestion,
     },
     LateContext, LateLintPass, LintContext,
 };
@@ -140,6 +140,17 @@ declare_lint_pass!(DropForgetUseless => [DROPPING_REFERENCES, FORGETTING_REFEREN
 
 impl<'tcx> LateLintPass<'tcx> for DropForgetUseless {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        fn remove_entirely(expr: &Expr<'_>) -> bool {
+            match expr.kind {
+                ExprKind::Cast(inner, ..) => remove_entirely(inner),
+                ExprKind::Type(inner, ..) => remove_entirely(inner),
+                ExprKind::Field(inner, ..) => remove_entirely(inner),
+                ExprKind::Path(..) => true,
+                ExprKind::AddrOf(.., inner) => remove_entirely(inner),
+                _ => false,
+            }
+        }
+
         if let ExprKind::Call(path, [arg]) = expr.kind
             && let ExprKind::Path(ref qpath) = path.kind
             && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
@@ -148,62 +159,93 @@ impl<'tcx> LateLintPass<'tcx> for DropForgetUseless {
             let arg_ty = cx.typeck_results().expr_ty(arg);
             let is_copy = arg_ty.is_copy_modulo_regions(cx.tcx, cx.param_env);
             let drop_is_single_call_in_arm = is_single_call_in_arm(cx, arg, expr);
-            let let_underscore_ignore_sugg = || {
-                if let Some((_, node)) = cx.tcx.hir().parent_iter(expr.hir_id).nth(0)
+            let sugg = || {
+                let start_end = if let Some((_, node)) =
+                    cx.tcx.hir().parent_iter(expr.hir_id).nth(0)
                     && let Node::Stmt(stmt) = node
                     && let StmtKind::Semi(e) = stmt.kind
                     && e.hir_id == expr.hir_id
                 {
-                    UseLetUnderscoreIgnoreSuggestion::Suggestion {
-                        start_span: expr.span.shrink_to_lo().until(arg.span),
-                        end_span: arg.span.shrink_to_hi().until(expr.span.shrink_to_hi()),
-                    }
+                    Some((
+                        expr.span.shrink_to_lo().until(arg.span),
+                        arg.span.shrink_to_hi().until(expr.span.shrink_to_hi()),
+                        stmt.span,
+                    ))
                 } else {
-                    UseLetUnderscoreIgnoreSuggestion::Note
+                    None
+                };
+                fn is_uninteresting(ty: Ty<'_>) -> bool {
+                    match ty.kind() {
+                        ty::Never => true,
+                        ty::Tuple(list) if list.is_empty() => true,
+                        _ => false,
+                    }
+                }
+                match (remove_entirely(arg), is_uninteresting(arg_ty), start_end) {
+                    (true, _, Some((_, _, stmt))) => {
+                        IgnoreSuggestion::RemoveEntirelySuggestion { span: stmt }
+                    }
+                    (true, _, None) => IgnoreSuggestion::RemoveEntirelyNote,
+                    (false, true, Some((start, end, _))) => {
+                        IgnoreSuggestion::RemoveFunctionCallSuggestion {
+                            start_span: start,
+                            end_span: end,
+                        }
+                    }
+                    (false, true, None) => IgnoreSuggestion::RemoveFunctionCallNote,
+                    (false, false, Some((start, end, _))) => {
+                        IgnoreSuggestion::UseLetUnderscoreSuggestion {
+                            start_span: start,
+                            end_span: end,
+                        }
+                    }
+                    (false, false, None) => IgnoreSuggestion::UseLetUnderscoreNote,
                 }
             };
-            match fn_name {
-                sym::mem_drop if arg_ty.is_ref() && !drop_is_single_call_in_arm => {
+            match (fn_name, arg_ty.ref_mutability()) {
+                (sym::mem_drop, Some(Mutability::Not)) if !drop_is_single_call_in_arm => {
                     cx.emit_span_lint(
                         DROPPING_REFERENCES,
                         expr.span,
-                        DropRefDiag { arg_ty, label: arg.span, sugg: let_underscore_ignore_sugg() },
+                        DropRefDiag { arg_ty, label: arg.span, sugg: sugg() },
                     );
                 }
-                sym::mem_forget if arg_ty.is_ref() => {
+                (sym::mem_drop, Some(Mutability::Mut)) if !drop_is_single_call_in_arm => {
+                    cx.emit_span_lint(
+                        DROPPING_REFERENCES,
+                        expr.span,
+                        DropMutRefDiag { arg_ty, label: arg.span, sugg: sugg() },
+                    );
+                }
+                (sym::mem_forget, Some(Mutability::Not)) => {
                     cx.emit_span_lint(
                         FORGETTING_REFERENCES,
                         expr.span,
-                        ForgetRefDiag {
-                            arg_ty,
-                            label: arg.span,
-                            sugg: let_underscore_ignore_sugg(),
-                        },
+                        ForgetRefDiag { arg_ty, label: arg.span, sugg: sugg() },
+                    );
+                }
+                (sym::mem_forget, Some(Mutability::Mut)) => {
+                    cx.emit_span_lint(
+                        FORGETTING_REFERENCES,
+                        expr.span,
+                        ForgetMutRefDiag { arg_ty, label: arg.span, sugg: sugg() },
                     );
                 }
-                sym::mem_drop if is_copy && !drop_is_single_call_in_arm => {
+                (sym::mem_drop, _) if is_copy && !drop_is_single_call_in_arm => {
                     cx.emit_span_lint(
                         DROPPING_COPY_TYPES,
                         expr.span,
-                        DropCopyDiag {
-                            arg_ty,
-                            label: arg.span,
-                            sugg: let_underscore_ignore_sugg(),
-                        },
+                        DropCopyDiag { arg_ty, label: arg.span, sugg: sugg() },
                     );
                 }
-                sym::mem_forget if is_copy => {
+                (sym::mem_forget, _) if is_copy => {
                     cx.emit_span_lint(
                         FORGETTING_COPY_TYPES,
                         expr.span,
-                        ForgetCopyDiag {
-                            arg_ty,
-                            label: arg.span,
-                            sugg: let_underscore_ignore_sugg(),
-                        },
+                        ForgetCopyDiag { arg_ty, label: arg.span, sugg: sugg() },
                     );
                 }
-                sym::mem_drop
+                (sym::mem_drop, _)
                     if let ty::Adt(adt, _) = arg_ty.kind()
                         && adt.is_manually_drop() =>
                 {

compiler/rustc_lint/src/lints.rs

@@ -657,15 +657,39 @@ pub struct ForLoopsOverFalliblesSuggestion<'a> {
 }
 
 #[derive(Subdiagnostic)]
-pub enum UseLetUnderscoreIgnoreSuggestion {
+pub enum IgnoreSuggestion {
+    #[note(lint_remove_entirely_suggestion)]
+    RemoveEntirelyNote,
+    #[multipart_suggestion(
+        lint_remove_entirely_suggestion,
+        style = "verbose",
+        applicability = "maybe-incorrect"
+    )]
+    RemoveEntirelySuggestion {
+        #[suggestion_part(code = "")]
+        span: Span,
+    },
+    #[note(lint_remove_function_call_suggestion)]
+    RemoveFunctionCallNote,
+    #[multipart_suggestion(
+        lint_remove_function_call_suggestion,
+        style = "verbose",
+        applicability = "maybe-incorrect"
+    )]
+    RemoveFunctionCallSuggestion {
+        #[suggestion_part(code = "")]
+        start_span: Span,
+        #[suggestion_part(code = "")]
+        end_span: Span,
+    },
     #[note(lint_use_let_underscore_ignore_suggestion)]
-    Note,
+    UseLetUnderscoreNote,
     #[multipart_suggestion(
         lint_use_let_underscore_ignore_suggestion,
         style = "verbose",
         applicability = "maybe-incorrect"
     )]
-    Suggestion {
+    UseLetUnderscoreSuggestion {
         #[suggestion_part(code = "let _ = ")]
         start_span: Span,
         #[suggestion_part(code = "")]
@@ -681,7 +705,17 @@ pub struct DropRefDiag<'a> {
     #[label]
     pub label: Span,
     #[subdiagnostic]
-    pub sugg: UseLetUnderscoreIgnoreSuggestion,
+    pub sugg: IgnoreSuggestion,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(lint_dropping_mutable_references)]
+pub struct DropMutRefDiag<'a> {
+    pub arg_ty: Ty<'a>,
+    #[label]
+    pub label: Span,
+    #[subdiagnostic]
+    pub sugg: IgnoreSuggestion,
 }
 
 #[derive(LintDiagnostic)]
@@ -691,7 +725,7 @@ pub struct DropCopyDiag<'a> {
     #[label]
     pub label: Span,
     #[subdiagnostic]
-    pub sugg: UseLetUnderscoreIgnoreSuggestion,
+    pub sugg: IgnoreSuggestion,
 }
 
 #[derive(LintDiagnostic)]
@@ -701,7 +735,17 @@ pub struct ForgetRefDiag<'a> {
     #[label]
     pub label: Span,
     #[subdiagnostic]
-    pub sugg: UseLetUnderscoreIgnoreSuggestion,
+    pub sugg: IgnoreSuggestion,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(lint_forgetting_mutable_references)]
+pub struct ForgetMutRefDiag<'a> {
+    pub arg_ty: Ty<'a>,
+    #[label]
+    pub label: Span,
+    #[subdiagnostic]
+    pub sugg: IgnoreSuggestion,
 }
 
 #[derive(LintDiagnostic)]
@@ -711,7 +755,7 @@ pub struct ForgetCopyDiag<'a> {
     #[label]
     pub label: Span,
     #[subdiagnostic]
-    pub sugg: UseLetUnderscoreIgnoreSuggestion,
+    pub sugg: IgnoreSuggestion,
 }
 
 #[derive(LintDiagnostic)]

tests/ui/lint/dropping_copy_types-issue-125189-can-not-fixed.stderr

@@ -6,7 +6,7 @@ LL |         0 => drop(y),
    |                   |
    |                   argument has type `i32`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+   = note: remove the whole expression
 note: the lint level is defined here
   --> $DIR/dropping_copy_types-issue-125189-can-not-fixed.rs:3:9
    |
@@ -21,7 +21,7 @@ LL |         1 => drop(z),
    |                   |
    |                   argument has type `i32`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+   = note: remove the whole expression
 
 error: calls to `std::mem::drop` with a value that implements `Copy` does nothing
   --> $DIR/dropping_copy_types-issue-125189-can-not-fixed.rs:11:14

tests/ui/lint/dropping_copy_types-issue-125189.fixed

@@ -3,8 +3,9 @@
 
 #![deny(dropping_copy_types)]
 
+#[allow(unused_variables)]
 fn main() {
     let y = 1;
     let _ = 3.2; //~ ERROR calls to `std::mem::drop`
-    let _ = y; //~ ERROR calls to `std::mem::drop`
+     //~ ERROR calls to `std::mem::drop`
 }

tests/ui/lint/dropping_copy_types-issue-125189.rs

@@ -3,6 +3,7 @@
 
 #![deny(dropping_copy_types)]
 
+#[allow(unused_variables)]
 fn main() {
     let y = 1;
     drop(3.2); //~ ERROR calls to `std::mem::drop`

tests/ui/lint/dropping_copy_types-issue-125189.stderr

@@ -1,5 +1,5 @@
 error: calls to `std::mem::drop` with a value that implements `Copy` does nothing
-  --> $DIR/dropping_copy_types-issue-125189.rs:8:5
+  --> $DIR/dropping_copy_types-issue-125189.rs:9:5
    |
 LL |     drop(3.2);
    |     ^^^^^---^
@@ -18,17 +18,17 @@ LL +     let _ = 3.2;
    |
 
 error: calls to `std::mem::drop` with a value that implements `Copy` does nothing
-  --> $DIR/dropping_copy_types-issue-125189.rs:9:5
+  --> $DIR/dropping_copy_types-issue-125189.rs:10:5
    |
 LL |     drop(y);
    |     ^^^^^-^
    |          |
    |          argument has type `i32`
    |
-help: use `let _ = ...` to ignore the expression or result
+help: remove the whole expression
    |
 LL -     drop(y);
-LL +     let _ = y;
+LL +
    |
 
 error: aborting due to 2 previous errors

tests/ui/lint/dropping_copy_types.rs

@@ -51,6 +51,8 @@ fn main() {
     drop(a3);
     drop(a4); //~ WARN calls to `std::mem::drop`
     drop(a5);
+
+    drop(drop(String::new())); //~ WARN calls to `std::mem::drop`
 }
 
 #[allow(unused)]