Auto merge of #11869 - PartiallyTyped:result-filter-map, r=Alexendoo

New Lint: `result_filter_map` / Mirror of `option_filter_map`

Added the `Result` mirror of `option_filter_map`.

changelog: New Lint: [`result_filter_map`]

I had to move around some code because the function def was too long 🙃.

I have also added some pattern checks on `option_filter_map`

clippy_lints/src/methods/filter_map.rs

@@ -14,7 +14,7 @@ use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::Span;
 use std::borrow::Cow;
 
-use super::{MANUAL_FILTER_MAP, MANUAL_FIND_MAP, OPTION_FILTER_MAP};
+use super::{MANUAL_FILTER_MAP, MANUAL_FIND_MAP, OPTION_FILTER_MAP, RESULT_FILTER_MAP};
 
 fn is_method(cx: &LateContext<'_>, expr: &hir::Expr<'_>, method_name: Symbol) -> bool {
     match &expr.kind {
@@ -46,6 +46,9 @@ fn is_method(cx: &LateContext<'_>, expr: &hir::Expr<'_>, method_name: Symbol) ->
 fn is_option_filter_map(cx: &LateContext<'_>, filter_arg: &hir::Expr<'_>, map_arg: &hir::Expr<'_>) -> bool {
     is_method(cx, map_arg, sym::unwrap) && is_method(cx, filter_arg, sym!(is_some))
 }
+fn is_ok_filter_map(cx: &LateContext<'_>, filter_arg: &hir::Expr<'_>, map_arg: &hir::Expr<'_>) -> bool {
+    is_method(cx, map_arg, sym::unwrap) && is_method(cx, filter_arg, sym!(is_ok))
+}
 
 #[derive(Debug, Copy, Clone)]
 enum OffendingFilterExpr<'tcx> {
@@ -273,6 +276,18 @@ fn is_filter_some_map_unwrap(
     (iterator || option) && is_option_filter_map(cx, filter_arg, map_arg)
 }
 
+/// is `filter(|x| x.is_ok()).map(|x| x.unwrap())`
+fn is_filter_ok_map_unwrap(
+    cx: &LateContext<'_>,
+    expr: &hir::Expr<'_>,
+    filter_arg: &hir::Expr<'_>,
+    map_arg: &hir::Expr<'_>,
+) -> bool {
+    // result has no filter, so we only check for iterators
+    let iterator = is_trait_method(cx, expr, sym::Iterator);
+    iterator && is_ok_filter_map(cx, filter_arg, map_arg)
+}
+
 /// lint use of `filter().map()` or `find().map()` for `Iterators`
 #[allow(clippy::too_many_arguments)]
 pub(super) fn check(
@@ -300,30 +315,21 @@ pub(super) fn check(
         return;
     }
 
-    if is_trait_method(cx, map_recv, sym::Iterator)
+    if is_filter_ok_map_unwrap(cx, expr, filter_arg, map_arg) {
+        span_lint_and_sugg(
+            cx,
+            RESULT_FILTER_MAP,
+            filter_span.with_hi(expr.span.hi()),
+            "`filter` for `Ok` followed by `unwrap`",
+            "consider using `flatten` instead",
+            reindent_multiline(Cow::Borrowed("flatten()"), true, indent_of(cx, map_span)).into_owned(),
+            Applicability::MachineApplicable,
+        );
 
-        // filter(|x| ...is_some())...
-        && let ExprKind::Closure(&Closure { body: filter_body_id, .. }) = filter_arg.kind
-        && let filter_body = cx.tcx.hir().body(filter_body_id)
-        && let [filter_param] = filter_body.params
-        // optional ref pattern: `filter(|&x| ..)`
-        && let (filter_pat, is_filter_param_ref) = if let PatKind::Ref(ref_pat, _) = filter_param.pat.kind {
-            (ref_pat, true)
-        } else {
-            (filter_param.pat, false)
-        }
+        return;
+    }
 
-        && let PatKind::Binding(_, filter_param_id, _, None) = filter_pat.kind
-        && let Some(mut offending_expr) = OffendingFilterExpr::hir(cx, filter_body.value, filter_param_id)
-
-        && let ExprKind::Closure(&Closure { body: map_body_id, .. }) = map_arg.kind
-        && let map_body = cx.tcx.hir().body(map_body_id)
-        && let [map_param] = map_body.params
-        && let PatKind::Binding(_, map_param_id, map_param_ident, None) = map_param.pat.kind
-
-        && let Some(check_result) =
-            offending_expr.check_map_call(cx, map_body, map_param_id, filter_param_id, is_filter_param_ref)
-    {
+    if let Some((map_param_ident, check_result)) = is_find_or_filter(cx, map_recv, filter_arg, map_arg) {
         let span = filter_span.with_hi(expr.span.hi());
         let (filter_name, lint) = if is_find {
             ("find", MANUAL_FIND_MAP)
@@ -395,6 +401,40 @@ pub(super) fn check(
     }
 }
 
+fn is_find_or_filter<'a>(
+    cx: &LateContext<'a>,
+    map_recv: &hir::Expr<'_>,
+    filter_arg: &hir::Expr<'_>,
+    map_arg: &hir::Expr<'_>,
+) -> Option<(Ident, CheckResult<'a>)> {
+    if is_trait_method(cx, map_recv, sym::Iterator)
+        // filter(|x| ...is_some())...
+        && let ExprKind::Closure(&Closure { body: filter_body_id, .. }) = filter_arg.kind
+        && let filter_body = cx.tcx.hir().body(filter_body_id)
+        && let [filter_param] = filter_body.params
+        // optional ref pattern: `filter(|&x| ..)`
+        && let (filter_pat, is_filter_param_ref) = if let PatKind::Ref(ref_pat, _) = filter_param.pat.kind {
+            (ref_pat, true)
+        } else {
+            (filter_param.pat, false)
+        }
+
+        && let PatKind::Binding(_, filter_param_id, _, None) = filter_pat.kind
+        && let Some(mut offending_expr) = OffendingFilterExpr::hir(cx, filter_body.value, filter_param_id)
+
+        && let ExprKind::Closure(&Closure { body: map_body_id, .. }) = map_arg.kind
+        && let map_body = cx.tcx.hir().body(map_body_id)
+        && let [map_param] = map_body.params
+        && let PatKind::Binding(_, map_param_id, map_param_ident, None) = map_param.pat.kind
+
+        && let Some(check_result) =
+            offending_expr.check_map_call(cx, map_body, map_param_id, filter_param_id, is_filter_param_ref)
+    {
+        return Some((map_param_ident, check_result));
+    }
+    None
+}
+
 fn acceptable_methods(method: &PathSegment<'_>) -> bool {
     let methods: [Symbol; 8] = [
         sym::clone,

clippy_lints/src/methods/mod.rs

@@ -1175,7 +1175,8 @@ declare_clippy_lint! {
 
 declare_clippy_lint! {
     /// ### What it does
-    /// Checks for indirect collection of populated `Option`
+    /// Checks for iterators of `Option`s using ``.filter(Option::is_some).map(Option::unwrap)` that may
+    /// be replaced with a `.flatten()` call.
     ///
     /// ### Why is this bad?
     /// `Option` is like a collection of 0-1 things, so `flatten`
@@ -3752,6 +3753,29 @@ declare_clippy_lint! {
     "using `Option.map_or(Err(_), Ok)`, which is more succinctly expressed as `Option.ok_or(_)`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for iterators of `Result`s using ``.filter(Result::is_ok).map(Result::unwrap)` that may
+    /// be replaced with a `.flatten()` call.
+    ///
+    /// ### Why is this bad?
+    /// `Result` implements `IntoIterator<Item = T>`. This means that `Result` can be flattened
+    /// automatically without suspicious-looking `unwrap` calls.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let _ = std::iter::empty::<Result<i32, ()>>().filter(Result::is_ok).map(Result::unwrap);
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let _ = std::iter::empty::<Result<i32, ()>>().flatten();
+    /// ```
+    #[clippy::version = "1.76.0"]
+    pub RESULT_FILTER_MAP,
+    complexity,
+    "filtering `Result` for `Ok` then force-unwrapping, which can be one type-safe operation"
+}
+
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Msrv,
@@ -3903,6 +3927,7 @@ impl_lint_pass!(Methods => [
     UNNECESSARY_FALLIBLE_CONVERSIONS,
     JOIN_ABSOLUTE_PATHS,
     OPTION_MAP_OR_ERR_OK,
+    RESULT_FILTER_MAP,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.