large_assignments: Allow moves into functions

Moves into functions are typically implemented with pointer passing
rather than memcpy's at the llvm-ir level, so allow moves into
functions.

Author: Enselic

compiler/rustc_monomorphize/src/collector.rs

@@ -666,7 +666,15 @@ impl<'a, 'tcx> MirUsedCollector<'a, 'tcx> {
         debug!(?def_id, ?fn_span);
 
         for arg in args {
-            if let Some(too_large_size) = self.operand_size_if_too_large(limit, &arg.node) {
+            // Moving args into functions is typically implemented with pointer
+            // passing at the llvm-ir level and not by memcpy's. So always allow
+            // moving args into functions.
+            let operand: &mir::Operand<'tcx> = &arg.node;
+            if let mir::Operand::Move(_) = operand {
+                continue;
+            }
+
+            if let Some(too_large_size) = self.operand_size_if_too_large(limit, operand) {
                 self.lint_large_assignment(limit.0, too_large_size, location, arg.span);
             };
         }

tests/ui/lint/large_assignments/copy_into_box_rc_arc.rs

@@ -17,6 +17,9 @@ fn main() {
     let _ = Arc::new(data); // OK!
     let _ = Box::new(data); // OK!
     let _ = Rc::new(data); // OK!
+
+    // Looking at --emit llvm-ir, we can see that a memcpy is involved in the
+    // parameter passing. So we want the lint to trigger here.
     let _ = NotBox::new(data); //~ ERROR large_assignments
 }
 
@@ -26,6 +29,8 @@ struct NotBox {
 
 impl NotBox {
     fn new(data: [u8; 9999]) -> Self {
+        // Looking at --emit llvm-ir, we can see that a memcpy is involved.
+        // So we want the lint to trigger here.
         Self { //~ ERROR large_assignments
             data,
         }

tests/ui/lint/large_assignments/copy_into_box_rc_arc.stderr

@@ -1,5 +1,5 @@
 error: moving 9999 bytes
-  --> $DIR/copy_into_box_rc_arc.rs:20:25
+  --> $DIR/copy_into_box_rc_arc.rs:23:25
    |
 LL |     let _ = NotBox::new(data);
    |                         ^^^^ value moved from here
@@ -12,7 +12,7 @@ LL | #![deny(large_assignments)]
    |         ^^^^^^^^^^^^^^^^^
 
 error: moving 9999 bytes
-  --> $DIR/copy_into_box_rc_arc.rs:29:9
+  --> $DIR/copy_into_box_rc_arc.rs:34:9
    |
 LL | /         Self {
 LL | |             data,

tests/ui/lint/large_assignments/move_into_box_rc_arc.rs

@@ -11,11 +11,16 @@ use std::{sync::Arc, rc::Rc};
 
 fn main() {
     // Looking at --emit mir, we can see that all parameters below are passed
-    // with by move.
+    // by move.
     let _ = Arc::new([0; 9999]); // OK!
     let _ = Box::new([0; 9999]); // OK!
     let _ = Rc::new([0; 9999]); // OK!
-    let _ = NotBox::new([0; 9999]); //~ ERROR large_assignments
+
+    // Looking at --emit llvm-ir, we can see that no memcpy is involved in the
+    // parameter passing. Instead, a pointer is passed. This is typically what
+    // we get when moving parameter into functions. So we don't want the lint to
+    // trigger here.
+    let _ = NotBox::new([0; 9999]); // OK (compare with copy_into_box_rc_arc.rs)
 }
 
 struct NotBox {
@@ -25,6 +30,8 @@ struct NotBox {
 impl NotBox {
     fn new(data: [u8; 9999]) -> Self {
         Self {
+            // Looking at --emit llvm-ir, we can see that a memcpy is involved.
+            // So we want the lint to trigger here.
             data, //~ ERROR large_assignments
         }
     }

tests/ui/lint/large_assignments/move_into_box_rc_arc.stderr

@@ -1,8 +1,8 @@
 error: moving 9999 bytes
-  --> $DIR/move_into_box_rc_arc.rs:18:25
+  --> $DIR/move_into_box_rc_arc.rs:35:13
    |
-LL |     let _ = NotBox::new([0; 9999]);
-   |                         ^^^^^^^^^ value moved from here
+LL |             data,
+   |             ^^^^ value moved from here
    |
    = note: The current maximum size is 1000, but it can be customized with the move_size_limit attribute: `#![move_size_limit = "..."]`
 note: the lint level is defined here
@@ -11,13 +11,5 @@ note: the lint level is defined here
 LL | #![deny(large_assignments)]
    |         ^^^^^^^^^^^^^^^^^
 
-error: moving 9999 bytes
-  --> $DIR/move_into_box_rc_arc.rs:28:13
-   |
-LL |             data,
-   |             ^^^^ value moved from here
-   |
-   = note: The current maximum size is 1000, but it can be customized with the move_size_limit attribute: `#![move_size_limit = "..."]`
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 

tests/ui/lint/large_assignments/move_into_fn.rs

@@ -0,0 +1,22 @@
+// build-fail
+
+#![feature(large_assignments)]
+#![move_size_limit = "1000"]
+#![deny(large_assignments)]
+#![allow(unused)]
+
+// Note: This type does not implement Copy.
+struct Data([u8; 9999]);
+
+fn main() {
+    // Looking at llvm-ir output, we can see a memcpy'd into Data, so we want
+    // the lint to trigger here.
+    let data = Data([100; 9999]); //~ ERROR large_assignments
+
+    // Looking at llvm-ir output, we can see that there is no memcpy involved in
+    // this function call. Instead, just a pointer is passed to the function. So
+    // the lint shall not trigger here.
+    take_data(data);
+}
+
+fn take_data(data: Data) {}

tests/ui/lint/large_assignments/move_into_fn.stderr

@@ -0,0 +1,15 @@
+error: moving 9999 bytes
+  --> $DIR/move_into_fn.rs:14:16
+   |
+LL |     let data = Data([100; 9999]);
+   |                ^^^^^^^^^^^^^^^^^ value moved from here
+   |
+   = note: The current maximum size is 1000, but it can be customized with the move_size_limit attribute: `#![move_size_limit = "..."]`
+note: the lint level is defined here
+  --> $DIR/move_into_fn.rs:5:9
+   |
+LL | #![deny(large_assignments)]
+   |         ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+