Fix unused lint error in macro

compiler/rustc_lint/src/unused.rs

@@ -539,10 +539,19 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                     );
                 }
                 MustUsePath::Def(span, def_id, reason) => {
-                    let span = span.find_ancestor_not_from_macro().unwrap_or(*span);
+                    let ancenstor_span = span.find_ancestor_not_from_macro().unwrap_or(*span);
+                    let is_redundant_let_ignore = cx
+                        .sess()
+                        .source_map()
+                        .span_to_prev_source(ancenstor_span)
+                        .ok()
+                        .map(|prev| prev.trim_end().ends_with("let _ ="))
+                        .unwrap_or(false);
+                    let suggestion_span =
+                        if is_redundant_let_ignore { *span } else { ancenstor_span };
                     cx.emit_span_lint(
                         UNUSED_MUST_USE,
-                        span,
+                        ancenstor_span,
                         UnusedDef {
                             pre: descr_pre,
                             post: descr_post,
@@ -551,11 +560,13 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                             note: *reason,
                             suggestion: (!is_inner).then_some(if expr_is_from_block {
                                 UnusedDefSuggestion::BlockTailExpr {
-                                    before_span: span.shrink_to_lo(),
-                                    after_span: span.shrink_to_hi(),
+                                    before_span: suggestion_span.shrink_to_lo(),
+                                    after_span: suggestion_span.shrink_to_hi(),
                                 }
                             } else {
-                                UnusedDefSuggestion::NormalExpr { span: span.shrink_to_lo() }
+                                UnusedDefSuggestion::NormalExpr {
+                                    span: suggestion_span.shrink_to_lo(),
+                                }
                             }),
                         },
                     );

tests/ui/lint/unused/lint-unsed-in-macro-issue-151269.rs

@@ -0,0 +1,15 @@
+#![deny(unused_must_use)]
+
+fn error() -> Result<(), ()> {
+    Err(())
+}
+
+macro_rules! foo {
+    () => {{
+        error();
+    }};
+}
+
+fn main() {
+    let _ = foo!(); //~ ERROR unused `Result` that must be used
+}

tests/ui/lint/unused/lint-unsed-in-macro-issue-151269.stderr

@@ -0,0 +1,19 @@
+error: unused `Result` that must be used
+  --> $DIR/lint-unsed-in-macro-issue-151269.rs:14:13
+   |
+LL |     let _ = foo!();
+   |             ^^^^^^
+   |
+   = note: this `Result` may be an `Err` variant, which should be handled
+note: the lint level is defined here
+  --> $DIR/lint-unsed-in-macro-issue-151269.rs:1:9
+   |
+LL | #![deny(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+help: use `let _ = ...` to ignore the resulting value
+   |
+LL |         let _ = error();
+   |         +++++++
+
+error: aborting due to 1 previous error
+