fix(missing_trait_methods): MSRV/unstable awareness

changelog: [`missing_trait_methods`]: do not diagnose unimplemented unstable trait fn's unless unstable feature is enabled

Simply reusing the stability check that is already used by [`len_zero`].
I suspect that more lints need a fix similar to this one,
likely everything that handles MSRV.

As per comments in the PR, the simple check for a feature is insufficient,
so to avoid false-positives, support for unstable is dropped.

Inability to write hermetic tests is *very* problematic here,
it's basically impossible to test that we actually don't lint the
unstable methods. This is bad.

Author: LebedevRI

clippy_lints/src/len_zero.rs

@@ -9,7 +9,7 @@ use clippy_utils::{parent_item_name, peel_ref_operators, sym};
 use rustc_ast::ast::LitKind;
 use rustc_errors::Applicability;
 use rustc_hir::def_id::DefId;
-use rustc_hir::{BinOpKind, Expr, ExprKind, PatExprKind, PatKind, RustcVersion, StabilityLevel, StableSince};
+use rustc_hir::{BinOpKind, Expr, ExprKind, PatExprKind, PatKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty::{self, Ty};
 use rustc_session::impl_lint_pass;
@@ -298,21 +298,7 @@ fn has_is_empty(cx: &LateContext<'_>, expr: &Expr<'_>, msrv: Msrv) -> bool {
         if item.is_fn() {
             let sig = cx.tcx.fn_sig(item.def_id).skip_binder();
             let ty = sig.skip_binder();
-            ty.inputs().len() == 1
-                && cx.tcx.lookup_stability(item.def_id).is_none_or(|stability| {
-                    if let StabilityLevel::Stable { since, .. } = stability.level {
-                        let version = match since {
-                            StableSince::Version(version) => version,
-                            StableSince::Current => RustcVersion::CURRENT,
-                            StableSince::Err(_) => return false,
-                        };
-
-                        msrv.meets(cx, version)
-                    } else {
-                        // Unstable fn, check if the feature is enabled.
-                        cx.tcx.features().enabled(stability.feature) && msrv.current(cx).is_none()
-                    }
-                })
+            ty.inputs().len() == 1 && msrv.is_stable(cx, item.def_id)
         } else {
             false
         }

clippy_lints/src/lib.rs

@@ -764,7 +764,7 @@ rustc_lint::late_lint_methods!(
         BoolToIntWithIf: bool_to_int_with_if::BoolToIntWithIf = bool_to_int_with_if::BoolToIntWithIf,
         BoxDefault: box_default::BoxDefault = box_default::BoxDefault,
         ImplicitSaturatingAdd: implicit_saturating_add::ImplicitSaturatingAdd = implicit_saturating_add::ImplicitSaturatingAdd,
-        MissingTraitMethods: missing_trait_methods::MissingTraitMethods = missing_trait_methods::MissingTraitMethods,
+        MissingTraitMethods: missing_trait_methods::MissingTraitMethods = missing_trait_methods::MissingTraitMethods::new(conf),
         FromRawWithVoidPtr: from_raw_with_void_ptr::FromRawWithVoidPtr = from_raw_with_void_ptr::FromRawWithVoidPtr,
         ConfusingXorAndPow: suspicious_xor_used_as_pow::ConfusingXorAndPow = suspicious_xor_used_as_pow::ConfusingXorAndPow,
         ManualIsAsciiCheck: manual_is_ascii_check::ManualIsAsciiCheck = manual_is_ascii_check::ManualIsAsciiCheck::new(conf),

clippy_lints/src/missing_trait_methods.rs

@@ -1,11 +1,13 @@
+use clippy_config::Conf;
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::is_lint_allowed;
 use clippy_utils::macros::span_is_local;
+use clippy_utils::msrvs::Msrv;
 use clippy_utils::source::snippet_opt;
 use rustc_hir::def_id::DefIdSet;
 use rustc_hir::{Impl, Item, ItemKind};
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_session::declare_lint_pass;
+use rustc_session::impl_lint_pass;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -56,7 +58,17 @@ declare_clippy_lint! {
     "trait implementation uses default provided method"
 }
 
-declare_lint_pass!(MissingTraitMethods => [MISSING_TRAIT_METHODS]);
+impl_lint_pass!(MissingTraitMethods => [MISSING_TRAIT_METHODS]);
+
+pub struct MissingTraitMethods {
+    msrv: Msrv,
+}
+
+impl MissingTraitMethods {
+    pub fn new(conf: &'static Conf) -> Self {
+        Self { msrv: conf.msrv }
+    }
+}
 
 impl<'tcx> LateLintPass<'tcx> for MissingTraitMethods {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'tcx>) {
@@ -79,6 +91,7 @@ impl<'tcx> LateLintPass<'tcx> for MissingTraitMethods {
                 .tcx
                 .provided_trait_methods(trait_id)
                 .filter(|assoc| !trait_item_ids.contains(&assoc.def_id))
+                .filter(|assoc| self.msrv.is_stable(cx, assoc.def_id))
             {
                 span_lint_and_then(
                     cx,

clippy_utils/src/msrvs.rs

@@ -3,7 +3,8 @@ use rustc_ast::Attribute;
 use rustc_ast::attr::AttributeExt;
 use rustc_attr_parsing::parse_version;
 use rustc_data_structures::smallvec::SmallVec;
-use rustc_hir::{HirId, RustcVersion};
+use rustc_hir::def_id::DefId;
+use rustc_hir::{HirId, RustcVersion, StabilityLevel, StableSince};
 use rustc_lint::LateContext;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::Session;
@@ -175,6 +176,25 @@ impl Msrv {
             _ => {},
         }
     }
+
+    pub fn is_stable(self, cx: &LateContext<'_>, def_id: DefId) -> bool {
+        cx.tcx.lookup_stability(def_id).is_none_or(|stability| {
+            if let StabilityLevel::Stable { since, .. } = stability.level {
+                let version = match since {
+                    StableSince::Version(version) => version,
+                    StableSince::Current => RustcVersion::CURRENT,
+                    StableSince::Err(_) => return false,
+                };
+
+                self.meets(cx, version)
+            } else {
+                // Unstable fn.
+                // FIXME: can we check that the feature is enabled?
+                // Please see https://github.com/rust-lang/rust-clippy/pull/17309#discussion_r3486693263 for false-positive concerns.
+                false
+            }
+        })
+    }
 }
 
 /// Tracks the current MSRV from `clippy.toml`, `Cargo.toml` or set via `#[clippy::msrv]` in early

tests/ui/len_zero_unstable.fixed

@@ -1,5 +1,6 @@
 #![warn(clippy::missing_trait_methods)]
 #![expect(clippy::needless_lifetimes)]
+#![allow(clippy::derive_ord_xor_partial_ord)]
 
 trait A {
     fn provided() {}
@@ -69,3 +70,29 @@ impl PartialEq<Partial> for Partial {
         todo!()
     }
 }
+
+#[clippy::msrv = "1.20.0"]
+fn msrv0() {
+    #[derive(PartialEq, Eq, PartialOrd)]
+    struct S {}
+
+    impl Ord for S {
+        fn cmp(&self, other: &S) -> std::cmp::Ordering {
+            unreachable!()
+        }
+    }
+}
+
+#[clippy::msrv = "1.21.0"]
+fn msrv1() {
+    #[derive(PartialEq, Eq, PartialOrd)]
+    struct S {}
+
+    impl Ord for S {
+        //~^ missing_trait_methods
+        //~| missing_trait_methods
+        fn cmp(&self, other: &S) -> std::cmp::Ordering {
+            unreachable!()
+        }
+    }
+}

tests/ui/len_zero_unstable.rs

@@ -1,72 +1,88 @@
 error: missing trait method provided by default: `provided`
-  --> tests/ui/missing_trait_methods.rs:22:1
+  --> tests/ui/missing_trait_methods.rs:23:1
    |
 LL | impl A for Partial {}
    | ^^^^^^^^^^^^^^^^^^
    |
 help: implement the method
-  --> tests/ui/missing_trait_methods.rs:5:5
+  --> tests/ui/missing_trait_methods.rs:6:5
    |
 LL |     fn provided() {}
    |     ^^^^^^^^^^^^^
    = note: `-D clippy::missing-trait-methods` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::missing_trait_methods)]`
 
 error: missing trait method provided by default: `b`
-  --> tests/ui/missing_trait_methods.rs:25:1
+  --> tests/ui/missing_trait_methods.rs:26:1
    |
 LL | impl B for Partial {
    | ^^^^^^^^^^^^^^^^^^
    |
 help: implement the method
-  --> tests/ui/missing_trait_methods.rs:15:5
+  --> tests/ui/missing_trait_methods.rs:16:5
    |
 LL |     fn b<'a, T: AsRef<[u8]>>(a: &'a T) -> &'a [u8] {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: missing trait method provided by default: `one`
-  --> tests/ui/missing_trait_methods.rs:59:1
+  --> tests/ui/missing_trait_methods.rs:60:1
    |
 LL | impl MissingMultiple for Partial {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 help: implement the method
-  --> tests/ui/missing_trait_methods.rs:54:5
+  --> tests/ui/missing_trait_methods.rs:55:5
    |
 LL |     fn one() {}
    |     ^^^^^^^^
 
 error: missing trait method provided by default: `two`
-  --> tests/ui/missing_trait_methods.rs:59:1
+  --> tests/ui/missing_trait_methods.rs:60:1
    |
 LL | impl MissingMultiple for Partial {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 help: implement the method
-  --> tests/ui/missing_trait_methods.rs:55:5
+  --> tests/ui/missing_trait_methods.rs:56:5
    |
 LL |     fn two() {}
    |     ^^^^^^^^
 
 error: missing trait method provided by default: `three`
-  --> tests/ui/missing_trait_methods.rs:59:1
+  --> tests/ui/missing_trait_methods.rs:60:1
    |
 LL | impl MissingMultiple for Partial {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 help: implement the method
-  --> tests/ui/missing_trait_methods.rs:56:5
+  --> tests/ui/missing_trait_methods.rs:57:5
    |
 LL |     fn three() {}
    |     ^^^^^^^^^^
 
 error: missing trait method provided by default: `ne`
-  --> tests/ui/missing_trait_methods.rs:67:1
+  --> tests/ui/missing_trait_methods.rs:68:1
    |
 LL | impl PartialEq<Partial> for Partial {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: implement the missing `ne` method of the `PartialEq<Partial>` trait
 
-error: aborting due to 6 previous errors
+error: missing trait method provided by default: `max`
+  --> tests/ui/missing_trait_methods.rs:91:5
+   |
+LL |     impl Ord for S {
+   |     ^^^^^^^^^^^^^^
+   |
+   = help: implement the missing `max` method of the `Ord` trait
+
+error: missing trait method provided by default: `min`
+  --> tests/ui/missing_trait_methods.rs:91:5
+   |
+LL |     impl Ord for S {
+   |     ^^^^^^^^^^^^^^
+   |
+   = help: implement the missing `min` method of the `Ord` trait
+
+error: aborting due to 8 previous errors