Rollup merge of #109158 - Ezrashaw:expand-sugg-for-unused-lint, r=Nilstrieb

error-msg: expand suggestion for `unused_def` lint

Fixes #108885

Expands `let _ = ..` suggestion into more positions.

9 files changed, 138 insertions, 31 deletions

diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index 20ab0af5856..308c02929ca 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/compiler/rustc_lint/src/lints.rs
@@ -1390,7 +1390,7 @@ pub struct UnusedOp<'a> {
     pub op: &'a str,
     #[label]
     pub label: Span,
-    #[suggestion(style = "verbose", code = "let _ = ", applicability = "machine-applicable")]
+    #[suggestion(style = "verbose", code = "let _ = ", applicability = "maybe-incorrect")]
     pub suggestion: Span,
 }
 
@@ -1434,17 +1434,15 @@ pub struct UnusedDef<'a, 'b> {
 }
 
 #[derive(Subdiagnostic)]
-pub enum UnusedDefSuggestion {
-    #[suggestion(
-        lint_suggestion,
-        style = "verbose",
-        code = "let _ = ",
-        applicability = "machine-applicable"
-    )]
-    Default {
-        #[primary_span]
-        span: Span,
-    },
+#[suggestion(
+    lint_suggestion,
+    style = "verbose",
+    code = "let _ = ",
+    applicability = "maybe-incorrect"
+)]
+pub struct UnusedDefSuggestion {
+    #[primary_span]
+    pub span: Span,
 }
 
 // Needed because of def_path_str
diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
index 2fab82d55cd..faca61fc29b 100644
--- a/compiler/rustc_lint/src/unused.rs
+++ b/compiler/rustc_lint/src/unused.rs
@@ -123,7 +123,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
         let must_use_result = is_ty_must_use(cx, ty, &expr, expr.span);
         let type_lint_emitted_or_suppressed = match must_use_result {
             Some(path) => {
-                emit_must_use_untranslated(cx, &path, "", "", 1);
+                emit_must_use_untranslated(cx, &path, "", "", 1, false);
                 true
             }
             None => false,
@@ -358,6 +358,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                         descr_pre_path,
                         descr_post_path,
                         1,
+                        false,
                     )
                 })
                 .is_some()
@@ -370,6 +371,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
             descr_pre: &str,
             descr_post: &str,
             plural_len: usize,
+            is_inner: bool,
         ) {
             let plural_suffix = pluralize!(plural_len);
 
@@ -377,20 +379,22 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                 MustUsePath::Suppressed => {}
                 MustUsePath::Boxed(path) => {
                     let descr_pre = &format!("{}boxed ", descr_pre);
-                    emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
+                    emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len, true);
                 }
                 MustUsePath::Opaque(path) => {
                     let descr_pre = &format!("{}implementer{} of ", descr_pre, plural_suffix);
-                    emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
+                    emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len, true);
                 }
                 MustUsePath::TraitObject(path) => {
                     let descr_post = &format!(" trait object{}{}", plural_suffix, descr_post);
-                    emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
+                    emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len, true);
                 }
                 MustUsePath::TupleElement(elems) => {
                     for (index, path) in elems {
                         let descr_post = &format!(" in tuple element {}", index);
-                        emit_must_use_untranslated(cx, path, descr_pre, descr_post, plural_len);
+                        emit_must_use_untranslated(
+                            cx, path, descr_pre, descr_post, plural_len, true,
+                        );
                     }
                 }
                 MustUsePath::Array(path, len) => {
@@ -401,6 +405,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                         descr_pre,
                         descr_post,
                         plural_len.saturating_add(usize::try_from(*len).unwrap_or(usize::MAX)),
+                        true,
                     );
                 }
                 MustUsePath::Closure(span) => {
@@ -418,19 +423,6 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                     );
                 }
                 MustUsePath::Def(span, def_id, reason) => {
-                    let suggestion = if matches!(
-                        cx.tcx.get_diagnostic_name(*def_id),
-                        Some(sym::add)
-                            | Some(sym::sub)
-                            | Some(sym::mul)
-                            | Some(sym::div)
-                            | Some(sym::rem)
-                            | Some(sym::neg),
-                    ) {
-                        Some(UnusedDefSuggestion::Default { span: span.shrink_to_lo() })
-                    } else {
-                        None
-                    };
                     cx.emit_spanned_lint(
                         UNUSED_MUST_USE,
                         *span,
@@ -440,7 +432,8 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                             cx,
                             def_id: *def_id,
                             note: *reason,
-                            suggestion,
+                            suggestion: (!is_inner)
+                                .then_some(UnusedDefSuggestion { span: span.shrink_to_lo() }),
                         },
                     );
                 }
diff --git a/tests/ui/conditional-compilation/cfg-attr-multi-true.stderr b/tests/ui/conditional-compilation/cfg-attr-multi-true.stderr
index fbfcd45652f..123ce71727f 100644
--- a/tests/ui/conditional-compilation/cfg-attr-multi-true.stderr
+++ b/tests/ui/conditional-compilation/cfg-attr-multi-true.stderr
@@ -35,6 +35,10 @@ note: the lint level is defined here
    |
 LL | #![warn(unused_must_use)]
    |         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = MustUseDeprecated::new();
+   |     +++++++
 
 warning: 5 warnings emitted
 
diff --git a/tests/ui/lint/fn_must_use.stderr b/tests/ui/lint/fn_must_use.stderr
index 657f23c6085..e88c1a9b8a9 100644
--- a/tests/ui/lint/fn_must_use.stderr
+++ b/tests/ui/lint/fn_must_use.stderr
@@ -10,12 +10,21 @@ note: the lint level is defined here
    |
 LL | #![warn(unused_must_use)]
    |         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = need_to_use_this_value();
+   |     +++++++
 
 warning: unused return value of `MyStruct::need_to_use_this_method_value` that must be used
   --> $DIR/fn_must_use.rs:60:5
    |
 LL |     m.need_to_use_this_method_value();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = m.need_to_use_this_method_value();
+   |     +++++++
 
 warning: unused return value of `EvenNature::is_even` that must be used
   --> $DIR/fn_must_use.rs:61:5
@@ -24,24 +33,43 @@ LL |     m.is_even(); // trait method!
    |     ^^^^^^^^^^^
    |
    = note: no side effects
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = m.is_even(); // trait method!
+   |     +++++++
 
 warning: unused return value of `MyStruct::need_to_use_this_associated_function_value` that must be used
   --> $DIR/fn_must_use.rs:64:5
    |
 LL |     MyStruct::need_to_use_this_associated_function_value();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = MyStruct::need_to_use_this_associated_function_value();
+   |     +++++++
 
 warning: unused return value of `std::cmp::PartialEq::eq` that must be used
   --> $DIR/fn_must_use.rs:70:5
    |
 LL |     2.eq(&3);
    |     ^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = 2.eq(&3);
+   |     +++++++
 
 warning: unused return value of `std::cmp::PartialEq::eq` that must be used
   --> $DIR/fn_must_use.rs:71:5
    |
 LL |     m.eq(&n);
    |     ^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = m.eq(&n);
+   |     +++++++
 
 warning: unused comparison that must be used
   --> $DIR/fn_must_use.rs:74:5
diff --git a/tests/ui/lint/unused/must-use-box-from-raw.stderr b/tests/ui/lint/unused/must-use-box-from-raw.stderr
index 47ab613bec2..4898db7fe3d 100644
--- a/tests/ui/lint/unused/must-use-box-from-raw.stderr
+++ b/tests/ui/lint/unused/must-use-box-from-raw.stderr
@@ -10,6 +10,10 @@ note: the lint level is defined here
    |
 LL | #![warn(unused_must_use)]
    |         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = Box::from_raw(ptr);
+   |     +++++++
 
 warning: 1 warning emitted
 
diff --git a/tests/ui/lint/unused/must_use-unit.stderr b/tests/ui/lint/unused/must_use-unit.stderr
index 9fcbc5074ea..993a19e5f04 100644
--- a/tests/ui/lint/unused/must_use-unit.stderr
+++ b/tests/ui/lint/unused/must_use-unit.stderr
@@ -9,12 +9,21 @@ note: the lint level is defined here
    |
 LL | #![deny(unused_must_use)]
    |         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = foo();
+   |     +++++++
 
 error: unused return value of `bar` that must be used
   --> $DIR/must_use-unit.rs:15:5
    |
 LL |     bar();
    |     ^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = bar();
+   |     +++++++
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/lint/unused/unused-async.stderr b/tests/ui/lint/unused/unused-async.stderr
index 4bcb26dc165..1c3702ba265 100644
--- a/tests/ui/lint/unused/unused-async.stderr
+++ b/tests/ui/lint/unused/unused-async.stderr
@@ -16,12 +16,22 @@ error: unused return value of `foo` that must be used
    |
 LL |     foo();
    |     ^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = foo();
+   |     +++++++
 
 error: unused output of future returned by `foo` that must be used
   --> $DIR/unused-async.rs:33:5
    |
 LL |     foo().await;
    |     ^^^^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = foo().await;
+   |     +++++++
 
 error: unused implementer of `Future` that must be used
   --> $DIR/unused-async.rs:34:5
@@ -36,12 +46,22 @@ error: unused return value of `bar` that must be used
    |
 LL |     bar();
    |     ^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = bar();
+   |     +++++++
 
 error: unused output of future returned by `bar` that must be used
   --> $DIR/unused-async.rs:36:5
    |
 LL |     bar().await;
    |     ^^^^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = bar().await;
+   |     +++++++
 
 error: unused implementer of `Future` that must be used
   --> $DIR/unused-async.rs:37:5
diff --git a/tests/ui/lint/unused/unused-result.stderr b/tests/ui/lint/unused/unused-result.stderr
index 4e1ba1fd959..f42995a65d1 100644
--- a/tests/ui/lint/unused/unused-result.stderr
+++ b/tests/ui/lint/unused/unused-result.stderr
@@ -9,6 +9,10 @@ note: the lint level is defined here
    |
 LL | #![deny(unused_results, unused_must_use)]
    |                         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = foo::<MustUse>();
+   |     +++++++
 
 error: unused `MustUseMsg` that must be used
   --> $DIR/unused-result.rs:22:5
@@ -17,6 +21,10 @@ LL |     foo::<MustUseMsg>();
    |     ^^^^^^^^^^^^^^^^^^^
    |
    = note: some message
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = foo::<MustUseMsg>();
+   |     +++++++
 
 error: unused result of type `isize`
   --> $DIR/unused-result.rs:34:5
@@ -35,6 +43,11 @@ error: unused `MustUse` that must be used
    |
 LL |     foo::<MustUse>();
    |     ^^^^^^^^^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = foo::<MustUse>();
+   |     +++++++
 
 error: unused `MustUseMsg` that must be used
   --> $DIR/unused-result.rs:36:5
@@ -43,6 +56,10 @@ LL |     foo::<MustUseMsg>();
    |     ^^^^^^^^^^^^^^^^^^^
    |
    = note: some message
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = foo::<MustUseMsg>();
+   |     +++++++
 
 error: aborting due to 5 previous errors
 
diff --git a/tests/ui/lint/unused/unused_attributes-must_use.stderr b/tests/ui/lint/unused/unused_attributes-must_use.stderr
index 0f699429e02..9633767c442 100644
--- a/tests/ui/lint/unused/unused_attributes-must_use.stderr
+++ b/tests/ui/lint/unused/unused_attributes-must_use.stderr
@@ -146,42 +146,76 @@ note: the lint level is defined here
    |
 LL | #![deny(unused_attributes, unused_must_use)]
    |                            ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = X;
+   |     +++++++
 
 error: unused `Y` that must be used
   --> $DIR/unused_attributes-must_use.rs:104:5
    |
 LL |     Y::Z;
    |     ^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = Y::Z;
+   |     +++++++
 
 error: unused `U` that must be used
   --> $DIR/unused_attributes-must_use.rs:105:5
    |
 LL |     U { unit: () };
    |     ^^^^^^^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = U { unit: () };
+   |     +++++++
 
 error: unused return value of `U::method` that must be used
   --> $DIR/unused_attributes-must_use.rs:106:5
    |
 LL |     U::method();
    |     ^^^^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = U::method();
+   |     +++++++
 
 error: unused return value of `foo` that must be used
   --> $DIR/unused_attributes-must_use.rs:107:5
    |
 LL |     foo();
    |     ^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = foo();
+   |     +++++++
 
 error: unused return value of `foreign_foo` that must be used
   --> $DIR/unused_attributes-must_use.rs:110:9
    |
 LL |         foreign_foo();
    |         ^^^^^^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |         let _ = foreign_foo();
+   |         +++++++
 
 error: unused return value of `Use::get_four` that must be used
   --> $DIR/unused_attributes-must_use.rs:118:5
    |
 LL |     ().get_four();
    |     ^^^^^^^^^^^^^
+   |
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |     let _ = ().get_four();
+   |     +++++++
 
 error: aborting due to 28 previous errors