Make lint: lint_dropping_references lint_forgetting_copy_types lint_forgetting_references give suggestion if possible.

compiler/rustc_lint/messages.ftl

@@ -232,12 +232,9 @@ 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}`
-    .note = use `let _ = ...` to ignore the expression or result
-    .suggestion = use `let _ = ...` to ignore the expression or result
 
 lint_dropping_references = calls to `std::mem::drop` with a reference instead of an owned value does nothing
     .label = argument has type `{$arg_ty}`
-    .note = use `let _ = ...` to ignore the expression or result
 
 lint_duplicate_macro_attribute =
     duplicated attribute
@@ -272,10 +269,9 @@ 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 `{$arg_ty}`
-    .note = use `let _ = ...` to ignore the expression or result
+
 lint_forgetting_references = calls to `std::mem::forget` with a reference instead of an owned value does nothing
     .label = argument has type `{$arg_ty}`
-    .note = use `let _ = ...` to ignore the expression or result
 
 lint_hidden_glob_reexport = private item shadows public glob re-export
     .note_glob_reexport = the name `{$name}` in the {$namespace} namespace is supposed to be publicly re-exported here
@@ -894,6 +890,8 @@ lint_unused_op = unused {$op} that must be used
 
 lint_unused_result = unused result of type `{$ty}`
 
+lint_use_let_underscore_ignore_suggestion = use `let _ = ...` to ignore the expression or result
+
 lint_variant_size_differences =
     enum variant is more than three times larger ({$largest} bytes) than the next largest
 

compiler/rustc_lint/src/drop_forget_useless.rs

@@ -5,8 +5,8 @@ use rustc_span::sym;
 
 use crate::{
     lints::{
-        DropCopyDiag, DropCopySuggestion, DropRefDiag, ForgetCopyDiag, ForgetRefDiag,
-        UndroppedManuallyDropsDiag, UndroppedManuallyDropsSuggestion,
+        DropCopyDiag, DropRefDiag, ForgetCopyDiag, ForgetRefDiag, UndroppedManuallyDropsDiag,
+        UndroppedManuallyDropsSuggestion, UseLetUnderscoreIgnoreSuggestion,
     },
     LateContext, LateLintPass, LintContext,
 };
@@ -148,46 +148,59 @@ 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 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()),
+                    }
+                } else {
+                    UseLetUnderscoreIgnoreSuggestion::Note
+                }
+            };
             match fn_name {
                 sym::mem_drop if arg_ty.is_ref() && !drop_is_single_call_in_arm => {
                     cx.emit_span_lint(
                         DROPPING_REFERENCES,
                         expr.span,
-                        DropRefDiag { arg_ty, label: arg.span },
+                        DropRefDiag { arg_ty, label: arg.span, sugg: let_underscore_ignore_sugg() },
                     );
                 }
                 sym::mem_forget if arg_ty.is_ref() => {
                     cx.emit_span_lint(
                         FORGETTING_REFERENCES,
                         expr.span,
-                        ForgetRefDiag { arg_ty, label: arg.span },
+                        ForgetRefDiag {
+                            arg_ty,
+                            label: arg.span,
+                            sugg: let_underscore_ignore_sugg(),
+                        },
                     );
                 }
                 sym::mem_drop if is_copy && !drop_is_single_call_in_arm => {
-                    let sugg = 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
-                    {
-                        DropCopySuggestion::Suggestion {
-                            start_span: expr.span.shrink_to_lo().until(arg.span),
-                            end_span: arg.span.shrink_to_hi().until(expr.span.shrink_to_hi()),
-                        }
-                    } else {
-                        DropCopySuggestion::Note
-                    };
-
                     cx.emit_span_lint(
                         DROPPING_COPY_TYPES,
                         expr.span,
-                        DropCopyDiag { arg_ty, label: arg.span, sugg },
+                        DropCopyDiag {
+                            arg_ty,
+                            label: arg.span,
+                            sugg: let_underscore_ignore_sugg(),
+                        },
                     );
                 }
                 sym::mem_forget if is_copy => {
                     cx.emit_span_lint(
                         FORGETTING_COPY_TYPES,
                         expr.span,
-                        ForgetCopyDiag { arg_ty, label: arg.span },
+                        ForgetCopyDiag {
+                            arg_ty,
+                            label: arg.span,
+                            sugg: let_underscore_ignore_sugg(),
+                        },
                     );
                 }
                 sym::mem_drop

compiler/rustc_lint/src/lints.rs

@@ -656,14 +656,32 @@ pub struct ForLoopsOverFalliblesSuggestion<'a> {
     pub end_span: Span,
 }
 
+#[derive(Subdiagnostic)]
+pub enum UseLetUnderscoreIgnoreSuggestion {
+    #[note(lint_use_let_underscore_ignore_suggestion)]
+    Note,
+    #[multipart_suggestion(
+        lint_use_let_underscore_ignore_suggestion,
+        style = "verbose",
+        applicability = "maybe-incorrect"
+    )]
+    Suggestion {
+        #[suggestion_part(code = "let _ = ")]
+        start_span: Span,
+        #[suggestion_part(code = "")]
+        end_span: Span,
+    },
+}
+
 // drop_forget_useless.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_dropping_references)]
-#[note]
 pub struct DropRefDiag<'a> {
     pub arg_ty: Ty<'a>,
     #[label]
     pub label: Span,
+    #[subdiagnostic]
+    pub sugg: UseLetUnderscoreIgnoreSuggestion,
 }
 
 #[derive(LintDiagnostic)]
@@ -673,38 +691,27 @@ pub struct DropCopyDiag<'a> {
     #[label]
     pub label: Span,
     #[subdiagnostic]
-    pub sugg: DropCopySuggestion,
-}
-
-#[derive(Subdiagnostic)]
-pub enum DropCopySuggestion {
-    #[note(lint_note)]
-    Note,
-    #[multipart_suggestion(lint_suggestion, style = "verbose", applicability = "maybe-incorrect")]
-    Suggestion {
-        #[suggestion_part(code = "let _ = ")]
-        start_span: Span,
-        #[suggestion_part(code = "")]
-        end_span: Span,
-    },
+    pub sugg: UseLetUnderscoreIgnoreSuggestion,
 }
 
 #[derive(LintDiagnostic)]
 #[diag(lint_forgetting_references)]
-#[note]
 pub struct ForgetRefDiag<'a> {
     pub arg_ty: Ty<'a>,
     #[label]
     pub label: Span,
+    #[subdiagnostic]
+    pub sugg: UseLetUnderscoreIgnoreSuggestion,
 }
 
 #[derive(LintDiagnostic)]
 #[diag(lint_forgetting_copy_types)]
-#[note]
 pub struct ForgetCopyDiag<'a> {
     pub arg_ty: Ty<'a>,
     #[label]
     pub label: Span,
+    #[subdiagnostic]
+    pub sugg: UseLetUnderscoreIgnoreSuggestion,
 }
 
 #[derive(LintDiagnostic)]

tests/ui/lint/dropping_copy_types.stderr

@@ -39,8 +39,12 @@ LL |     drop(s3);
    |          |
    |          argument has type `&SomeStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
    = note: `#[warn(dropping_references)]` on by default
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL -     drop(s3);
+LL +     let _ = s3;
+   |
 
 warning: calls to `std::mem::drop` with a value that implements `Copy` does nothing
   --> $DIR/dropping_copy_types.rs:37:5
@@ -64,7 +68,11 @@ LL |     drop(s5);
    |          |
    |          argument has type `&SomeStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL -     drop(s5);
+LL +     let _ = s5;
+   |
 
 warning: calls to `std::mem::drop` with a reference instead of an owned value does nothing
   --> $DIR/dropping_copy_types.rs:50:5
@@ -74,7 +82,11 @@ LL |     drop(a2);
    |          |
    |          argument has type `&AnotherStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL -     drop(a2);
+LL +     let _ = a2;
+   |
 
 warning: calls to `std::mem::drop` with a reference instead of an owned value does nothing
   --> $DIR/dropping_copy_types.rs:52:5
@@ -84,7 +96,11 @@ LL |     drop(a4);
    |          |
    |          argument has type `&AnotherStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL -     drop(a4);
+LL +     let _ = a4;
+   |
 
 warning: calls to `std::mem::drop` with a value that implements `Copy` does nothing
   --> $DIR/dropping_copy_types.rs:71:13

tests/ui/lint/dropping_references-can-fixed.fixed

@@ -0,0 +1,31 @@
+//@ check-fail
+//@ run-rustfix
+
+#![deny(dropping_references)]
+
+struct SomeStruct;
+
+fn main() {
+    let _ = &SomeStruct; //~ ERROR calls to `std::mem::drop`
+
+    let mut owned1 = SomeStruct;
+    let _ = &owned1; //~ ERROR calls to `std::mem::drop`
+    let _ = &&owned1; //~ ERROR calls to `std::mem::drop`
+    let _ = &mut owned1; //~ ERROR calls to `std::mem::drop`
+    drop(owned1);
+
+    let reference1 = &SomeStruct;
+    let _ = reference1; //~ ERROR calls to `std::mem::drop`
+
+    let reference2 = &mut SomeStruct;
+    let _ = reference2; //~ ERROR calls to `std::mem::drop`
+
+    let ref reference3 = SomeStruct;
+    let _ = reference3; //~ ERROR calls to `std::mem::drop`
+}
+
+#[allow(dead_code)]
+fn test_generic_fn_drop<T>(val: T) {
+    let _ = &val; //~ ERROR calls to `std::mem::drop`
+    drop(val);
+}

tests/ui/lint/dropping_references-can-fixed.rs

@@ -0,0 +1,31 @@
+//@ check-fail
+//@ run-rustfix
+
+#![deny(dropping_references)]
+
+struct SomeStruct;
+
+fn main() {
+    drop(&SomeStruct); //~ ERROR calls to `std::mem::drop`
+
+    let mut owned1 = SomeStruct;
+    drop(&owned1); //~ ERROR calls to `std::mem::drop`
+    drop(&&owned1); //~ ERROR calls to `std::mem::drop`
+    drop(&mut owned1); //~ ERROR calls to `std::mem::drop`
+    drop(owned1);
+
+    let reference1 = &SomeStruct;
+    drop(reference1); //~ ERROR calls to `std::mem::drop`
+
+    let reference2 = &mut SomeStruct;
+    drop(reference2); //~ ERROR calls to `std::mem::drop`
+
+    let ref reference3 = SomeStruct;
+    drop(reference3); //~ ERROR calls to `std::mem::drop`
+}
+
+#[allow(dead_code)]
+fn test_generic_fn_drop<T>(val: T) {
+    drop(&val); //~ ERROR calls to `std::mem::drop`
+    drop(val);
+}