A small diagnostic improvement for dropping_copy_types

fixes rust-lang#125189

Author: surechen

Diff for: compiler/rustc_lint/messages.ftl

@@ -197,6 +197,7 @@ 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}`

Diff for: compiler/rustc_lint/src/drop_forget_useless.rs

@@ -1,11 +1,11 @@
-use rustc_hir::{Arm, Expr, ExprKind, Node};
+use rustc_hir::{Arm, Expr, ExprKind, Node, StmtKind};
 use rustc_middle::ty;
 use rustc_span::sym;
 
 use crate::{
     lints::{
-        DropCopyDiag, DropRefDiag, ForgetCopyDiag, ForgetRefDiag, UndroppedManuallyDropsDiag,
-        UndroppedManuallyDropsSuggestion,
+        DropCopyDiag, DropCopySuggestion, DropRefDiag, ForgetCopyDiag, ForgetRefDiag,
+        UndroppedManuallyDropsDiag, UndroppedManuallyDropsSuggestion,
     },
     LateContext, LateLintPass, LintContext,
 };
@@ -163,10 +163,24 @@ impl<'tcx> LateLintPass<'tcx> for DropForgetUseless {
                     );
                 }
                 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
+                        && let Ok(value) = cx.sess().source_map().span_to_snippet(arg.span)
+                    {
+                        DropCopySuggestion::Suggestion {
+                            sugg: expr.span,
+                            replace: format!("let _ = {}", value),
+                        }
+                    } else {
+                        DropCopySuggestion::Note
+                    };
+
                     cx.emit_span_lint(
                         DROPPING_COPY_TYPES,
                         expr.span,
-                        DropCopyDiag { arg_ty, label: arg.span },
+                        DropCopyDiag { arg_ty, label: arg.span, sugg },
                     );
                 }
                 sym::mem_forget if is_copy => {

Diff for: compiler/rustc_lint/src/lints.rs

@@ -669,11 +669,29 @@ pub struct DropRefDiag<'a> {
 
 #[derive(LintDiagnostic)]
 #[diag(lint_dropping_copy_types)]
-#[note]
 pub struct DropCopyDiag<'a> {
     pub arg_ty: Ty<'a>,
     #[label]
     pub label: Span,
+    #[subdiagnostic]
+    pub sugg: DropCopySuggestion,
+}
+
+#[derive(Subdiagnostic)]
+pub enum DropCopySuggestion {
+    #[note(lint_note)]
+    Note,
+    #[suggestion(
+        lint_suggestion,
+        style = "verbose",
+        code = "{replace}",
+        applicability = "maybe-incorrect"
+    )]
+    Suggestion {
+        #[primary_span]
+        sugg: Span,
+        replace: String,
+    },
 }
 
 #[derive(LintDiagnostic)]

Diff for: tests/ui/associated-types/defaults-unsound-62211-1.next.stderr

@@ -6,8 +6,11 @@ LL |     drop(origin);
    |          |
    |          argument has type `<T as UncheckedCopy>::Output`
    |
-   = note: use `let _ = ...` to ignore the expression or result
    = note: `#[warn(dropping_copy_types)]` on by default
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL |     let _ = origin;
+   |     ~~~~~~~~~~~~~~
 
 warning: 1 warning emitted
 

Diff for: tests/ui/associated-types/defaults-unsound-62211-2.next.stderr

@@ -6,8 +6,11 @@ LL |     drop(origin);
    |          |
    |          argument has type `<T as UncheckedCopy>::Output`
    |
-   = note: use `let _ = ...` to ignore the expression or result
    = note: `#[warn(dropping_copy_types)]` on by default
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL |     let _ = origin;
+   |     ~~~~~~~~~~~~~~
 
 warning: 1 warning emitted
 

Diff for: tests/ui/lint/dropping_copy_types-issue-125189.fixed

@@ -0,0 +1,9 @@
+//@ run-rustfix
+
+#![deny(dropping_copy_types)]
+
+fn main() {
+   let y = 1;
+    let _ = 3.2; //~ ERROR calls to `std::mem::drop`
+    let _ = y; //~ ERROR calls to `std::mem::drop`
+}

Diff for: tests/ui/lint/dropping_copy_types-issue-125189.rs

@@ -0,0 +1,9 @@
+//@ run-rustfix
+
+#![deny(dropping_copy_types)]
+
+fn main() {
+   let y = 1;
+    drop(3.2); //~ ERROR calls to `std::mem::drop`
+    drop(y); //~ ERROR calls to `std::mem::drop`
+}

Diff for: tests/ui/lint/dropping_copy_types-issue-125189.stderr

@@ -0,0 +1,33 @@
+error: calls to `std::mem::drop` with a value that implements `Copy` does nothing
+  --> $DIR/dropping_copy_types-issue-125189.rs:7:5
+   |
+LL |     drop(3.2);
+   |     ^^^^^---^
+   |          |
+   |          argument has type `f64`
+   |
+note: the lint level is defined here
+  --> $DIR/dropping_copy_types-issue-125189.rs:3:9
+   |
+LL | #![deny(dropping_copy_types)]
+   |         ^^^^^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the expression or result
+   |
+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:8:5
+   |
+LL |     drop(y);
+   |     ^^^^^-^
+   |          |
+   |          argument has type `i32`
+   |
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL |     let _ = y;
+   |     ~~~~~~~~~
+
+error: aborting due to 2 previous errors
+

Diff for: tests/ui/lint/dropping_copy_types.stderr

@@ -6,12 +6,15 @@ LL |     drop(s1);
    |          |
    |          argument has type `SomeStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
 note: the lint level is defined here
   --> $DIR/dropping_copy_types.rs:3:9
    |
 LL | #![warn(dropping_copy_types)]
    |         ^^^^^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL |     let _ = s1;
+   |     ~~~~~~~~~~
 
 warning: calls to `std::mem::drop` with a value that implements `Copy` does nothing
   --> $DIR/dropping_copy_types.rs:35:5
@@ -21,7 +24,10 @@ LL |     drop(s2);
    |          |
    |          argument has type `SomeStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL |     let _ = s2;
+   |     ~~~~~~~~~~
 
 warning: calls to `std::mem::drop` with a reference instead of an owned value does nothing
   --> $DIR/dropping_copy_types.rs:36:5
@@ -42,7 +48,10 @@ LL |     drop(s4);
    |          |
    |          argument has type `SomeStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL |     let _ = s4;
+   |     ~~~~~~~~~~
 
 warning: calls to `std::mem::drop` with a reference instead of an owned value does nothing
   --> $DIR/dropping_copy_types.rs:38:5
@@ -82,7 +91,10 @@ LL |             drop(println_and(13));
    |                  |
    |                  argument has type `i32`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+help: use `let _ = ...` to ignore the expression or result
+   |
+LL |             let _ = println_and(13);
+   |             ~~~~~~~~~~~~~~~~~~~~~~~
 
 warning: calls to `std::mem::drop` with a value that implements `Copy` does nothing
   --> $DIR/dropping_copy_types.rs:74:14