From 953104e60cac57929846a3c3b776f12135c4b398 Mon Sep 17 00:00:00 2001
From: Aaron Hill <aa1ronham@gmail.com>
Date: Sun, 21 Jun 2020 21:27:34 -0400
Subject: [PATCH] Fix spurious 'value moved here in previous iteration of loop'
 messages

Fixes #46099

Previously, we would check the 'move' and 'use' spans to see if we
should emit this message. However, this can give false positives when
macros are involved, since two distinct expressions may end up with the
same span.

Instead, we check the actual MIR `Location`, which eliminates false
positives.
---
 .../borrow_check/diagnostics/conflict_errors.rs   |  2 +-
 src/test/ui/moves/issue-46099-move-in-macro.rs    | 15 +++++++++++++++
 .../ui/moves/issue-46099-move-in-macro.stderr     | 14 ++++++++++++++
 src/test/ui/moves/move-in-guard-2.stderr          |  5 ++++-
 4 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 src/test/ui/moves/issue-46099-move-in-macro.rs
 create mode 100644 src/test/ui/moves/issue-46099-move-in-macro.stderr

diff --git a/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs b/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs
index 2e897647a3beb..b12ab5a727cbb 100644
--- a/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs
+++ b/src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs
@@ -139,7 +139,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
                 let move_msg = if move_spans.for_closure() { " into closure" } else { "" };
 
-                if span == move_span {
+                if location == move_out.source {
                     err.span_label(
                         span,
                         format!("value moved{} here, in previous iteration of loop", move_msg),
diff --git a/src/test/ui/moves/issue-46099-move-in-macro.rs b/src/test/ui/moves/issue-46099-move-in-macro.rs
new file mode 100644
index 0000000000000..576fe1f4c8905
--- /dev/null
+++ b/src/test/ui/moves/issue-46099-move-in-macro.rs
@@ -0,0 +1,15 @@
+// Regression test for issue #46099
+// Tests that we don't emit spurious
+// 'value moved in previous iteration of loop' message
+
+macro_rules! test {
+    ($v:expr) => {{
+        drop(&$v);
+        $v
+    }}
+}
+
+fn main() {
+    let b = Box::new(true);
+    test!({b}); //~ ERROR use of moved value
+}
diff --git a/src/test/ui/moves/issue-46099-move-in-macro.stderr b/src/test/ui/moves/issue-46099-move-in-macro.stderr
new file mode 100644
index 0000000000000..83c99db870951
--- /dev/null
+++ b/src/test/ui/moves/issue-46099-move-in-macro.stderr
@@ -0,0 +1,14 @@
+error[E0382]: use of moved value: `b`
+  --> $DIR/issue-46099-move-in-macro.rs:14:12
+   |
+LL |     let b = Box::new(true);
+   |         - move occurs because `b` has type `std::boxed::Box<bool>`, which does not implement the `Copy` trait
+LL |     test!({b});
+   |            ^
+   |            |
+   |            value moved here
+   |            value used here after move
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0382`.
diff --git a/src/test/ui/moves/move-in-guard-2.stderr b/src/test/ui/moves/move-in-guard-2.stderr
index 8bd405279c526..00d89f550714c 100644
--- a/src/test/ui/moves/move-in-guard-2.stderr
+++ b/src/test/ui/moves/move-in-guard-2.stderr
@@ -5,7 +5,10 @@ LL |     let x: Box<_> = box 1;
    |         - move occurs because `x` has type `std::boxed::Box<i32>`, which does not implement the `Copy` trait
 ...
 LL |         (_, 2) if take(x) => (),
-   |                        ^ value moved here, in previous iteration of loop
+   |                        ^
+   |                        |
+   |                        value moved here
+   |                        value used here after move
 
 error: aborting due to previous error