4 files changed, 36 insertions, 3 deletions
diff --git a/compiler/rustc_error_messages/locales/en-US/borrowck.ftl b/compiler/rustc_error_messages/locales/en-US/borrowck.ftl
index 0021638c102..fe77cf23e8f 100644
--- a/compiler/rustc_error_messages/locales/en-US/borrowck.ftl
+++ b/compiler/rustc_error_messages/locales/en-US/borrowck.ftl
@@ -33,7 +33,7 @@ borrowck_var_here_defined = variable defined here
 
 borrowck_var_here_captured = variable captured here
 
-borrowck_closure_inferred_mut =  inferred to be a `FnMut` closure
+borrowck_closure_inferred_mut = inferred to be a `FnMut` closure
 
 borrowck_returned_closure_escaped =
     returns a closure that contains a reference to a captured variable, which then escapes the closure body
diff --git a/compiler/rustc_error_messages/locales/en-US/lint.ftl b/compiler/rustc_error_messages/locales/en-US/lint.ftl
index dca678dff7a..b1e7cc69a80 100644
--- a/compiler/rustc_error_messages/locales/en-US/lint.ftl
+++ b/compiler/rustc_error_messages/locales/en-US/lint.ftl
@@ -309,6 +309,7 @@ lint_unused_generator =
     .note = generators are lazy and do nothing unless resumed
 
 lint_unused_def = unused {$pre}`{$def}`{$post} that must be used
+    .suggestion = use `let _ = ...` to ignore the resulting value
 
 lint_path_statement_drop = path statement drops value
     .suggestion = use `drop` to clarify the intent
diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index 0c1019545f3..2e447b900e1 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/compiler/rustc_lint/src/lints.rs
@@ -1402,6 +1402,21 @@ pub struct UnusedDef<'a, 'b> {
     pub cx: &'a LateContext<'b>,
     pub def_id: DefId,
     pub note: Option<Symbol>,
+    pub suggestion: Option<UnusedDefSuggestion>,
+}
+
+#[derive(Subdiagnostic)]
+pub enum UnusedDefSuggestion {
+    #[suggestion(
+        suggestion,
+        style = "verbose",
+        code = "let _ = ",
+        applicability = "machine-applicable"
+    )]
+    Default {
+        #[primary_span]
+        span: Span,
+    },
 }
 
 // Needed because of def_path_str
@@ -1417,6 +1432,9 @@ impl<'a> DecorateLint<'a, ()> for UnusedDef<'_, '_> {
         if let Some(note) = self.note {
             diag.note(note.as_str());
         }
+        if let Some(sugg) = self.suggestion {
+            diag.subdiagnostic(sugg);
+        }
         diag
     }
 
diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
index 4c9b3df2dbd..622601224ff 100644
--- a/compiler/rustc_lint/src/unused.rs
+++ b/compiler/rustc_lint/src/unused.rs
@@ -1,7 +1,7 @@
 use crate::lints::{
     PathStatementDrop, PathStatementDropSub, PathStatementNoEffect, UnusedAllocationDiag,
-    UnusedAllocationMutDiag, UnusedClosure, UnusedDef, UnusedDelim, UnusedDelimSuggestion,
-    UnusedGenerator, UnusedImportBracesDiag, UnusedOp, UnusedResult,
+    UnusedAllocationMutDiag, UnusedClosure, UnusedDef, UnusedDefSuggestion, UnusedDelim,
+    UnusedDelimSuggestion, UnusedGenerator, UnusedImportBracesDiag, UnusedOp, UnusedResult,
 };
 use crate::Lint;
 use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
@@ -418,6 +418,19 @@ 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,
@@ -427,6 +440,7 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                             cx,
                             def_id: *def_id,
                             note: *reason,
+                            suggestion,
                         },
                     );
                 }