add testcase for rust-lang#8878

tests/ui/map_flatten_fixable.fixed

@@ -28,4 +28,22 @@ fn main() {
 
     // mapping to Result on Result
     let _: Result<_, &str> = (Ok(Ok(1))).and_then(|x| x);
+
+    issue8878();
+}
+
+#[allow(clippy::bind_instead_of_map)] // map + flatten will be suggested to `and_then`, but afterwards `map` is suggested again
+#[rustfmt::skip] // whitespace is important for this one
+fn issue8878() {
+    std::collections::HashMap::<u32, u32>::new()
+        .get(&0)
+        .and_then(|_| {
+// we need some newlines
+// so that the span is big enough
+// we need some newlines
+// so that the span is big enough
+// for a splitted output of the diagnostic
+            Some("")
+ // whitespace beforehand is important as well
+        });
 }

tests/ui/map_flatten_fixable.rs

@@ -28,4 +28,21 @@ fn main() {
 
     // mapping to Result on Result
     let _: Result<_, &str> = (Ok(Ok(1))).map(|x| x).flatten();
+
+    issue8878();
+}
+
+#[allow(clippy::bind_instead_of_map)] // map + flatten will be suggested to `and_then`, but afterwards `map` is suggested again
+#[rustfmt::skip] // whitespace is important for this one
+fn issue8878() {
+    std::collections::HashMap::<u32, u32>::new()
+        .get(&0)
+        .map(|_| {
+// we need some newlines
+// so that the span is big enough
+// for a splitted output of the diagnostic
+            Some("")
+ // whitespace beforehand is important as well
+        })
+        .flatten();
 }

tests/ui/map_flatten_fixable.stderr

@@ -76,5 +76,31 @@ help: try replacing `map` with `and_then`, and remove the `.flatten()`
 LL |     let _: Result<_, &str> = (Ok(Ok(1))).and_then(|x| x);
    |                                          ~~~~~~~~~~~~~~~
 
-error: aborting due to 7 previous errors
+error: called `map(..).flatten()` on `Option`
+  --> $DIR/map_flatten_fixable.rs:40:10
+   |
+LL |           .map(|_| {
+   |  __________^
+LL | | // we need some newlines
+LL | | // so that the span is big enough
+LL | | // for a splitted output of the diagnostic
+...  |
+LL | |         })
+LL | |         .flatten();
+   | |__________________^
+   |
+help: try replacing `map` with `and_then`
+   |
+LL ~         .and_then(|_| {
+LL + // we need some newlines
+LL + // so that the span is big enough
+   |
+help: and remove the `.flatten()`
+   |
+LL +             Some("")
+LL +  // whitespace beforehand is important as well
+LL ~         });
+   |
+
+error: aborting due to 8 previous errors