Auto merge of rust-lang#12278 - GabrielBFern:master, r=Jarcho

[`mem_replace_with_default`] No longer triggers on unused expression

changelog:[`mem_replace_with_default`]: No longer triggers on unused expression

Change [`mem_replace_with_default`] to not trigger on unused expression because the lint from `#[must_use]` handle this case better.

fixes: rust-lang#5586

Author: bors

clippy_lints/src/mem_replace.rs

@@ -3,7 +3,9 @@ use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg, span_lin
 use clippy_utils::source::{snippet, snippet_with_applicability};
 use clippy_utils::sugg::Sugg;
 use clippy_utils::ty::is_non_aggregate_primitive_type;
-use clippy_utils::{is_default_equivalent, is_res_lang_ctor, path_res, peel_ref_operators, std_or_core};
+use clippy_utils::{
+    is_default_equivalent, is_expr_used_or_unified, is_res_lang_ctor, path_res, peel_ref_operators, std_or_core,
+};
 use rustc_errors::Applicability;
 use rustc_hir::LangItem::OptionNone;
 use rustc_hir::{Expr, ExprKind};
@@ -232,7 +234,7 @@ impl<'tcx> LateLintPass<'tcx> for MemReplace {
             // Check that second argument is `Option::None`
             if is_res_lang_ctor(cx, path_res(cx, src), OptionNone) {
                 check_replace_option_with_none(cx, dest, expr.span);
-            } else if self.msrv.meets(msrvs::MEM_TAKE) {
+            } else if self.msrv.meets(msrvs::MEM_TAKE) && is_expr_used_or_unified(cx.tcx, expr) {
                 check_replace_with_default(cx, src, dest, expr.span);
             }
             check_replace_with_uninit(cx, src, dest, expr.span);

tests/ui/mem_replace.fixed

@@ -71,6 +71,14 @@ fn dont_lint_primitive() {
     let _ = std::mem::replace(&mut pint, 0);
 }
 
+// lint is disabled for expressions that are not used because changing to `take` is not the
+// recommended fix. Additionally, the `replace` is #[must_use], so that lint will provide
+// the correct suggestion
+fn dont_lint_not_used() {
+    let mut s = String::from("foo");
+    std::mem::replace(&mut s, String::default());
+}
+
 fn main() {
     replace_option_with_none();
     replace_with_default();

tests/ui/mem_replace.rs

@@ -71,6 +71,14 @@ fn dont_lint_primitive() {
     let _ = std::mem::replace(&mut pint, 0);
 }
 
+// lint is disabled for expressions that are not used because changing to `take` is not the
+// recommended fix. Additionally, the `replace` is #[must_use], so that lint will provide
+// the correct suggestion
+fn dont_lint_not_used() {
+    let mut s = String::from("foo");
+    std::mem::replace(&mut s, String::default());
+}
+
 fn main() {
     replace_option_with_none();
     replace_with_default();

tests/ui/mem_replace.stderr

@@ -119,31 +119,31 @@ LL |     let _ = std::mem::replace(&mut slice, &[]);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut slice)`
 
 error: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
-  --> $DIR/mem_replace.rs:89:13
+  --> $DIR/mem_replace.rs:97:13
    |
 LL |     let _ = std::mem::replace(&mut s, String::default());
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut s)`
 
 error: replacing an `Option` with `None`
-  --> $DIR/mem_replace.rs:119:13
+  --> $DIR/mem_replace.rs:127:13
    |
 LL |     let _ = std::mem::replace(&mut f.0, None);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `f.0.take()`
 
 error: replacing an `Option` with `None`
-  --> $DIR/mem_replace.rs:120:13
+  --> $DIR/mem_replace.rs:128:13
    |
 LL |     let _ = std::mem::replace(&mut *f, None);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `(*f).take()`
 
 error: replacing an `Option` with `None`
-  --> $DIR/mem_replace.rs:121:13
+  --> $DIR/mem_replace.rs:129:13
    |
 LL |     let _ = std::mem::replace(&mut b.opt, None);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider `Option::take()` instead: `b.opt.take()`
 
 error: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
-  --> $DIR/mem_replace.rs:123:13
+  --> $DIR/mem_replace.rs:131:13
    |
 LL |     let _ = std::mem::replace(&mut b.val, String::default());
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using: `std::mem::take(&mut b.val)`

tests/ui/mem_replace_macro.rs

@@ -7,6 +7,6 @@ use proc_macros::{external, inline_macros};
 #[inline_macros]
 fn main() {
     let s = &mut String::from("foo");
-    inline!(std::mem::replace($s, Default::default()));
-    external!(std::mem::replace($s, Default::default()));
+    let _ = inline!(std::mem::replace($s, Default::default()));
+    let _ = external!(std::mem::replace($s, Default::default()));
 }

tests/ui/mem_replace_macro.stderr

@@ -1,8 +1,8 @@
 error: replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`
-  --> $DIR/mem_replace_macro.rs:10:13
+  --> $DIR/mem_replace_macro.rs:10:21
    |
-LL |     inline!(std::mem::replace($s, Default::default()));
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     let _ = inline!(std::mem::replace($s, Default::default()));
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `-D clippy::mem-replace-with-default` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::mem_replace_with_default)]`