initial draft. fixes rust-lang#6061.

CHANGELOG.md

@@ -1988,6 +1988,7 @@ Released 2018-09-13
 [`op_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#op_ref
 [`option_as_ref_deref`]: https://rust-lang.github.io/rust-clippy/master/index.html#option_as_ref_deref
 [`option_env_unwrap`]: https://rust-lang.github.io/rust-clippy/master/index.html#option_env_unwrap
+[`option_filter_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#option_filter_map
 [`option_if_let_else`]: https://rust-lang.github.io/rust-clippy/master/index.html#option_if_let_else
 [`option_map_or_none`]: https://rust-lang.github.io/rust-clippy/master/index.html#option_map_or_none
 [`option_map_unit_fn`]: https://rust-lang.github.io/rust-clippy/master/index.html#option_map_unit_fn

clippy_lints/src/lib.rs

@@ -714,6 +714,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &methods::NEW_RET_NO_SELF,
         &methods::OK_EXPECT,
         &methods::OPTION_AS_REF_DEREF,
+        &methods::OPTION_FILTER_MAP,
         &methods::OPTION_MAP_OR_NONE,
         &methods::OR_FUN_CALL,
         &methods::RESULT_MAP_OR_INTO_OPTION,
@@ -1469,6 +1470,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&methods::NEW_RET_NO_SELF),
         LintId::of(&methods::OK_EXPECT),
         LintId::of(&methods::OPTION_AS_REF_DEREF),
+        LintId::of(&methods::OPTION_FILTER_MAP),
         LintId::of(&methods::OPTION_MAP_OR_NONE),
         LintId::of(&methods::OR_FUN_CALL),
         LintId::of(&methods::RESULT_MAP_OR_INTO_OPTION),
@@ -1668,6 +1670,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&methods::MAP_COLLECT_RESULT_UNIT),
         LintId::of(&methods::NEW_RET_NO_SELF),
         LintId::of(&methods::OK_EXPECT),
+        LintId::of(&methods::OPTION_FILTER_MAP),
         LintId::of(&methods::OPTION_MAP_OR_NONE),
         LintId::of(&methods::RESULT_MAP_OR_INTO_OPTION),
         LintId::of(&methods::SHOULD_IMPLEMENT_TRAIT),

clippy_lints/src/methods/mod.rs

@@ -1,6 +1,7 @@
 mod bind_instead_of_map;
 mod inefficient_to_string;
 mod manual_saturating_arithmetic;
+mod option_filter_map;
 mod option_map_unwrap_or;
 mod unnecessary_filter_map;
 mod unnecessary_lazy_eval;
@@ -816,6 +817,28 @@ declare_clippy_lint! {
     "using `Iterator::step_by(0)`, which will panic at runtime"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for indirect collection of populated `Option`
+    ///
+    /// **Why is this bad?** `Option` is like a collection of 0-1 things, so `flatten`
+    /// automatically does this without suspicious-looking `unwrap` calls.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    ///
+    /// ```rust
+    /// my_iter.filter(Option::is_some).map(Option::unwrap)
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// my_iter.flatten()
+    /// ```
+    pub OPTION_FILTER_MAP,
+    style,
+    "filtering `Option` for `Some` then force-unwrapping, which can be one type-safe operation"
+}
+
 declare_clippy_lint! {
     /// **What it does:** Checks for the use of `iter.nth(0)`.
     ///
@@ -1441,6 +1464,7 @@ impl_lint_pass!(Methods => [
     FILTER_NEXT,
     SKIP_WHILE_NEXT,
     FILTER_MAP,
+    OPTION_FILTER_MAP,
     FILTER_MAP_NEXT,
     FLAT_MAP_IDENTITY,
     FIND_MAP,
@@ -2912,14 +2936,26 @@ fn lint_skip_while_next<'tcx>(
 fn lint_filter_map<'tcx>(
     cx: &LateContext<'tcx>,
     expr: &'tcx hir::Expr<'_>,
-    _filter_args: &'tcx [hir::Expr<'_>],
-    _map_args: &'tcx [hir::Expr<'_>],
+    filter_args: &'tcx [hir::Expr<'_>],
+    map_args: &'tcx [hir::Expr<'_>],
 ) {
-    // lint if caller of `.filter().map()` is an Iterator
-    if match_trait_method(cx, expr, &paths::ITERATOR) {
-        let msg = "called `filter(..).map(..)` on an `Iterator`";
-        let hint = "this is more succinctly expressed by calling `.filter_map(..)` instead";
-        span_lint_and_help(cx, FILTER_MAP, expr.span, msg, None, hint);
+    // is it specifically `.filter(Option::is_some).map(Option::unwrap)`?
+    if option_filter_map::in_scope(filter_args, map_args) {
+        span_lint_and_help(
+            cx,
+            OPTION_FILTER_MAP,
+            expr.span,
+            "`filter` for `Some` followed by `unwrap`",
+            None,
+            "consider using `flatten` instead",
+        );
+    } else {
+        // lint if caller of `.filter().map()` is an Iterator
+        if match_trait_method(cx, expr, &paths::ITERATOR) {
+            let msg = "called `filter(..).map(..)` on an `Iterator`";
+            let hint = "this is more succinctly expressed by calling `.filter_map(..)` instead";
+            span_lint_and_help(cx, FILTER_MAP, expr.span, msg, None, hint);
+        }
     }
 }
 

clippy_lints/src/methods/option_filter_map.rs

@@ -0,0 +1,14 @@
+use crate::utils::{match_qpath, paths};
+use rustc_hir::{self, Expr, ExprKind};
+use std::iter;
+
+fn calls_fn<'tcx>(expr: &'tcx [Expr<'_>], fn_name: &str) -> bool {
+    expr.get(1).map_or(false, |x| match &x.kind {
+        ExprKind::Path(qp) => match_qpath(qp, &paths::OPTION.iter().cloned().chain(iter::once(fn_name)).collect::<Vec<&str>>()),
+        _ => false,
+    })
+}
+
+pub fn in_scope<'tcx>(filter_args: &'tcx [Expr<'_>], map_args: &'tcx [Expr<'_>]) -> bool {
+    calls_fn(map_args, "unwrap") && calls_fn(filter_args, "is_some")
+}

tests/ui/option_filter_map.rs

@@ -0,0 +1,16 @@
+fn odds_out(x: i32) -> Option<i32> {
+    if x % 2 == 0 {
+        Some(x)
+    } else {
+        None
+    }
+}
+
+fn main() {
+    let evens: Vec<i32> = vec![1, 2, 3]
+        .into_iter()
+        .map(odds_out)
+        .filter(Option::is_some)
+        .map(Option::unwrap)
+        .collect();
+}

tests/ui/option_filter_map.stderr

@@ -0,0 +1,16 @@
+error: `filter` for `Some` followed by `unwrap`
+  --> $DIR/option_filter_map.rs:10:27
+   |
+LL |       let evens: Vec<i32> = vec![1, 2, 3]
+   |  ___________________________^
+LL | |         .into_iter()
+LL | |         .map(odds_out)
+LL | |         .filter(Option::is_some)
+LL | |         .map(Option::unwrap)
+   | |____________________________^
+   |
+   = note: `-D clippy::option-filter-map` implied by `-D warnings`
+   = help: consider using `flatten` instead
+
+error: aborting due to previous error
+