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

8 files changed, 215 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index f54bfbfa472..239631777e9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4647,6 +4647,7 @@ Released 2018-09-13
 [`let_underscore_untyped`]: https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_untyped
 [`let_unit_value`]: https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value
 [`let_with_type_underscore`]: https://rust-lang.github.io/rust-clippy/master/index.html#let_with_type_underscore
+[`lines_filter_map_ok`]: https://rust-lang.github.io/rust-clippy/master/index.html#lines_filter_map_ok
 [`linkedlist`]: https://rust-lang.github.io/rust-clippy/master/index.html#linkedlist
 [`logic_bug`]: https://rust-lang.github.io/rust-clippy/master/index.html#logic_bug
 [`lossy_float_literal`]: https://rust-lang.github.io/rust-clippy/master/index.html#lossy_float_literal
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);
+                    });
+                }
+        }
+    }
+}
diff --git a/clippy_utils/src/paths.rs b/clippy_utils/src/paths.rs
index c919575bfe9..9be2d0eae80 100644
--- a/clippy_utils/src/paths.rs
+++ b/clippy_utils/src/paths.rs
@@ -23,6 +23,7 @@ pub const CLONE_TRAIT_METHOD: [&str; 4] = ["core", "clone", "Clone", "clone"];
 pub const CORE_ITER_CLONED: [&str; 6] = ["core", "iter", "traits", "iterator", "Iterator", "cloned"];
 pub const CORE_ITER_COPIED: [&str; 6] = ["core", "iter", "traits", "iterator", "Iterator", "copied"];
 pub const CORE_ITER_FILTER: [&str; 6] = ["core", "iter", "traits", "iterator", "Iterator", "filter"];
+pub const CORE_RESULT_OK_METHOD: [&str; 4] = ["core", "result", "Result", "ok"];
 pub const CSTRING_AS_C_STR: [&str; 5] = ["alloc", "ffi", "c_str", "CString", "as_c_str"];
 pub const DEFAULT_TRAIT_METHOD: [&str; 4] = ["core", "default", "Default", "default"];
 pub const DEREF_MUT_TRAIT_METHOD: [&str; 5] = ["core", "ops", "deref", "DerefMut", "deref_mut"];
@@ -113,6 +114,7 @@ pub const STDERR: [&str; 4] = ["std", "io", "stdio", "stderr"];
 pub const STDOUT: [&str; 4] = ["std", "io", "stdio", "stdout"];
 pub const CONVERT_IDENTITY: [&str; 3] = ["core", "convert", "identity"];
 pub const STD_FS_CREATE_DIR: [&str; 3] = ["std", "fs", "create_dir"];
+pub const STD_IO_LINES: [&str; 3] = ["std", "io", "Lines"];
 pub const STD_IO_SEEK: [&str; 3] = ["std", "io", "Seek"];
 pub const STD_IO_SEEK_FROM_CURRENT: [&str; 4] = ["std", "io", "SeekFrom", "Current"];
 pub const STD_IO_SEEKFROM_START: [&str; 4] = ["std", "io", "SeekFrom", "Start"];
diff --git a/tests/ui/lines_filter_map_ok.fixed b/tests/ui/lines_filter_map_ok.fixed
new file mode 100644
index 00000000000..f4033cd8ed8
--- /dev/null
+++ b/tests/ui/lines_filter_map_ok.fixed
@@ -0,0 +1,29 @@
+// run-rustfix
+
+#![allow(unused, clippy::map_identity)]
+#![warn(clippy::lines_filter_map_ok)]
+
+use std::io::{self, BufRead, BufReader};
+
+fn main() -> io::Result<()> {
+    let f = std::fs::File::open("/")?;
+    // Lint
+    BufReader::new(f).lines().map_while(Result::ok).for_each(|_| ());
+    // Lint
+    let f = std::fs::File::open("/")?;
+    BufReader::new(f).lines().map_while(Result::ok).for_each(|_| ());
+    let s = "foo\nbar\nbaz\n";
+    // Lint
+    io::stdin().lines().map_while(Result::ok).for_each(|_| ());
+    // Lint
+    io::stdin().lines().map_while(Result::ok).for_each(|_| ());
+    // Do not lint (not a `Lines` iterator)
+    io::stdin()
+        .lines()
+        .map(std::convert::identity)
+        .filter_map(|x| x.ok())
+        .for_each(|_| ());
+    // Do not lint (not a `Result::ok()` extractor)
+    io::stdin().lines().filter_map(|x| x.err()).for_each(|_| ());
+    Ok(())
+}
diff --git a/tests/ui/lines_filter_map_ok.rs b/tests/ui/lines_filter_map_ok.rs
new file mode 100644
index 00000000000..7e11816b2ac
--- /dev/null
+++ b/tests/ui/lines_filter_map_ok.rs
@@ -0,0 +1,29 @@
+// run-rustfix
+
+#![allow(unused, clippy::map_identity)]
+#![warn(clippy::lines_filter_map_ok)]
+
+use std::io::{self, BufRead, BufReader};
+
+fn main() -> io::Result<()> {
+    let f = std::fs::File::open("/")?;
+    // Lint
+    BufReader::new(f).lines().filter_map(Result::ok).for_each(|_| ());
+    // Lint
+    let f = std::fs::File::open("/")?;
+    BufReader::new(f).lines().flat_map(Result::ok).for_each(|_| ());
+    let s = "foo\nbar\nbaz\n";
+    // Lint
+    io::stdin().lines().filter_map(Result::ok).for_each(|_| ());
+    // Lint
+    io::stdin().lines().filter_map(|x| x.ok()).for_each(|_| ());
+    // Do not lint (not a `Lines` iterator)
+    io::stdin()
+        .lines()
+        .map(std::convert::identity)
+        .filter_map(|x| x.ok())
+        .for_each(|_| ());
+    // Do not lint (not a `Result::ok()` extractor)
+    io::stdin().lines().filter_map(|x| x.err()).for_each(|_| ());
+    Ok(())
+}
diff --git a/tests/ui/lines_filter_map_ok.stderr b/tests/ui/lines_filter_map_ok.stderr
new file mode 100644
index 00000000000..cddd403d589
--- /dev/null
+++ b/tests/ui/lines_filter_map_ok.stderr
@@ -0,0 +1,51 @@
+error: `filter_map()` will run forever if the iterator repeatedly produces an `Err`
+  --> $DIR/lines_filter_map_ok.rs:11:31
+   |
+LL |     BufReader::new(f).lines().filter_map(Result::ok).for_each(|_| ());
+   |                               ^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `map_while(Result::ok)`
+   |
+note: this expression returning a `std::io::Lines` may produce an infinite number of `Err` in case of a read error
+  --> $DIR/lines_filter_map_ok.rs:11:5
+   |
+LL |     BufReader::new(f).lines().filter_map(Result::ok).for_each(|_| ());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: `-D clippy::lines-filter-map-ok` implied by `-D warnings`
+
+error: `flat_map()` will run forever if the iterator repeatedly produces an `Err`
+  --> $DIR/lines_filter_map_ok.rs:14:31
+   |
+LL |     BufReader::new(f).lines().flat_map(Result::ok).for_each(|_| ());
+   |                               ^^^^^^^^^^^^^^^^^^^^ help: replace with: `map_while(Result::ok)`
+   |
+note: this expression returning a `std::io::Lines` may produce an infinite number of `Err` in case of a read error
+  --> $DIR/lines_filter_map_ok.rs:14:5
+   |
+LL |     BufReader::new(f).lines().flat_map(Result::ok).for_each(|_| ());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `filter_map()` will run forever if the iterator repeatedly produces an `Err`
+  --> $DIR/lines_filter_map_ok.rs:17:25
+   |
+LL |     io::stdin().lines().filter_map(Result::ok).for_each(|_| ());
+   |                         ^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `map_while(Result::ok)`
+   |
+note: this expression returning a `std::io::Lines` may produce an infinite number of `Err` in case of a read error
+  --> $DIR/lines_filter_map_ok.rs:17:5
+   |
+LL |     io::stdin().lines().filter_map(Result::ok).for_each(|_| ());
+   |     ^^^^^^^^^^^^^^^^^^^
+
+error: `filter_map()` will run forever if the iterator repeatedly produces an `Err`
+  --> $DIR/lines_filter_map_ok.rs:19:25
+   |
+LL |     io::stdin().lines().filter_map(|x| x.ok()).for_each(|_| ());
+   |                         ^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `map_while(Result::ok)`
+   |
+note: this expression returning a `std::io::Lines` may produce an infinite number of `Err` in case of a read error
+  --> $DIR/lines_filter_map_ok.rs:19:5
+   |
+LL |     io::stdin().lines().filter_map(|x| x.ok()).for_each(|_| ());
+   |     ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+