Auto merge of #10534 - samueltardieu:lines-filter-map-ok, r=llogiq

Flag `bufreader.lines().filter_map(Result::ok)` as suspicious

This lint detects a problem that happened recently in https://github.com/uutils/coreutils and is described in https://github.com/rust-lang/rust/issues/64144.

changelog: [`lines_filter_map_ok`]: new lint

3 files changed, 103 insertions, 0 deletions

diff --git a/clippy_lints/src/declared_lints.rs b/clippy_lints/src/declared_lints.rs
index f07c7f0534c..7836e5ccb9a 100644
--- a/clippy_lints/src/declared_lints.rs
+++ b/clippy_lints/src/declared_lints.rs
@@ -232,6 +232,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::let_with_type_underscore::LET_WITH_TYPE_UNDERSCORE_INFO,
     crate::lifetimes::EXTRA_UNUSED_LIFETIMES_INFO,
     crate::lifetimes::NEEDLESS_LIFETIMES_INFO,
+    crate::lines_filter_map_ok::LINES_FILTER_MAP_OK_INFO,
     crate::literal_representation::DECIMAL_LITERAL_REPRESENTATION_INFO,
     crate::literal_representation::INCONSISTENT_DIGIT_GROUPING_INFO,
     crate::literal_representation::LARGE_DIGIT_GROUPS_INFO,
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 75c53a09d79..ce055f16240 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -171,6 +171,7 @@ mod let_if_seq;
 mod let_underscore;
 mod let_with_type_underscore;
 mod lifetimes;
+mod lines_filter_map_ok;
 mod literal_representation;
 mod loops;
 mod macro_use;
@@ -949,6 +950,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
             avoid_breaking_exported_api,
         ))
     });
+    store.register_late_pass(|_| Box::new(lines_filter_map_ok::LinesFilterMapOk));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 
diff --git a/clippy_lints/src/lines_filter_map_ok.rs b/clippy_lints/src/lines_filter_map_ok.rs
new file mode 100644
index 00000000000..b0f9276475d
--- /dev/null
+++ b/clippy_lints/src/lines_filter_map_ok.rs
@@ -0,0 +1,100 @@
+use clippy_utils::{
+    diagnostics::span_lint_and_then, is_diag_item_method, is_trait_method, match_def_path, path_to_local_id, paths,
+    ty::match_type,
+};
+use rustc_errors::Applicability;
+use rustc_hir::{Body, Closure, Expr, ExprKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Detect uses of `lines.filter_map(Result::ok)` or `lines.flat_map(Result::ok)`
+    /// when `lines` has type `std::io::Lines`.
+    ///
+    /// ### Why is this bad?
+    /// `Lines` instances might produce a never-ending stream of `Err`, in which case
+    /// `filter_map(Result::ok)` will enter an infinite loop while waiting for an
+    /// `Ok` variant. Calling `next()` once is sufficient to enter the infinite loop,
+    /// even in the absence of explicit loops in the user code.
+    ///
+    /// This situation can arise when working with user-provided paths. On some platforms,
+    /// `std::fs::File::open(path)` might return `Ok(fs)` even when `path` is a directory,
+    /// but any later attempt to read from `fs` will return an error.
+    ///
+    /// ### Known problems
+    /// This lint suggests replacing `filter_map()` or `flat_map()` applied to a `Lines`
+    /// instance in all cases. There two cases where the suggestion might not be
+    /// appropriate or necessary:
+    ///
+    /// - If the `Lines` instance can never produce any error, or if an error is produced
+    ///   only once just before terminating the iterator, using `map_while()` is not
+    ///   necessary but will not do any harm.
+    /// - If the `Lines` instance can produce intermittent errors then recover and produce
+    ///   successful results, using `map_while()` would stop at the first error.
+    ///
+    /// ### Example
+    /// ```rust
+    /// # use std::{fs::File, io::{self, BufRead, BufReader}};
+    /// # let _ = || -> io::Result<()> {
+    /// let mut lines = BufReader::new(File::open("some-path")?).lines().filter_map(Result::ok);
+    /// // If "some-path" points to a directory, the next statement never terminates:
+    /// let first_line: Option<String> = lines.next();
+    /// # Ok(()) };
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// # use std::{fs::File, io::{self, BufRead, BufReader}};
+    /// # let _ = || -> io::Result<()> {
+    /// let mut lines = BufReader::new(File::open("some-path")?).lines().map_while(Result::ok);
+    /// let first_line: Option<String> = lines.next();
+    /// # Ok(()) };
+    /// ```
+    #[clippy::version = "1.70.0"]
+    pub LINES_FILTER_MAP_OK,
+    suspicious,
+    "filtering `std::io::Lines` with `filter_map()` or `flat_map()` might cause an infinite loop"
+}
+declare_lint_pass!(LinesFilterMapOk => [LINES_FILTER_MAP_OK]);
+
+impl LateLintPass<'_> for LinesFilterMapOk {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        if let ExprKind::MethodCall(fm_method, fm_receiver, [fm_arg], fm_span) = expr.kind &&
+            is_trait_method(cx, expr, sym::Iterator) &&
+            (fm_method.ident.as_str() == "filter_map" || fm_method.ident.as_str() == "flat_map") &&
+            match_type(cx, cx.typeck_results().expr_ty_adjusted(fm_receiver), &paths::STD_IO_LINES)
+        {
+            let lint = match &fm_arg.kind {
+                // Detect `Result::ok`
+                ExprKind::Path(qpath) =>
+                    cx.qpath_res(qpath, fm_arg.hir_id).opt_def_id().map(|did|
+                        match_def_path(cx, did, &paths::CORE_RESULT_OK_METHOD)).unwrap_or_default(),
+                // Detect `|x| x.ok()`
+                ExprKind::Closure(Closure { body, .. }) =>
+                    if let Body { params: [param], value, .. } = cx.tcx.hir().body(*body) &&
+                        let ExprKind::MethodCall(method, receiver, [], _) = value.kind &&
+                        path_to_local_id(receiver, param.pat.hir_id) &&
+                        let Some(method_did) = cx.typeck_results().type_dependent_def_id(value.hir_id)
+                    {
+                        is_diag_item_method(cx, method_did, sym::Result) && method.ident.as_str() == "ok"
+                    } else {
+                        false
+                    }
+                _ => false,
+            };
+            if lint {
+                span_lint_and_then(cx,
+                    LINES_FILTER_MAP_OK,
+                    fm_span,
+                    &format!("`{}()` will run forever if the iterator repeatedly produces an `Err`", fm_method.ident),
+                    |diag| {
+                        diag.span_note(
+                            fm_receiver.span,
+                            "this expression returning a `std::io::Lines` may produce an infinite number of `Err` in case of a read error");
+                        diag.span_suggestion(fm_span, "replace with", "map_while(Result::ok)", Applicability::MaybeIncorrect);
+                    });
+                }
+        }
+    }
+}