A small diagnostic improvement for dropping_copy_types

fixes #125189

Author: surechen

compiler/rustc_lint/messages.ftl

@@ -196,7 +196,7 @@ 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 `{$arg_ty}`
-    .note = use `let _ = ...` to ignore the expression or result
+    .note = use `{$replace}` to ignore the value instead of dropping directly
 
 lint_dropping_references = calls to `std::mem::drop` with a reference instead of an owned value does nothing
     .label = argument has type `{$arg_ty}`

compiler/rustc_lint/src/drop_forget_useless.rs

@@ -1,4 +1,4 @@
-use rustc_hir::{Arm, Expr, ExprKind, Node};
+use rustc_hir::{Arm, Expr, ExprKind, Node, StmtKind};
 use rustc_middle::ty;
 use rustc_span::sym;
 
@@ -163,10 +163,25 @@ impl<'tcx> LateLintPass<'tcx> for DropForgetUseless {
                     );
                 }
                 sym::mem_drop if is_copy && !drop_is_single_call_in_arm => {
+                    let replace = format!(
+                        "let _ = {}",
+                        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
+                        {
+                            cx.sess()
+                                .source_map()
+                                .span_to_snippet(arg.span)
+                                .unwrap_or("...".to_string())
+                        } else {
+                            "...".to_string()
+                        }
+                    );
                     cx.emit_span_lint(
                         DROPPING_COPY_TYPES,
                         expr.span,
-                        DropCopyDiag { arg_ty, label: arg.span },
+                        DropCopyDiag { arg_ty, label: arg.span, replace },
                     );
                 }
                 sym::mem_forget if is_copy => {

compiler/rustc_lint/src/lints.rs

@@ -674,6 +674,7 @@ pub struct DropCopyDiag<'a> {
     pub arg_ty: Ty<'a>,
     #[label]
     pub label: Span,
+    pub replace: String,
 }
 
 #[derive(LintDiagnostic)]

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

@@ -6,7 +6,7 @@ LL |     drop(origin);
    |          |
    |          argument has type `<T as UncheckedCopy>::Output`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+   = note: use `let _ = origin` to ignore the value instead of dropping directly
    = note: `#[warn(dropping_copy_types)]` on by default
 
 warning: 1 warning emitted

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

@@ -6,7 +6,7 @@ LL |     drop(origin);
    |          |
    |          argument has type `<T as UncheckedCopy>::Output`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+   = note: use `let _ = origin` to ignore the value instead of dropping directly
    = note: `#[warn(dropping_copy_types)]` on by default
 
 warning: 1 warning emitted

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

@@ -0,0 +1,17 @@
+//@ check-fail
+
+#![deny(dropping_copy_types)]
+
+fn main() {
+   let y = 1;
+   let z = 2;
+    match y {
+        1 => drop(3), //~ ERROR calls to `std::mem::drop`
+        2 => drop(y), //~ ERROR calls to `std::mem::drop`
+        3 => if drop(z) == () {}, //~ ERROR calls to `std::mem::drop`
+        _ => (),
+    }
+
+    drop(3.2); //~ ERROR calls to `std::mem::drop`
+    drop(y); //~ ERROR calls to `std::mem::drop`
+}

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

@@ -0,0 +1,57 @@
+error: calls to `std::mem::drop` with a value that implements `Copy` does nothing
+  --> $DIR/dropping_copy_types-issue-125189.rs:9:14
+   |
+LL |         1 => drop(3),
+   |              ^^^^^-^
+   |                   |
+   |                   argument has type `i32`
+   |
+   = note: use `let _ = ...` to ignore the value instead of dropping directly
+note: the lint level is defined here
+  --> $DIR/dropping_copy_types-issue-125189.rs:3:9
+   |
+LL | #![deny(dropping_copy_types)]
+   |         ^^^^^^^^^^^^^^^^^^^
+
+error: calls to `std::mem::drop` with a value that implements `Copy` does nothing
+  --> $DIR/dropping_copy_types-issue-125189.rs:10:14
+   |
+LL |         2 => drop(y),
+   |              ^^^^^-^
+   |                   |
+   |                   argument has type `i32`
+   |
+   = note: use `let _ = ...` to ignore the value instead of dropping directly
+
+error: calls to `std::mem::drop` with a value that implements `Copy` does nothing
+  --> $DIR/dropping_copy_types-issue-125189.rs:11:17
+   |
+LL |         3 => if drop(z) == () {},
+   |                 ^^^^^-^
+   |                      |
+   |                      argument has type `i32`
+   |
+   = note: use `let _ = ...` to ignore the value instead of dropping directly
+
+error: calls to `std::mem::drop` with a value that implements `Copy` does nothing
+  --> $DIR/dropping_copy_types-issue-125189.rs:15:5
+   |
+LL |     drop(3.2);
+   |     ^^^^^---^
+   |          |
+   |          argument has type `f64`
+   |
+   = note: use `let _ = 3.2` to ignore the value instead of dropping directly
+
+error: calls to `std::mem::drop` with a value that implements `Copy` does nothing
+  --> $DIR/dropping_copy_types-issue-125189.rs:16:5
+   |
+LL |     drop(y);
+   |     ^^^^^-^
+   |          |
+   |          argument has type `i32`
+   |
+   = note: use `let _ = y` to ignore the value instead of dropping directly
+
+error: aborting due to 5 previous errors
+

tests/ui/lint/dropping_copy_types.stderr

@@ -6,7 +6,7 @@ LL |     drop(s1);
    |          |
    |          argument has type `SomeStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+   = note: use `let _ = s1` to ignore the value instead of dropping directly
 note: the lint level is defined here
   --> $DIR/dropping_copy_types.rs:3:9
    |
@@ -21,7 +21,7 @@ LL |     drop(s2);
    |          |
    |          argument has type `SomeStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+   = note: use `let _ = s2` to ignore the value instead of dropping directly
 
 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 +42,7 @@ LL |     drop(s4);
    |          |
    |          argument has type `SomeStruct`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+   = note: use `let _ = s4` to ignore the value instead of dropping directly
 
 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 +82,7 @@ LL |             drop(println_and(13));
    |                  |
    |                  argument has type `i32`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+   = note: use `let _ = println_and(13)` to ignore the value instead of dropping directly
 
 warning: calls to `std::mem::drop` with a value that implements `Copy` does nothing
   --> $DIR/dropping_copy_types.rs:74:14
@@ -92,7 +92,7 @@ LL |         3 if drop(println_and(14)) == () => (),
    |                   |
    |                   argument has type `i32`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+   = note: use `let _ = ...` to ignore the value instead of dropping directly
 
 warning: calls to `std::mem::drop` with a value that implements `Copy` does nothing
   --> $DIR/dropping_copy_types.rs:76:14
@@ -102,7 +102,7 @@ LL |         4 => drop(2),
    |                   |
    |                   argument has type `i32`
    |
-   = note: use `let _ = ...` to ignore the expression or result
+   = note: use `let _ = ...` to ignore the value instead of dropping directly
 
 warning: 10 warnings emitted