Allow configuring the trivial copy size limit

clippy_lints/src/lib.rs

@@ -400,7 +400,10 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
     reg.register_late_lint_pass(box large_enum_variant::LargeEnumVariant::new(conf.enum_variant_size_threshold));
     reg.register_late_lint_pass(box explicit_write::Pass);
     reg.register_late_lint_pass(box needless_pass_by_value::NeedlessPassByValue);
-    reg.register_late_lint_pass(box trivially_copy_pass_by_ref::TriviallyCopyPassByRef);
+    reg.register_late_lint_pass(box trivially_copy_pass_by_ref::TriviallyCopyPassByRef::new(
+            conf.trivial_copy_size_limit,
+            &reg.sess.target,
+    ));
     reg.register_early_lint_pass(box literal_representation::LiteralDigitGrouping);
     reg.register_early_lint_pass(box literal_representation::LiteralRepresentation::new(
             conf.literal_representation_threshold

clippy_lints/src/trivially_copy_pass_by_ref.rs

@@ -1,8 +1,11 @@
+use std::cmp;
+
 use rustc::hir::*;
 use rustc::hir::map::*;
 use rustc::hir::intravisit::FnKind;
 use rustc::lint::*;
 use rustc::ty::TypeVariants;
+use rustc::session::config::Config as SessionConfig;
 use rustc_target::spec::abi::Abi;
 use rustc_target::abi::LayoutOf;
 use syntax::ast::NodeId;
@@ -17,6 +20,14 @@ use crate::utils::{in_macro, is_copy, is_self, span_lint_and_sugg, snippet};
 /// be passed through registers if they fit into two or less general purpose
 /// 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.
+///
+/// The configuration option `trivial_copy_size_limit` can be set to override
+/// this limit for a project.
+///
 /// **Example:**
 /// ```rust
 /// fn foo(v: &u32) {
@@ -33,7 +44,24 @@ declare_clippy_lint! {
     "functions taking small copyable arguments by reference"
 }
 
-pub struct TriviallyCopyPassByRef;
+pub struct TriviallyCopyPassByRef {
+    limit: u64,
+}
+
+impl TriviallyCopyPassByRef {
+    pub fn new(limit: Option<u64>, target: &SessionConfig) -> Self {
+        let limit = limit.unwrap_or_else(|| {
+            let bit_width = target.usize_ty.bit_width().expect("usize should have a width") as u64;
+            // 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);
+            let byte_width = bit_width / 8;
+            // Use a limit of 2 times the register bit width
+            byte_width * 2
+        });
+        Self { limit }
+    }
+}
 
 impl LintPass for TriviallyCopyPassByRef {
     fn get_lints(&self) -> LintArray {
@@ -94,7 +122,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TriviallyCopyPassByRef {
                 if let TypeVariants::TyRef(_, ty, Mutability::MutImmutable) = ty.sty;
                 if is_copy(cx, ty);
                 if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
-                if size < 16;
+                if size <= self.limit;
                 if let Ty_::TyRptr(_, MutTy { ty: ref decl_ty, .. }) = input.node;
                 then {
                     let value_type = if is_self(arg) {

clippy_lints/src/utils/conf.rs

@@ -156,6 +156,8 @@ define_Conf! {
     (verbose_bit_mask_threshold, "verbose_bit_mask_threshold", 1 => u64),
     /// Lint: DECIMAL_LITERAL_REPRESENTATION. The lower bound for linting decimal literals
     (literal_representation_threshold, "literal_representation_threshold", 16384 => u64),
+    /// Lint: TRIVIALLY_COPY_PASS_BY_REF. The maximum size (in bytes) to consider a `Copy` type for passing by value instead of by reference.
+    (trivial_copy_size_limit, "trivial_copy_size_limit", None => Option<u64>),
 }
 
 /// Search for the configuration file.

tests/ui-toml/toml_trivially_copy/clippy.toml

@@ -0,0 +1 @@
+trivial-copy-size-limit = 2

tests/ui-toml/toml_trivially_copy/test.rs

@@ -0,0 +1,19 @@
+#![allow(many_single_char_names)]
+
+#[derive(Copy, Clone)]
+struct Foo(u8);
+
+#[derive(Copy, Clone)]
+struct Bar(u32);
+
+fn good(a: &mut u32, b: u32, c: &Bar, d: &u32) {
+}
+
+fn bad(x: &u16, y: &Foo) {
+}
+
+fn main() {
+    let (mut a, b, c, d, x, y) = (0, 0, Bar(0), 0, 0, Foo(0));
+    good(&mut a, b, &c, &d);
+    bad(&x, &y);
+}

tests/ui-toml/toml_trivially_copy/test.stderr

@@ -0,0 +1,16 @@
+error: this argument is passed by reference, but would be more efficient if passed by value
+  --> $DIR/test.rs:12:11
+   |
+12 | fn bad(x: &u16, y: &Foo) {
+   |           ^^^^ help: consider passing by value instead: `u16`
+   |
+   = note: `-D trivially-copy-pass-by-ref` implied by `-D warnings`
+
+error: this argument is passed by reference, but would be more efficient if passed by value
+  --> $DIR/test.rs:12:20
+   |
+12 | fn bad(x: &u16, y: &Foo) {
+   |                    ^^^^ help: consider passing by value instead: `Foo`
+
+error: aborting due to 2 previous errors
+

tests/ui-toml/toml_unknown_key/conf_unknown_key.stderr

@@ -1,4 +1,4 @@
-error: error reading Clippy's configuration file `$DIR/clippy.toml`: unknown field `foobar`, expected one of `blacklisted-names`, `cyclomatic-complexity-threshold`, `doc-valid-idents`, `too-many-arguments-threshold`, `type-complexity-threshold`, `single-char-binding-names-threshold`, `too-large-for-stack`, `enum-variant-name-threshold`, `enum-variant-size-threshold`, `verbose-bit-mask-threshold`, `literal-representation-threshold`, `third-party`
+error: error reading Clippy's configuration file `$DIR/clippy.toml`: unknown field `foobar`, expected one of `blacklisted-names`, `cyclomatic-complexity-threshold`, `doc-valid-idents`, `too-many-arguments-threshold`, `type-complexity-threshold`, `single-char-binding-names-threshold`, `too-large-for-stack`, `enum-variant-name-threshold`, `enum-variant-size-threshold`, `verbose-bit-mask-threshold`, `literal-representation-threshold`, `trivial-copy-size-limit`, `third-party`
 
 error: aborting due to previous error
 

tests/ui/useless_asref.rs

@@ -1,5 +1,5 @@
 #![deny(useless_asref)]
-
+#![allow(trivially_copy_pass_by_ref)]
 use std::fmt::Debug;
 
 struct FakeAsRef;