diff --git a/compiler/rustc_typeck/src/check/upvar.rs b/compiler/rustc_typeck/src/check/upvar.rs
index cac9899ecd4b2..17d9e1fb24624 100644
--- a/compiler/rustc_typeck/src/check/upvar.rs
+++ b/compiler/rustc_typeck/src/check/upvar.rs
@@ -915,6 +915,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         reasons.auto_traits.extend(auto_trait_reasons);
         reasons.drop_order = drop_order;
 
+        // `auto_trait_reasons` are in hashset order, so sort them to put the
+        // diagnostics we emit later in a cross-platform-consistent order.
+        reasons.auto_traits.sort_unstable();
+
         reasons
     }
 
diff --git a/src/test/ui/closures/2229_closure_analysis/migrations/auto_traits.stderr b/src/test/ui/closures/2229_closure_analysis/migrations/auto_traits.stderr
index ee4907bb755cc..a8367766ae1cf 100644
--- a/src/test/ui/closures/2229_closure_analysis/migrations/auto_traits.stderr
+++ b/src/test/ui/closures/2229_closure_analysis/migrations/auto_traits.stderr
@@ -29,8 +29,8 @@ error: changes to closure capture in Rust 2021 will affect which traits the clos
 LL |     thread::spawn(move || unsafe {
    |                   ^^^^^^^^^^^^^^
    |                   |
-   |                   in Rust 2018, this closure implements `Sync` as `fptr` implements `Sync`, but in Rust 2021, this closure will no longer implement `Sync` because `fptr` is not fully captured and `fptr.0.0` does not implement `Sync`
    |                   in Rust 2018, this closure implements `Send` as `fptr` implements `Send`, but in Rust 2021, this closure will no longer implement `Send` because `fptr` is not fully captured and `fptr.0.0` does not implement `Send`
+   |                   in Rust 2018, this closure implements `Sync` as `fptr` implements `Sync`, but in Rust 2021, this closure will no longer implement `Sync` because `fptr` is not fully captured and `fptr.0.0` does not implement `Sync`
 ...
 LL |         *fptr.0.0 = 20;
    |         --------- in Rust 2018, this closure captures all of `fptr`, but in Rust 2021, it will only capture `fptr.0.0`
diff --git a/src/test/ui/closures/2229_closure_analysis/migrations/mir_calls_to_shims.stderr b/src/test/ui/closures/2229_closure_analysis/migrations/mir_calls_to_shims.stderr
index 6594ec316532a..2648b00435b31 100644
--- a/src/test/ui/closures/2229_closure_analysis/migrations/mir_calls_to_shims.stderr
+++ b/src/test/ui/closures/2229_closure_analysis/migrations/mir_calls_to_shims.stderr
@@ -4,8 +4,8 @@ error: changes to closure capture in Rust 2021 will affect which traits the clos
 LL |     let result = panic::catch_unwind(move || {
    |                                      ^^^^^^^
    |                                      |
-   |                                      in Rust 2018, this closure implements `UnwindSafe` as `f` implements `UnwindSafe`, but in Rust 2021, this closure will no longer implement `UnwindSafe` because `f` is not fully captured and `f.0` does not implement `UnwindSafe`
    |                                      in Rust 2018, this closure implements `RefUnwindSafe` as `f` implements `RefUnwindSafe`, but in Rust 2021, this closure will no longer implement `RefUnwindSafe` because `f` is not fully captured and `f.0` does not implement `RefUnwindSafe`
+   |                                      in Rust 2018, this closure implements `UnwindSafe` as `f` implements `UnwindSafe`, but in Rust 2021, this closure will no longer implement `UnwindSafe` because `f` is not fully captured and `f.0` does not implement `UnwindSafe`
 ...
 LL |         f.0()
    |         --- in Rust 2018, this closure captures all of `f`, but in Rust 2021, it will only capture `f.0`
diff --git a/src/test/ui/closures/2229_closure_analysis/migrations/multi_diagnostics.stderr b/src/test/ui/closures/2229_closure_analysis/migrations/multi_diagnostics.stderr
index 0008f1b2c07ed..483eae6bb4b1f 100644
--- a/src/test/ui/closures/2229_closure_analysis/migrations/multi_diagnostics.stderr
+++ b/src/test/ui/closures/2229_closure_analysis/migrations/multi_diagnostics.stderr
@@ -94,8 +94,8 @@ error: changes to closure capture in Rust 2021 will affect which traits the clos
 LL |     thread::spawn(move || unsafe {
    |                   ^^^^^^^^^^^^^^
    |                   |
-   |                   in Rust 2018, this closure implements `Sync` as `fptr1` implements `Sync`, but in Rust 2021, this closure will no longer implement `Sync` because `fptr1` is not fully captured and `fptr1.0.0` does not implement `Sync`
    |                   in Rust 2018, this closure implements `Send` as `fptr1` implements `Send`, but in Rust 2021, this closure will no longer implement `Send` because `fptr1` is not fully captured and `fptr1.0.0` does not implement `Send`
+   |                   in Rust 2018, this closure implements `Sync` as `fptr1` implements `Sync`, but in Rust 2021, this closure will no longer implement `Sync` because `fptr1` is not fully captured and `fptr1.0.0` does not implement `Sync`
    |                   in Rust 2018, this closure implements `Send` as `fptr2` implements `Send`, but in Rust 2021, this closure will no longer implement `Send` because `fptr2` is not fully captured and `fptr2.0` does not implement `Send`
 ...
 LL |         *fptr1.0.0 = 20;