Let lint_forgetting_references give the suggestion if possible

Author: surechen

compiler/rustc_lint/messages.ftl

@@ -237,8 +237,6 @@ lint_dropping_copy_types = calls to `std::mem::drop` with a value that implement
 
 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
-    .suggestion = use `let _ = ...` to ignore the expression or result
 
 lint_duplicate_macro_attribute =
     duplicated attribute
@@ -273,12 +271,9 @@ 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
-    .suggestion = 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
@@ -897,6 +892,8 @@ 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 +5,9 @@ use rustc_span::sym;
 
 use crate::{
     lints::{
-        DropCopyDiag, DropRefDiag, ForgetCopyDiag, ForgetRefDiag, IgnoreDropSuggestion,
+        DropCopyDiag, DropCopySuggestion, DropRefDiag, ForgetCopyDiag, ForgetRefDiag,
         UndroppedManuallyDropsDiag, UndroppedManuallyDropsSuggestion,
+        UseLetUnderscoreIgnoreSuggestion,
     },
     LateContext, LateLintPass, LintContext,
 };
@@ -148,31 +149,37 @@ 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 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
-            {
-                IgnoreDropSuggestion::Suggestion {
-                    start_span: expr.span.shrink_to_lo().until(arg.span),
-                    end_span: arg.span.shrink_to_hi().until(expr.span.shrink_to_hi()),
+            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
                 }
-            } else {
-                IgnoreDropSuggestion::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, sugg },
+                        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 => {
@@ -199,7 +206,11 @@ impl<'tcx> LateLintPass<'tcx> for DropForgetUseless {
                     cx.emit_span_lint(
                         FORGETTING_COPY_TYPES,
                         expr.span,
-                        ForgetCopyDiag { arg_ty, label: arg.span, sugg },
+                        ForgetCopyDiag {
+                            arg_ty,
+                            label: arg.span,
+                            sugg: let_underscore_ignore_sugg(),
+                        },
                     );
                 }
                 sym::mem_drop

compiler/rustc_lint/src/lints.rs

@@ -657,10 +657,14 @@ pub struct ForLoopsOverFalliblesSuggestion<'a> {
 }
 
 #[derive(Subdiagnostic)]
-pub enum IgnoreDropSuggestion {
-    #[note(lint_note)]
+pub enum UseLetUnderscoreIgnoreSuggestion {
+    #[note(lint_use_let_underscore_ignore_suggestion)]
     Note,
-    #[multipart_suggestion(lint_suggestion, style = "verbose", applicability = "maybe-incorrect")]
+    #[multipart_suggestion(
+        lint_use_let_underscore_ignore_suggestion,
+        style = "verbose",
+        applicability = "maybe-incorrect"
+    )]
     Suggestion {
         #[suggestion_part(code = "let _ = ")]
         start_span: Span,
@@ -677,7 +681,7 @@ pub struct DropRefDiag<'a> {
     #[label]
     pub label: Span,
     #[subdiagnostic]
-    pub sugg: IgnoreDropSuggestion,
+    pub sugg: UseLetUnderscoreIgnoreSuggestion,
 }
 
 #[derive(LintDiagnostic)]
@@ -705,11 +709,12 @@ pub enum DropCopySuggestion {
 
 #[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)]
@@ -719,7 +724,7 @@ pub struct ForgetCopyDiag<'a> {
     #[label]
     pub label: Span,
     #[subdiagnostic]
-    pub sugg: IgnoreDropSuggestion,
+    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/forgetting_copy_types.stderr

@@ -39,8 +39,12 @@ LL |     forget(s3);
    |            |
    |            argument has type `&SomeStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
    = note: `#[warn(forgetting_references)]` on by default
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL -     forget(s3);
+LL +     let _ = s3;
+   |
 
 warning: calls to `std::mem::forget` with a value that implements `Copy` does nothing
   --> $DIR/forgetting_copy_types.rs:37:5
@@ -64,7 +68,11 @@ LL |     forget(s5);
    |            |
    |            argument has type `&SomeStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL -     forget(s5);
+LL +     let _ = s5;
+   |
 
 warning: calls to `std::mem::forget` with a reference instead of an owned value does nothing
   --> $DIR/forgetting_copy_types.rs:50:5
@@ -74,7 +82,11 @@ LL |     forget(a2);
    |            |
    |            argument has type `&AnotherStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL -     forget(a2);
+LL +     let _ = a2;
+   |
 
 warning: calls to `std::mem::forget` with a reference instead of an owned value does nothing
   --> $DIR/forgetting_copy_types.rs:52:5
@@ -84,7 +96,11 @@ LL |     forget(a3);
    |            |
    |            argument has type `&AnotherStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL -     forget(a3);
+LL +     let _ = a3;
+   |
 
 warning: calls to `std::mem::forget` with a reference instead of an owned value does nothing
   --> $DIR/forgetting_copy_types.rs:53:5
@@ -94,7 +110,11 @@ LL |     forget(a4);
    |            |
    |            argument has type `&AnotherStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL -     forget(a4);
+LL +     let _ = a4;
+   |
 
 warning: 8 warnings emitted
 

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

@@ -0,0 +1,40 @@
+//@ check-fail
+//@ run-rustfix
+
+#![deny(forgetting_references)]
+
+use std::mem::forget;
+
+struct SomeStruct;
+
+fn main() {
+    let _ = &SomeStruct; //~ ERROR calls to `std::mem::forget`
+
+    let mut owned = SomeStruct;
+    let _ = &owned; //~ ERROR calls to `std::mem::forget`
+    let _ = &&owned; //~ ERROR calls to `std::mem::forget`
+    let _ = &mut owned; //~ ERROR calls to `std::mem::forget`
+    forget(owned);
+
+    let reference1 = &SomeStruct;
+    let _ = &*reference1; //~ ERROR calls to `std::mem::forget`
+
+    let reference2 = &mut SomeStruct;
+    let _ = reference2; //~ ERROR calls to `std::mem::forget`
+
+    let ref reference3 = SomeStruct;
+    let _ = reference3; //~ ERROR calls to `std::mem::forget`
+}
+
+#[allow(dead_code)]
+fn test_generic_fn_forget<T>(val: T) {
+    let _ = &val; //~ ERROR calls to `std::mem::forget`
+    forget(val);
+}
+
+#[allow(dead_code)]
+fn test_similarly_named_function() {
+    fn forget<T>(_val: T) {}
+    forget(&SomeStruct); //OK; call to unrelated function which happens to have the same name
+    let _ = &SomeStruct; //~ ERROR calls to `std::mem::forget`
+}

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

@@ -0,0 +1,40 @@
+//@ check-fail
+//@ run-rustfix
+
+#![deny(forgetting_references)]
+
+use std::mem::forget;
+
+struct SomeStruct;
+
+fn main() {
+    forget(&SomeStruct); //~ ERROR calls to `std::mem::forget`
+
+    let mut owned = SomeStruct;
+    forget(&owned); //~ ERROR calls to `std::mem::forget`
+    forget(&&owned); //~ ERROR calls to `std::mem::forget`
+    forget(&mut owned); //~ ERROR calls to `std::mem::forget`
+    forget(owned);
+
+    let reference1 = &SomeStruct;
+    forget(&*reference1); //~ ERROR calls to `std::mem::forget`
+
+    let reference2 = &mut SomeStruct;
+    forget(reference2); //~ ERROR calls to `std::mem::forget`
+
+    let ref reference3 = SomeStruct;
+    forget(reference3); //~ ERROR calls to `std::mem::forget`
+}
+
+#[allow(dead_code)]
+fn test_generic_fn_forget<T>(val: T) {
+    forget(&val); //~ ERROR calls to `std::mem::forget`
+    forget(val);
+}
+
+#[allow(dead_code)]
+fn test_similarly_named_function() {
+    fn forget<T>(_val: T) {}
+    forget(&SomeStruct); //OK; call to unrelated function which happens to have the same name
+    std::mem::forget(&SomeStruct); //~ ERROR calls to `std::mem::forget`
+}