Allow pass by reference if we return a reference

Currently this code will trigger `trivally_copy_pass_by_ref`:

```
struct OuterStruct {
    field: [u8; 8],
}

fn return_inner(outer: &OuterStruct) -> &[u8] {
    &outer.field
}
```

If we change the `outer` to be pass-by-value it will not live long
enough for us to return the reference. The above example is trivial but
I've hit this in real code that either returns a reference to either the
argument or in to `self`.

This suppresses the `trivally_copy_pass_by_ref` lint if we return a
reference and it has the same lifetime as the argument. This will likely
miss complex cases with multiple lifetimes bounded by each other but it
should cover the majority of cases with little effort.

clippy_lints/src/trivially_copy_pass_by_ref.rs

@@ -115,6 +115,14 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TriviallyCopyPassByRef {
         let fn_sig = cx.tcx.fn_sig(fn_def_id);
         let fn_sig = cx.tcx.erase_late_bound_regions(&fn_sig);
 
+        // Use lifetimes to determine if we're returning a reference to the argument. In that case
+        // we can't switch to pass-by-value as the argument will not live long enough.
+        let output_lt = if let TypeVariants::TyRef(output_lt, _, _) = fn_sig.output().sty {
+            Some(output_lt)
+        } else {
+            None
+        };
+
         for ((input, &ty), arg) in decl.inputs.iter().zip(fn_sig.inputs()).zip(&body.arguments) {
             // All spans generated from a proc-macro invocation are the same...
             if span == input.span {
@@ -122,7 +130,8 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for TriviallyCopyPassByRef {
             }
 
             if_chain! {
-                if let TypeVariants::TyRef(_, ty, Mutability::MutImmutable) = ty.sty;
+                if let TypeVariants::TyRef(input_lt, ty, Mutability::MutImmutable) = ty.sty;
+                if Some(input_lt) != output_lt;
                 if is_copy(cx, ty);
                 if let Some(size) = cx.layout_of(ty).ok().map(|l| l.size.bytes());
                 if size <= self.limit;

tests/ui/trivially_copy_pass_by_ref.rs

@@ -11,6 +11,15 @@ type Baz = u32;
 fn good(a: &mut u32, b: u32, c: &Bar) {
 }
 
+fn good_return_implicit_lt_ref(foo: &Foo) -> &u32 {
+    &foo.0
+}
+
+#[allow(needless_lifetimes)]
+fn good_return_explicit_lt_ref<'a>(foo: &'a Foo) -> &'a u32 {
+    &foo.0
+}
+
 fn bad(x: &u32, y: &Foo, z: &Baz) {
 }
 
@@ -46,6 +55,8 @@ 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();

tests/ui/trivially_copy_pass_by_ref.stderr

@@ -1,81 +1,81 @@
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:14:11
+  --> $DIR/trivially_copy_pass_by_ref.rs:23:11
    |
-14 | fn bad(x: &u32, y: &Foo, z: &Baz) {
+23 | fn bad(x: &u32, y: &Foo, z: &Baz) {
    |           ^^^^ help: consider passing by value instead: `u32`
    |
    = 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/trivially_copy_pass_by_ref.rs:14:20
+  --> $DIR/trivially_copy_pass_by_ref.rs:23:20
    |
-14 | fn bad(x: &u32, y: &Foo, z: &Baz) {
+23 | fn bad(x: &u32, y: &Foo, z: &Baz) {
    |                    ^^^^ help: consider passing by value instead: `Foo`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:14:29
+  --> $DIR/trivially_copy_pass_by_ref.rs:23:29
    |
-14 | fn bad(x: &u32, y: &Foo, z: &Baz) {
+23 | fn bad(x: &u32, y: &Foo, z: &Baz) {
    |                             ^^^^ help: consider passing by value instead: `Baz`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:24:12
+  --> $DIR/trivially_copy_pass_by_ref.rs:33:12
    |
-24 |     fn bad(&self, x: &u32, y: &Foo, z: &Baz) {
+33 |     fn bad(&self, x: &u32, y: &Foo, z: &Baz) {
    |            ^^^^^ help: consider passing by value instead: `self`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:24:22
+  --> $DIR/trivially_copy_pass_by_ref.rs:33:22
    |
-24 |     fn bad(&self, x: &u32, y: &Foo, z: &Baz) {
+33 |     fn bad(&self, x: &u32, y: &Foo, z: &Baz) {
    |                      ^^^^ help: consider passing by value instead: `u32`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:24:31
+  --> $DIR/trivially_copy_pass_by_ref.rs:33:31
    |
-24 |     fn bad(&self, x: &u32, y: &Foo, z: &Baz) {
+33 |     fn bad(&self, x: &u32, y: &Foo, z: &Baz) {
    |                               ^^^^ help: consider passing by value instead: `Foo`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:24:40
+  --> $DIR/trivially_copy_pass_by_ref.rs:33:40
    |
-24 |     fn bad(&self, x: &u32, y: &Foo, z: &Baz) {
+33 |     fn bad(&self, x: &u32, y: &Foo, z: &Baz) {
    |                                        ^^^^ help: consider passing by value instead: `Baz`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:27:16
+  --> $DIR/trivially_copy_pass_by_ref.rs:36:16
    |
-27 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
+36 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
    |                ^^^^ help: consider passing by value instead: `u32`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:27:25
+  --> $DIR/trivially_copy_pass_by_ref.rs:36:25
    |
-27 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
+36 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
    |                         ^^^^ help: consider passing by value instead: `Foo`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:27:34
+  --> $DIR/trivially_copy_pass_by_ref.rs:36:34
    |
-27 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
+36 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
    |                                  ^^^^ help: consider passing by value instead: `Baz`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:41:16
+  --> $DIR/trivially_copy_pass_by_ref.rs:50:16
    |
-41 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
+50 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
    |                ^^^^ help: consider passing by value instead: `u32`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:41:25
+  --> $DIR/trivially_copy_pass_by_ref.rs:50:25
    |
-41 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
+50 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
    |                         ^^^^ help: consider passing by value instead: `Foo`
 
 error: this argument is passed by reference, but would be more efficient if passed by value
-  --> $DIR/trivially_copy_pass_by_ref.rs:41:34
+  --> $DIR/trivially_copy_pass_by_ref.rs:50:34
    |
-41 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
+50 |     fn bad2(x: &u32, y: &Foo, z: &Baz) {
    |                                  ^^^^ help: consider passing by value instead: `Baz`
 
 error: aborting due to 13 previous errors