add owned_cow lint (rust-lang#13948)

Closes rust-lang#13697.

---

changelog: add [`owned_cow`] lint

Author: y21

CHANGELOG.md

@@ -5984,6 +5984,7 @@ Released 2018-09-13
 [`out_of_bounds_indexing`]: https://rust-lang.github.io/rust-clippy/master/index.html#out_of_bounds_indexing
 [`overflow_check_conditional`]: https://rust-lang.github.io/rust-clippy/master/index.html#overflow_check_conditional
 [`overly_complex_bool_expr`]: https://rust-lang.github.io/rust-clippy/master/index.html#overly_complex_bool_expr
+[`owned_cow`]: https://rust-lang.github.io/rust-clippy/master/index.html#owned_cow
 [`panic`]: https://rust-lang.github.io/rust-clippy/master/index.html#panic
 [`panic_in_result_fn`]: https://rust-lang.github.io/rust-clippy/master/index.html#panic_in_result_fn
 [`panic_params`]: https://rust-lang.github.io/rust-clippy/master/index.html#panic_params

book/src/lint_configuration.md

@@ -380,6 +380,7 @@ Suppress lints whenever the suggested change would cause breakage for other crat
 * [`linkedlist`](https://rust-lang.github.io/rust-clippy/master/index.html#linkedlist)
 * [`needless_pass_by_ref_mut`](https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut)
 * [`option_option`](https://rust-lang.github.io/rust-clippy/master/index.html#option_option)
+* [`owned_cow`](https://rust-lang.github.io/rust-clippy/master/index.html#owned_cow)
 * [`rc_buffer`](https://rust-lang.github.io/rust-clippy/master/index.html#rc_buffer)
 * [`rc_mutex`](https://rust-lang.github.io/rust-clippy/master/index.html#rc_mutex)
 * [`redundant_allocation`](https://rust-lang.github.io/rust-clippy/master/index.html#redundant_allocation)

clippy_config/src/conf.rs

@@ -438,6 +438,7 @@ define_Conf! {
         linkedlist,
         needless_pass_by_ref_mut,
         option_option,
+        owned_cow,
         rc_buffer,
         rc_mutex,
         redundant_allocation,

clippy_lints/src/declared_lints.rs

@@ -745,6 +745,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::types::BOX_COLLECTION_INFO,
     crate::types::LINKEDLIST_INFO,
     crate::types::OPTION_OPTION_INFO,
+    crate::types::OWNED_COW_INFO,
     crate::types::RC_BUFFER_INFO,
     crate::types::RC_MUTEX_INFO,
     crate::types::REDUNDANT_ALLOCATION_INFO,

clippy_lints/src/types/mod.rs

@@ -2,6 +2,7 @@ mod borrowed_box;
 mod box_collection;
 mod linked_list;
 mod option_option;
+mod owned_cow;
 mod rc_buffer;
 mod rc_mutex;
 mod redundant_allocation;
@@ -355,13 +356,63 @@ declare_clippy_lint! {
     "usage of `Rc<Mutex<T>>`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Detects needlessly owned `Cow` types.
+    ///
+    /// ### Why is this bad?
+    /// The borrowed types are usually more flexible, in that e.g. a
+    /// `Cow<'_, str>` can accept both `&str` and `String` while
+    /// `Cow<'_, String>` can only accept `&String` and `String`. In
+    /// particular, `&str` is more general, because it allows for string
+    /// literals while `&String` can only be borrowed from a heap-owned
+    /// `String`).
+    ///
+    /// ### Known Problems
+    /// The lint does not check for usage of the type. There may be external
+    /// interfaces that require the use of an owned type.
+    ///
+    /// At least the `CString` type also has a different API than `CStr`: The
+    /// former has an `as_bytes` method which the latter calls `to_bytes`.
+    /// There is no guarantee that other types won't gain additional methods
+    /// leading to a similar mismatch.
+    ///
+    /// In addition, the lint only checks for the known problematic types
+    /// `String`, `Vec<_>`, `CString`, `OsString` and `PathBuf`. Custom types
+    /// that implement `ToOwned` will not be detected.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let wrogn: std::borrow::Cow<'_, Vec<u8>>;
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// let right: std::borrow::Cow<'_, [u8]>;
+    /// ```
+    #[clippy::version = "1.85.0"]
+    pub OWNED_COW,
+    style,
+    "needlessly owned Cow type"
+}
+
 pub struct Types {
     vec_box_size_threshold: u64,
     type_complexity_threshold: u64,
     avoid_breaking_exported_api: bool,
 }
 
-impl_lint_pass!(Types => [BOX_COLLECTION, VEC_BOX, OPTION_OPTION, LINKEDLIST, BORROWED_BOX, REDUNDANT_ALLOCATION, RC_BUFFER, RC_MUTEX, TYPE_COMPLEXITY]);
+impl_lint_pass!(Types => [
+    BOX_COLLECTION,
+    VEC_BOX,
+    OPTION_OPTION,
+    LINKEDLIST,
+    BORROWED_BOX,
+    REDUNDANT_ALLOCATION,
+    RC_BUFFER,
+    RC_MUTEX,
+    TYPE_COMPLEXITY,
+    OWNED_COW
+]);
 
 impl<'tcx> LateLintPass<'tcx> for Types {
     fn check_fn(
@@ -561,6 +612,7 @@ impl Types {
                         triggered |= option_option::check(cx, hir_ty, qpath, def_id);
                         triggered |= linked_list::check(cx, hir_ty, def_id);
                         triggered |= rc_mutex::check(cx, hir_ty, qpath, def_id);
+                        triggered |= owned_cow::check(cx, qpath, def_id);
 
                         if triggered {
                             return;
@@ -612,6 +664,14 @@ impl Types {
                     QPath::LangItem(..) => {},
                 }
             },
+            TyKind::Path(ref qpath) => {
+                let res = cx.qpath_res(qpath, hir_ty.hir_id);
+                if let Some(def_id) = res.opt_def_id()
+                    && self.is_type_change_allowed(context)
+                {
+                    owned_cow::check(cx, qpath, def_id);
+                }
+            },
             TyKind::Ref(lt, ref mut_ty) => {
                 context.is_nested_call = true;
                 if !borrowed_box::check(cx, hir_ty, lt, mut_ty) {

clippy_lints/src/types/owned_cow.rs

@@ -0,0 +1,66 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::source::snippet_opt;
+use rustc_errors::Applicability;
+use rustc_hir::def_id::DefId;
+use rustc_hir::{self as hir};
+use rustc_lint::LateContext;
+use rustc_span::{Span, sym};
+
+pub(super) fn check(cx: &LateContext<'_>, qpath: &hir::QPath<'_>, def_id: DefId) -> bool {
+    if cx.tcx.is_diagnostic_item(sym::Cow, def_id)
+        && let hir::QPath::Resolved(_, path) = qpath
+        && let [.., last_seg] = path.segments
+        && let Some(args) = last_seg.args
+        && let [_lt, carg] = args.args
+        && let hir::GenericArg::Type(cty) = carg
+        && let Some((span, repl)) = replacement(cx, cty.as_unambig_ty())
+    {
+        span_lint_and_sugg(
+            cx,
+            super::OWNED_COW,
+            span,
+            "needlessly owned Cow type",
+            "use",
+            repl,
+            Applicability::Unspecified,
+        );
+        return true;
+    }
+    false
+}
+
+fn replacement(cx: &LateContext<'_>, cty: &hir::Ty<'_>) -> Option<(Span, String)> {
+    if clippy_utils::is_path_lang_item(cx, cty, hir::LangItem::String) {
+        return Some((cty.span, "str".into()));
+    }
+    if clippy_utils::is_path_diagnostic_item(cx, cty, sym::Vec) {
+        return if let hir::TyKind::Path(hir::QPath::Resolved(_, path)) = cty.kind
+            && let [.., last_seg] = path.segments
+            && let Some(args) = last_seg.args
+            && let [t, ..] = args.args
+            && let Some(snip) = snippet_opt(cx, t.span())
+        {
+            Some((cty.span, format!("[{snip}]")))
+        } else {
+            None
+        };
+    }
+    if clippy_utils::is_path_diagnostic_item(cx, cty, sym::cstring_type) {
+        return Some((
+            cty.span,
+            (if clippy_utils::is_no_std_crate(cx) {
+                "core::ffi::CStr"
+            } else {
+                "std::ffi::CStr"
+            })
+            .into(),
+        ));
+    }
+    // Neither OsString nor PathBuf are available outside std
+    for (diag, repl) in [(sym::OsString, "std::ffi::OsStr"), (sym::PathBuf, "std::path::Path")] {
+        if clippy_utils::is_path_diagnostic_item(cx, cty, diag) {
+            return Some((cty.span, repl.into()));
+        }
+    }
+    None
+}

tests/ui/owned_cow.fixed

@@ -0,0 +1,23 @@
+#![warn(clippy::owned_cow)]
+
+use std::borrow::Cow;
+use std::ffi::{CString, OsString};
+use std::path::PathBuf;
+
+fn main() {
+    let x: Cow<'static, str> = Cow::Owned(String::from("Hi!"));
+    //~^ ERROR: needlessly owned Cow type
+    let y: Cow<'_, [u8]> = Cow::Owned(vec![]);
+    //~^ ERROR: needlessly owned Cow type
+    let z: Cow<'_, [_]> = Cow::Owned(vec![2_i32]);
+    //~^ ERROR: needlessly owned Cow type
+    let o: Cow<'_, std::ffi::OsStr> = Cow::Owned(OsString::new());
+    //~^ ERROR: needlessly owned Cow type
+    let c: Cow<'_, std::ffi::CStr> = Cow::Owned(CString::new("").unwrap());
+    //~^ ERROR: needlessly owned Cow type
+    let p: Cow<'_, std::path::Path> = Cow::Owned(PathBuf::new());
+    //~^ ERROR: needlessly owned Cow type
+
+    // false positive: borrowed type
+    let b: Cow<'_, str> = Cow::Borrowed("Hi!");
+}

tests/ui/owned_cow.rs

@@ -0,0 +1,23 @@
+#![warn(clippy::owned_cow)]
+
+use std::borrow::Cow;
+use std::ffi::{CString, OsString};
+use std::path::PathBuf;
+
+fn main() {
+    let x: Cow<'static, String> = Cow::Owned(String::from("Hi!"));
+    //~^ ERROR: needlessly owned Cow type
+    let y: Cow<'_, Vec<u8>> = Cow::Owned(vec![]);
+    //~^ ERROR: needlessly owned Cow type
+    let z: Cow<'_, Vec<_>> = Cow::Owned(vec![2_i32]);
+    //~^ ERROR: needlessly owned Cow type
+    let o: Cow<'_, OsString> = Cow::Owned(OsString::new());
+    //~^ ERROR: needlessly owned Cow type
+    let c: Cow<'_, CString> = Cow::Owned(CString::new("").unwrap());
+    //~^ ERROR: needlessly owned Cow type
+    let p: Cow<'_, PathBuf> = Cow::Owned(PathBuf::new());
+    //~^ ERROR: needlessly owned Cow type
+
+    // false positive: borrowed type
+    let b: Cow<'_, str> = Cow::Borrowed("Hi!");
+}

tests/ui/owned_cow.stderr

@@ -0,0 +1,41 @@
+error: needlessly owned Cow type
+  --> tests/ui/owned_cow.rs:8:25
+   |
+LL |     let x: Cow<'static, String> = Cow::Owned(String::from("Hi!"));
+   |                         ^^^^^^ help: use: `str`
+   |
+   = note: `-D clippy::owned-cow` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::owned_cow)]`
+
+error: needlessly owned Cow type
+  --> tests/ui/owned_cow.rs:10:20
+   |
+LL |     let y: Cow<'_, Vec<u8>> = Cow::Owned(vec![]);
+   |                    ^^^^^^^ help: use: `[u8]`
+
+error: needlessly owned Cow type
+  --> tests/ui/owned_cow.rs:12:20
+   |
+LL |     let z: Cow<'_, Vec<_>> = Cow::Owned(vec![2_i32]);
+   |                    ^^^^^^ help: use: `[_]`
+
+error: needlessly owned Cow type
+  --> tests/ui/owned_cow.rs:14:20
+   |
+LL |     let o: Cow<'_, OsString> = Cow::Owned(OsString::new());
+   |                    ^^^^^^^^ help: use: `std::ffi::OsStr`
+
+error: needlessly owned Cow type
+  --> tests/ui/owned_cow.rs:16:20
+   |
+LL |     let c: Cow<'_, CString> = Cow::Owned(CString::new("").unwrap());
+   |                    ^^^^^^^ help: use: `std::ffi::CStr`
+
+error: needlessly owned Cow type
+  --> tests/ui/owned_cow.rs:18:20
+   |
+LL |     let p: Cow<'_, PathBuf> = Cow::Owned(PathBuf::new());
+   |                    ^^^^^^^ help: use: `std::path::Path`
+
+error: aborting due to 6 previous errors
+

tests/ui/unnecessary_to_owned.fixed

@@ -3,7 +3,8 @@
     clippy::needless_borrows_for_generic_args,
     clippy::ptr_arg,
     clippy::manual_async_fn,
-    clippy::needless_lifetimes
+    clippy::needless_lifetimes,
+    clippy::owned_cow
 )]
 #![warn(clippy::unnecessary_to_owned, clippy::redundant_clone)]
 

tests/ui/unnecessary_to_owned.rs

@@ -3,7 +3,8 @@
     clippy::needless_borrows_for_generic_args,
     clippy::ptr_arg,
     clippy::manual_async_fn,
-    clippy::needless_lifetimes
+    clippy::needless_lifetimes,
+    clippy::owned_cow
 )]
 #![warn(clippy::unnecessary_to_owned, clippy::redundant_clone)]