Make trivial-copy-size-limit consistently the size of the target pointer (#13319)

Fixes
https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy/topic/Ambiguous.20default.20value.20for.20.60trivial-copy-size-limit.60

The current situation is

| Target width | `trivial-copy-size-limit`|
|--------|--------|
| 8-bit | 2 |
| 16-bit | 4 |
| 32-bit | 8 |
| 64-bit | 8 |

~~Since practically speaking it's almost always 8, let's go with that as
the unconditional default to make it easier to understand~~

Now defaults to `target_pointer_width`

changelog: [`trivial-copy-size-limit`] now also defaults to the size of
a target pointer (unchanged for 64-bit targets)

Author: Jarcho

book/src/lint_configuration.md

@@ -1026,7 +1026,7 @@ The order of associated items in traits.
 The maximum size (in bytes) to consider a `Copy` type for passing by value instead of by
 reference.
 
-**Default Value:** `target_pointer_width * 2`
+**Default Value:** `target_pointer_width`
 
 ---
 **Affected lints:**

clippy_config/src/conf.rs

@@ -828,7 +828,7 @@ define_Conf! {
     trait_assoc_item_kinds_order: SourceItemOrderingTraitAssocItemKinds = DEFAULT_TRAIT_ASSOC_ITEM_KINDS_ORDER.into(),
     /// The maximum size (in bytes) to consider a `Copy` type for passing by value instead of by
     /// reference.
-    #[default_text = "target_pointer_width * 2"]
+    #[default_text = "target_pointer_width"]
     #[lints(trivially_copy_pass_by_ref)]
     trivial_copy_size_limit: Option<u64> = None,
     /// The maximum complexity a type can have

clippy_lints/src/pass_by_ref_or_value.rs

@@ -1,5 +1,3 @@
-use std::{cmp, iter};
-
 use clippy_config::Conf;
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::source::snippet;
@@ -20,6 +18,7 @@ use rustc_middle::ty::{self, RegionKind, TyCtxt};
 use rustc_session::impl_lint_pass;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::{Span, sym};
+use std::iter;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -33,10 +32,8 @@ declare_clippy_lint! {
     /// registers.
     ///
     /// ### Known problems
-    /// This lint is target register size dependent, it is
-    /// limited to 32-bit to try and reduce portability problems between 32 and
-    /// 64-bit, but if you are compiling for 8 or 16-bit targets then the limit
-    /// will be different.
+    /// This lint is target dependent, some cases will lint on 64-bit targets but
+    /// not 32-bit or lower targets.
     ///
     /// The configuration option `trivial_copy_size_limit` can be set to override
     /// this limit for a project.
@@ -112,16 +109,9 @@ pub struct PassByRefOrValue {
 
 impl PassByRefOrValue {
     pub fn new(tcx: TyCtxt<'_>, conf: &'static Conf) -> Self {
-        let ref_min_size = conf.trivial_copy_size_limit.unwrap_or_else(|| {
-            let bit_width = u64::from(tcx.sess.target.pointer_width);
-            // Cap the calculated bit width at 32-bits to reduce
-            // portability problems between 32 and 64-bit targets
-            let bit_width = cmp::min(bit_width, 32);
-            #[expect(clippy::integer_division)]
-            let byte_width = bit_width / 8;
-            // Use a limit of 2 times the register byte width
-            byte_width * 2
-        });
+        let ref_min_size = conf
+            .trivial_copy_size_limit
+            .unwrap_or_else(|| u64::from(tcx.sess.target.pointer_width / 8));
 
         Self {
             ref_min_size,

tests/ui/trivially_copy_pass_by_ref.fixed

@@ -0,0 +1,179 @@
+//@normalize-stderr-test: "\(\d+ byte\)" -> "(N byte)"
+//@normalize-stderr-test: "\(limit: \d+ byte\)" -> "(limit: N byte)"
+#![deny(clippy::trivially_copy_pass_by_ref)]
+#![allow(
+    clippy::disallowed_names,
+    clippy::extra_unused_lifetimes,
+    clippy::needless_lifetimes,
+    clippy::needless_pass_by_ref_mut,
+    clippy::redundant_field_names,
+    clippy::uninlined_format_args
+)]
+
+#[derive(Copy, Clone)]
+struct Foo(u32);
+
+#[derive(Copy, Clone)]
+struct Bar([u8; 24]);
+
+#[derive(Copy, Clone)]
+pub struct Color {
+    pub r: u8,
+    pub g: u8,
+    pub b: u8,
+    pub a: u8,
+}
+
+struct FooRef<'a> {
+    foo: &'a Foo,
+}
+
+type Baz = u32;
+
+fn good(a: &mut u32, b: u32, c: &Bar) {}
+
+fn good_return_implicit_lt_ref(foo: &Foo) -> &u32 {
+    &foo.0
+}
+
+#[allow(clippy::needless_lifetimes)]
+fn good_return_explicit_lt_ref<'a>(foo: &'a Foo) -> &'a u32 {
+    &foo.0
+}
+
+fn good_return_implicit_lt_struct(foo: &Foo) -> FooRef {
+    FooRef { foo }
+}
+
+#[allow(clippy::needless_lifetimes)]
+fn good_return_explicit_lt_struct<'a>(foo: &'a Foo) -> FooRef<'a> {
+    FooRef { foo }
+}
+
+fn bad(x: u32, y: Foo, z: Baz) {}
+//~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+//~| ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+//~| ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+
+impl Foo {
+    fn good(self, a: &mut u32, b: u32, c: &Bar) {}
+
+    fn good2(&mut self) {}
+
+    fn bad(self, x: u32, y: Foo, z: Baz) {}
+    //~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+    //~| ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+    //~| ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+    //~| ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+
+    fn bad2(x: u32, y: Foo, z: Baz) {}
+    //~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+    //~| ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+    //~| ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+
+    fn bad_issue7518(self, other: Self) {}
+    //~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if
+}
+
+impl AsRef<u32> for Foo {
+    fn as_ref(&self) -> &u32 {
+        &self.0
+    }
+}
+
+impl Bar {
+    fn good(&self, a: &mut u32, b: u32, c: &Bar) {}
+
+    fn bad2(x: u32, y: Foo, z: Baz) {}
+    //~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if
+    //~| ERROR: this argument (4 byte) is passed by reference, but would be more efficient if
+    //~| ERROR: this argument (4 byte) is passed by reference, but would be more efficient if
+}
+
+trait MyTrait {
+    fn trait_method(&self, foo: Foo);
+    //~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if
+}
+
+pub trait MyTrait2 {
+    fn trait_method2(&self, color: &Color);
+}
+
+trait MyTrait3 {
+    #[expect(clippy::trivially_copy_pass_by_ref)]
+    fn trait_method(&self, foo: &Foo);
+}
+
+// Trait impls should not warn
+impl MyTrait3 for Foo {
+    fn trait_method(&self, foo: &Foo) {
+        unimplemented!()
+    }
+}
+
+mod issue3992 {
+    pub trait A {
+        #[allow(clippy::trivially_copy_pass_by_ref)]
+        fn a(b: &u16) {}
+    }
+
+    #[allow(clippy::trivially_copy_pass_by_ref)]
+    pub fn c(d: &u16) {}
+}
+
+mod issue5876 {
+    // Don't lint here as it is always inlined
+    #[inline(always)]
+    fn foo_always(x: &i32) {
+        println!("{}", x);
+    }
+
+    #[inline(never)]
+    fn foo_never(x: i32) {
+        //~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+        println!("{}", x);
+    }
+
+    #[inline]
+    fn foo(x: i32) {
+        //~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+        println!("{}", x);
+    }
+}
+
+fn ref_to_opt_ref_implicit(x: &u32) -> Option<&u32> {
+    Some(x)
+}
+
+fn ref_to_opt_ref_explicit<'a>(x: &'a u32) -> Option<&'a u32> {
+    Some(x)
+}
+
+fn with_constraint<'a, 'b: 'a>(x: &'b u32, y: &'a u32) -> &'a u32 {
+    if true { x } else { y }
+}
+
+async fn async_implicit(x: &u32) -> &u32 {
+    x
+}
+
+async fn async_explicit<'a>(x: &'a u32) -> &'a u32 {
+    x
+}
+
+fn unrelated_lifetimes<'a, 'b>(_x: u32, y: &'b u32) -> &'b u32 {
+    //~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
+    y
+}
+
+fn return_ptr(x: &u32) -> *const u32 {
+    x
+}
+
+fn return_field_ptr(x: &(u32, u32)) -> *const u32 {
+    &x.0
+}
+
+fn return_field_ptr_addr_of(x: &(u32, u32)) -> *const u32 {
+    core::ptr::addr_of!(x.0)
+}

tests/ui/trivially_copy_pass_by_ref.rs

@@ -1,14 +1,15 @@
 //@normalize-stderr-test: "\(\d+ byte\)" -> "(N byte)"
-//@normalize-stderr-test: "\(limit: \d+ byte\)" -> "(limit: 8 byte)"
+//@normalize-stderr-test: "\(limit: \d+ byte\)" -> "(limit: N byte)"
 #![deny(clippy::trivially_copy_pass_by_ref)]
 #![allow(
     clippy::disallowed_names,
+    clippy::extra_unused_lifetimes,
     clippy::needless_lifetimes,
+    clippy::needless_pass_by_ref_mut,
     clippy::redundant_field_names,
-    clippy::uninlined_format_args,
-    clippy::needless_pass_by_ref_mut
+    clippy::uninlined_format_args
 )]
-//@no-rustfix
+
 #[derive(Copy, Clone)]
 struct Foo(u32);
 
@@ -90,21 +91,26 @@ impl Bar {
 }
 
 trait MyTrait {
-    fn trait_method(&self, _foo: &Foo);
+    fn trait_method(&self, foo: &Foo);
     //~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if
 }
 
 pub trait MyTrait2 {
-    fn trait_method2(&self, _color: &Color);
+    fn trait_method2(&self, color: &Color);
+}
+
+trait MyTrait3 {
+    #[expect(clippy::trivially_copy_pass_by_ref)]
+    fn trait_method(&self, foo: &Foo);
 }
 
-impl MyTrait for Foo {
-    fn trait_method(&self, _foo: &Foo) {
+// Trait impls should not warn
+impl MyTrait3 for Foo {
+    fn trait_method(&self, foo: &Foo) {
         unimplemented!()
     }
 }
 
-#[allow(unused_variables)]
 mod issue3992 {
     pub trait A {
         #[allow(clippy::trivially_copy_pass_by_ref)]
@@ -135,57 +141,39 @@ mod issue5876 {
     }
 }
 
-fn _ref_to_opt_ref_implicit(x: &u32) -> Option<&u32> {
+fn ref_to_opt_ref_implicit(x: &u32) -> Option<&u32> {
     Some(x)
 }
 
-#[allow(clippy::needless_lifetimes)]
-fn _ref_to_opt_ref_explicit<'a>(x: &'a u32) -> Option<&'a u32> {
+fn ref_to_opt_ref_explicit<'a>(x: &'a u32) -> Option<&'a u32> {
     Some(x)
 }
 
-fn _with_constraint<'a, 'b: 'a>(x: &'b u32, y: &'a u32) -> &'a u32 {
+fn with_constraint<'a, 'b: 'a>(x: &'b u32, y: &'a u32) -> &'a u32 {
     if true { x } else { y }
 }
 
-async fn _async_implicit(x: &u32) -> &u32 {
+async fn async_implicit(x: &u32) -> &u32 {
     x
 }
 
-#[allow(clippy::needless_lifetimes)]
-async fn _async_explicit<'a>(x: &'a u32) -> &'a u32 {
+async fn async_explicit<'a>(x: &'a u32) -> &'a u32 {
     x
 }
 
-fn _unrelated_lifetimes<'a, 'b>(_x: &'a u32, y: &'b u32) -> &'b u32 {
+fn unrelated_lifetimes<'a, 'b>(_x: &'a u32, y: &'b u32) -> &'b u32 {
     //~^ ERROR: this argument (4 byte) is passed by reference, but would be more efficient if passed by
     y
 }
 
-fn _return_ptr(x: &u32) -> *const u32 {
+fn return_ptr(x: &u32) -> *const u32 {
     x
 }
 
-fn _return_field_ptr(x: &(u32, u32)) -> *const u32 {
+fn return_field_ptr(x: &(u32, u32)) -> *const u32 {
     &x.0
 }
 
-fn _return_field_ptr_addr_of(x: &(u32, u32)) -> *const u32 {
+fn return_field_ptr_addr_of(x: &(u32, u32)) -> *const u32 {
     core::ptr::addr_of!(x.0)
 }
-
-fn main() {
-    let (mut foo, bar) = (Foo(0), Bar([0; 24]));
-    let (mut a, b, c, x, y, z) = (0, 0, Bar([0; 24]), 0, Foo(0), 0);
-    good(&mut a, b, &c);
-    good_return_implicit_lt_ref(&y);
-    good_return_explicit_lt_ref(&y);
-    bad(&x, &y, &z);
-    foo.good(&mut a, b, &c);
-    foo.good2();
-    foo.bad(&x, &y, &z);
-    Foo::bad2(&x, &y, &z);
-    bar.good(&mut a, b, &c);
-    Bar::bad2(&x, &y, &z);
-    foo.as_ref();
-}