New lint [`filter_map_bool_then`]

8 files changed, 171 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 02d12d60e73..251f32d2e6e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4844,6 +4844,7 @@ Released 2018-09-13
 [`field_reassign_with_default`]: https://rust-lang.github.io/rust-clippy/master/index.html#field_reassign_with_default
 [`filetype_is_file`]: https://rust-lang.github.io/rust-clippy/master/index.html#filetype_is_file
 [`filter_map`]: https://rust-lang.github.io/rust-clippy/master/index.html#filter_map
+[`filter_map_bool_then`]: https://rust-lang.github.io/rust-clippy/master/index.html#filter_map_bool_then
 [`filter_map_identity`]: https://rust-lang.github.io/rust-clippy/master/index.html#filter_map_identity
 [`filter_map_next`]: https://rust-lang.github.io/rust-clippy/master/index.html#filter_map_next
 [`filter_next`]: https://rust-lang.github.io/rust-clippy/master/index.html#filter_next
diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index 9b99a255423..e084471433d 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/clippy_lints/src/declared_lints.rs
@@ -337,6 +337,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::methods::EXPECT_USED_INFO,
     crate::methods::EXTEND_WITH_DRAIN_INFO,
     crate::methods::FILETYPE_IS_FILE_INFO,
+    crate::methods::FILTER_MAP_BOOL_THEN_INFO,
     crate::methods::FILTER_MAP_IDENTITY_INFO,
     crate::methods::FILTER_MAP_NEXT_INFO,
     crate::methods::FILTER_NEXT_INFO,
diff --git a/clippy_lints/src/methods/filter_map_bool_then.rs b/clippy_lints/src/methods/filter_map_bool_then.rs
new file mode 100644
index 00000000000..e5d8be21f3d
--- /dev/null
+++ b/clippy_lints/src/methods/filter_map_bool_then.rs
@@ -0,0 +1,39 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::paths::BOOL_THEN;
+use clippy_utils::source::snippet_opt;
+use clippy_utils::{is_from_proc_macro, match_def_path, peel_blocks};
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_span::Span;
+
+use super::FILTER_MAP_BOOL_THEN;
+
+pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>, arg: &Expr<'_>, call_span: Span) {
+    if !in_external_macro(cx.sess(), expr.span)
+        && let ExprKind::Closure(closure) = arg.kind
+        && let body = peel_blocks(cx.tcx.hir().body(closure.body).value)
+        && let ExprKind::MethodCall(_, recv, [then_arg], _) = body.kind
+        && let ExprKind::Closure(then_closure) = then_arg.kind
+        && let then_body = peel_blocks(cx.tcx.hir().body(then_closure.body).value)
+        && let Some(def_id) = cx.typeck_results().type_dependent_def_id(body.hir_id)
+        && match_def_path(cx, def_id, &BOOL_THEN)
+        && !is_from_proc_macro(cx, expr)
+        && let Some(decl_snippet) = closure.fn_arg_span.and_then(|s| snippet_opt(cx, s))
+        // NOTE: This will include the `()` (parenthesis) around it. Maybe there's some utils method
+        // to remove them? `unused_parens` will already take care of this but it may be nice.
+        && let Some(filter) = snippet_opt(cx, recv.span)
+        && let Some(map) = snippet_opt(cx, then_body.span)
+    {
+        span_lint_and_sugg(
+            cx,
+            FILTER_MAP_BOOL_THEN,
+            call_span,
+            "usage of `bool::then` in `filter_map`",
+            "use `filter` then `map` instead",
+            format!("filter({decl_snippet} {filter}).map({decl_snippet} {map})"),
+            Applicability::MachineApplicable,
+        );
+    }
+}
diff --git a/clippy_lints/src/methods/mod.rs b/clippy_lints/src/methods/mod.rs
index 1058289679b..e774450018e 100644
--- a/clippy_lints/src/methods/mod.rs
+++ b/clippy_lints/src/methods/mod.rs
@@ -21,6 +21,7 @@ mod expect_used;
 mod extend_with_drain;
 mod filetype_is_file;
 mod filter_map;
+mod filter_map_bool_then;
 mod filter_map_identity;
 mod filter_map_next;
 mod filter_next;
@@ -3475,6 +3476,37 @@ declare_clippy_lint! {
     "disallows `.skip(0)`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usage of `bool::then` in `Iterator::filter_map`.
+    ///
+    /// ### Why is this bad?
+    /// This can be written with `filter` then `map` instead, which would reduce nesting and
+    /// separates the filtering from the transformation phase. This comes with no cost to
+    /// performance and is just cleaner.
+    ///
+    /// ### Limitations
+    /// Does not lint `bool::then_some`, as it eagerly evaluates its arguments rather than lazily.
+    /// This can create differing behavior, so better safe than sorry.
+    ///
+    /// ### Example
+    /// ```rust
+    /// # fn really_expensive_fn(i: i32) -> i32 { i }
+    /// # let v = vec![];
+    /// _ = v.into_iter().filter_map(|i| (i % 2 == 0).then(|| really_expensive_fn(i)));
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// # fn really_expensive_fn(i: i32) -> i32 { i }
+    /// # let v = vec![];
+    /// _ = v.into_iter().filter(|i| i % 2 == 0).map(|i| really_expensive_fn(i));
+    /// ```
+    #[clippy::version = "1.72.0"]
+    pub FILTER_MAP_BOOL_THEN,
+    style,
+    "checks for usage of `bool::then` in `Iterator::filter_map`"
+}
+
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Msrv,
@@ -3612,6 +3644,7 @@ impl_lint_pass!(Methods => [
     FORMAT_COLLECT,
     STRING_LIT_CHARS_ANY,
     ITER_SKIP_ZERO,
+    FILTER_MAP_BOOL_THEN,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -3889,6 +3922,7 @@ impl Methods {
                 },
                 ("filter_map", [arg]) => {
                     unnecessary_filter_map::check(cx, expr, arg, name);
+                    filter_map_bool_then::check(cx, expr, arg, call_span);
                     filter_map_identity::check(cx, expr, arg, span);
                 },
                 ("find_map", [arg]) => {
diff --git a/clippy_utils/src/paths.rs b/clippy_utils/src/paths.rs
index 8d96d3cfe50..914ea85ac28 100644
--- a/clippy_utils/src/paths.rs
+++ b/clippy_utils/src/paths.rs
@@ -163,3 +163,5 @@ pub const OPTION_EXPECT: [&str; 4] = ["core", "option", "Option", "expect"];
 pub const FORMATTER: [&str; 3] = ["core", "fmt", "Formatter"];
 pub const DEBUG_STRUCT: [&str; 4] = ["core", "fmt", "builders", "DebugStruct"];
 pub const ORD_CMP: [&str; 4] = ["core", "cmp", "Ord", "cmp"];
+#[expect(clippy::invalid_paths)] // not sure why it thinks this, it works so
+pub const BOOL_THEN: [&str; 4] = ["core", "bool", "<impl bool>", "then"];
diff --git a/tests/ui/filter_map_bool_then.fixed b/tests/ui/filter_map_bool_then.fixed
new file mode 100644
index 00000000000..0db0b0a8a28
--- /dev/null
+++ b/tests/ui/filter_map_bool_then.fixed
@@ -0,0 +1,33 @@
+//@run-rustfix
+//@aux-build:proc_macros.rs:proc-macro
+#![allow(clippy::clone_on_copy, clippy::unnecessary_lazy_evaluations, unused)]
+#![warn(clippy::filter_map_bool_then)]
+
+#[macro_use]
+extern crate proc_macros;
+
+fn main() {
+    let v = vec![1, 2, 3, 4, 5, 6];
+    v.clone().into_iter().filter(|i| (i % 2 == 0)).map(|i| i + 1);
+    v.clone()
+        .into_iter()
+        .filter(|i| (i % 2 == 0)).map(|i| i + 1);
+    v.clone()
+        .into_iter()
+        .filter(|&i| i != 1000)
+        .filter(|i| (i % 2 == 0)).map(|i| i + 1);
+    v.iter()
+        .copied()
+        .filter(|&i| i != 1000)
+        .filter(|i| (i.clone() % 2 == 0)).map(|i| i + 1);
+    // Do not lint
+    external! {
+        let v = vec![1, 2, 3, 4, 5, 6];
+        v.clone().into_iter().filter_map(|i| (i % 2 == 0).then(|| i + 1));
+    }
+    with_span! {
+        span
+        let v = vec![1, 2, 3, 4, 5, 6];
+        v.clone().into_iter().filter_map(|i| (i % 2 == 0).then(|| i + 1));
+    }
+}
diff --git a/tests/ui/filter_map_bool_then.rs b/tests/ui/filter_map_bool_then.rs
new file mode 100644
index 00000000000..764f9c4a85e
--- /dev/null
+++ b/tests/ui/filter_map_bool_then.rs
@@ -0,0 +1,33 @@
+//@run-rustfix
+//@aux-build:proc_macros.rs:proc-macro
+#![allow(clippy::clone_on_copy, clippy::unnecessary_lazy_evaluations, unused)]
+#![warn(clippy::filter_map_bool_then)]
+
+#[macro_use]
+extern crate proc_macros;
+
+fn main() {
+    let v = vec![1, 2, 3, 4, 5, 6];
+    v.clone().into_iter().filter_map(|i| (i % 2 == 0).then(|| i + 1));
+    v.clone()
+        .into_iter()
+        .filter_map(|i| -> Option<_> { (i % 2 == 0).then(|| i + 1) });
+    v.clone()
+        .into_iter()
+        .filter(|&i| i != 1000)
+        .filter_map(|i| (i % 2 == 0).then(|| i + 1));
+    v.iter()
+        .copied()
+        .filter(|&i| i != 1000)
+        .filter_map(|i| (i.clone() % 2 == 0).then(|| i + 1));
+    // Do not lint
+    external! {
+        let v = vec![1, 2, 3, 4, 5, 6];
+        v.clone().into_iter().filter_map(|i| (i % 2 == 0).then(|| i + 1));
+    }
+    with_span! {
+        span
+        let v = vec![1, 2, 3, 4, 5, 6];
+        v.clone().into_iter().filter_map(|i| (i % 2 == 0).then(|| i + 1));
+    }
+}
diff --git a/tests/ui/filter_map_bool_then.stderr b/tests/ui/filter_map_bool_then.stderr
new file mode 100644
index 00000000000..79bc7ba02f7
--- /dev/null
+++ b/tests/ui/filter_map_bool_then.stderr
@@ -0,0 +1,28 @@
+error: usage of `bool::then` in `filter_map`
+  --> $DIR/filter_map_bool_then.rs:11:27
+   |
+LL |     v.clone().into_iter().filter_map(|i| (i % 2 == 0).then(|| i + 1));
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `filter` then `map` instead: `filter(|i| (i % 2 == 0)).map(|i| i + 1)`
+   |
+   = note: `-D clippy::filter-map-bool-then` implied by `-D warnings`
+
+error: usage of `bool::then` in `filter_map`
+  --> $DIR/filter_map_bool_then.rs:14:10
+   |
+LL |         .filter_map(|i| -> Option<_> { (i % 2 == 0).then(|| i + 1) });
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `filter` then `map` instead: `filter(|i| (i % 2 == 0)).map(|i| i + 1)`
+
+error: usage of `bool::then` in `filter_map`
+  --> $DIR/filter_map_bool_then.rs:18:10
+   |
+LL |         .filter_map(|i| (i % 2 == 0).then(|| i + 1));
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `filter` then `map` instead: `filter(|i| (i % 2 == 0)).map(|i| i + 1)`
+
+error: usage of `bool::then` in `filter_map`
+  --> $DIR/filter_map_bool_then.rs:22:10
+   |
+LL |         .filter_map(|i| (i.clone() % 2 == 0).then(|| i + 1));
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `filter` then `map` instead: `filter(|i| (i.clone() % 2 == 0)).map(|i| i + 1)`
+
+error: aborting due to 4 previous errors
+