A small diagnostic improvement for dropping_copy_types

fixes #125189

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