Add lint logic.

clippy_lints/src/methods/mod.rs

@@ -1275,6 +1275,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Methods {
             ["add"] | ["offset"] | ["sub"] | ["wrapping_offset"] | ["wrapping_add"] | ["wrapping_sub"] => {
                 check_pointer_offset(cx, expr, arg_lists[0])
             },
+            ["is_file", ..] => lint_filetype_is_file(cx, expr, arg_lists[0]),
             _ => {},
         }
 
@@ -3259,3 +3260,26 @@ fn check_pointer_offset(cx: &LateContext<'_, '_>, expr: &hir::Expr<'_>, args: &[
         }
     }
 }
+
+fn lint_filetype_is_file(cx: &LateContext<'_, '_>, expr: &hir::Expr<'_>, args: &[hir::Expr<'_>]) {
+    let ty = cx.tables.expr_ty(&args[0]);
+
+    if !match_type(cx, ty, &paths::FILE_TYPE) {
+        return;
+    }
+
+    if_chain! {
+        if let Some(parent) = get_parent_expr(cx, expr);
+        if let hir::ExprKind::Unary(op, _) = parent.kind;
+        if op == hir::UnNot;
+        then {
+            let lint_msg = "`!FileType::is_file()` does not deny all readable file types";
+            let help_msg = "use `FileType::is_dir()` instead";
+            span_help_and_lint(cx, FILETYPE_IS_FILE, expr.span, lint_msg, help_msg);
+        } else {
+            let lint_msg = "`FileType::is_file()` does not cover all readable file types";
+            let help_msg = "use `!FileType::is_dir()` instead";
+            span_help_and_lint(cx, FILETYPE_IS_FILE, expr.span, lint_msg, help_msg);
+        }
+    }
+}

clippy_lints/src/utils/paths.rs

@@ -28,6 +28,7 @@ pub const DROP_TRAIT: [&str; 4] = ["core", "ops", "drop", "Drop"];
 pub const DURATION: [&str; 3] = ["core", "time", "Duration"];
 pub const EARLY_CONTEXT: [&str; 4] = ["rustc", "lint", "context", "EarlyContext"];
 pub const EXIT: [&str; 3] = ["std", "process", "exit"];
+pub const FILE_TYPE: [&str; 3] = ["std", "fs", "FileType"];
 pub const FMT_ARGUMENTS_NEW_V1: [&str; 4] = ["core", "fmt", "Arguments", "new_v1"];
 pub const FMT_ARGUMENTS_NEW_V1_FORMATTED: [&str; 4] = ["core", "fmt", "Arguments", "new_v1_formatted"];
 pub const FMT_ARGUMENTV1_NEW: [&str; 4] = ["core", "fmt", "ArgumentV1", "new"];

tests/ui/filetype_is_file.stderr

@@ -0,0 +1,27 @@
+error: `FileType::is_file()` does not cover all readable file types
+  --> $DIR/filetype_is_file.rs:8:8
+   |
+LL |     if fs::metadata("foo.txt")?.file_type().is_file() {
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::filetype-is-file` implied by `-D warnings`
+   = help: use `!FileType::is_dir()` instead
+
+error: `!FileType::is_file()` does not deny all readable file types
+  --> $DIR/filetype_is_file.rs:13:9
+   |
+LL |     if !fs::metadata("foo.txt")?.file_type().is_file() {
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use `FileType::is_dir()` instead
+
+error: `FileType::is_file()` does not cover all readable file types
+  --> $DIR/filetype_is_file.rs:18:9
+   |
+LL |     if !fs::metadata("foo.txt")?.file_type().is_file().bitor(true) {
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use `!FileType::is_dir()` instead
+
+error: aborting due to 3 previous errors
+