Auto merge of #12562 - m-rph:12501, r=y21

Allow `filter_map_identity` when the closure is typed

This extends the `filter_map_identity` lint to support typed closures.

For untyped closures, we know that the program compiles, and therefore we can safely suggest using flatten.

For typed closures, they may participate in type resolution. In this case we use `Applicability::MaybeIncorrect`.

Details:
https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy/topic/Should.20.60filter_map_identity.60.20lint.20when.20closures.20are.20typed.3F

changelog: `filter_map_identity` will now suggest using flatten for typed closures.

r? `@y21` && `@Centri3`

clippy_lints/src/methods/filter_map_identity.rs

@@ -1,5 +1,5 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::{is_expr_untyped_identity_function, is_trait_method};
+use clippy_utils::{is_expr_identity_function, is_expr_untyped_identity_function, is_trait_method};
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_lint::LateContext;
@@ -7,8 +7,20 @@ use rustc_span::{sym, Span};
 
 use super::FILTER_MAP_IDENTITY;
 
+fn is_identity(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> Option<Applicability> {
+    if is_expr_untyped_identity_function(cx, expr) {
+        return Some(Applicability::MachineApplicable);
+    }
+    if is_expr_identity_function(cx, expr) {
+        return Some(Applicability::Unspecified);
+    }
+    None
+}
+
 pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, filter_map_arg: &hir::Expr<'_>, filter_map_span: Span) {
-    if is_trait_method(cx, expr, sym::Iterator) && is_expr_untyped_identity_function(cx, filter_map_arg) {
+    if is_trait_method(cx, expr, sym::Iterator)
+        && let Some(applicability) = is_identity(cx, filter_map_arg)
+    {
         span_lint_and_sugg(
             cx,
             FILTER_MAP_IDENTITY,
@@ -16,7 +28,7 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, filter_map_arg:
             "use of `filter_map` with an identity function",
             "try",
             "flatten()".to_string(),
-            Applicability::MachineApplicable,
+            applicability,
         );
     }
 }