Add note clarifying why a closure no longer implements a trait

Author: roxelo

compiler/rustc_typeck/src/check/upvar.rs

@@ -535,6 +535,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 captured_names,
                             ));
                         }
+
+                        if reasons.contains("closure trait implementation") {
+                            let closure_body_span = self.tcx.hir().span(body_id.hir_id);
+                            let closure_ending_span = self.tcx.sess.source_map().guess_head_span(closure_body_span).shrink_to_lo();
+
+                            let missing_trait = &reasons[..reasons.find("closure trait implementation").unwrap() - 1];
+
+                            diagnostics_builder.span_label(closure_ending_span, format!("in Rust 2018, this closure would implement {} as `{}` implements {}, but in Rust 2021, this closure will no longer implement {} as {} does not implement {}",
+                                missing_trait,
+                                self.tcx.hir().name(*var_hir_id),
+                                missing_trait,
+                                missing_trait,
+                                captured_names,
+                                missing_trait,
+                            ));
+                        }
                     }
                     diagnostics_builder.note("for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>");
                     let closure_body_span = self.tcx.hir().span(body_id.hir_id);

src/test/ui/closures/2229_closure_analysis/migrations/auto_traits.fixed

@@ -13,6 +13,7 @@ fn test_send_trait() {
     let fptr = SendPointer(&mut f as *mut i32);
     thread::spawn(move || { let _ = &fptr; unsafe {
         //~^ ERROR: `Send` closure trait implementation
+        //~| NOTE: in Rust 2018, this closure would implement `Send` as `fptr` implements `Send`, but in Rust 2021, this closure will no longer implement `Send` as `fptr.0` does not implement `Send`
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `fptr` to be fully captured
         *fptr.0 = 20;
@@ -32,6 +33,7 @@ fn test_sync_trait() {
     let fptr = SyncPointer(f);
     thread::spawn(move || { let _ = &fptr; unsafe {
         //~^ ERROR: `Sync`, `Send` closure trait implementation
+        //~| NOTE: in Rust 2018, this closure would implement `Sync`, `Send` as `fptr` implements `Sync`, `Send`, but in Rust 2021, this closure will no longer implement `Sync`, `Send` as `fptr.0.0` does not implement `Sync`, `Send`
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `fptr` to be fully captured
         *fptr.0.0 = 20;
@@ -55,6 +57,7 @@ fn test_clone_trait() {
     let f = U(S(String::from("Hello World")), T(0));
     let c = || { let _ = &f; 
         //~^ ERROR: `Clone` closure trait implementation, and drop order
+        //~| NOTE: in Rust 2018, this closure would implement `Clone` as `f` implements `Clone`, but in Rust 2021, this closure will no longer implement `Clone` as `f.1` does not implement `Clone`
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `f` to be fully captured
         let f_1 = f.1;

src/test/ui/closures/2229_closure_analysis/migrations/auto_traits.rs

@@ -13,6 +13,7 @@ fn test_send_trait() {
     let fptr = SendPointer(&mut f as *mut i32);
     thread::spawn(move || unsafe {
         //~^ ERROR: `Send` closure trait implementation
+        //~| NOTE: in Rust 2018, this closure would implement `Send` as `fptr` implements `Send`, but in Rust 2021, this closure will no longer implement `Send` as `fptr.0` does not implement `Send`
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `fptr` to be fully captured
         *fptr.0 = 20;
@@ -32,6 +33,7 @@ fn test_sync_trait() {
     let fptr = SyncPointer(f);
     thread::spawn(move || unsafe {
         //~^ ERROR: `Sync`, `Send` closure trait implementation
+        //~| NOTE: in Rust 2018, this closure would implement `Sync`, `Send` as `fptr` implements `Sync`, `Send`, but in Rust 2021, this closure will no longer implement `Sync`, `Send` as `fptr.0.0` does not implement `Sync`, `Send`
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `fptr` to be fully captured
         *fptr.0.0 = 20;
@@ -55,6 +57,7 @@ fn test_clone_trait() {
     let f = U(S(String::from("Hello World")), T(0));
     let c = || {
         //~^ ERROR: `Clone` closure trait implementation, and drop order
+        //~| NOTE: in Rust 2018, this closure would implement `Clone` as `f` implements `Clone`, but in Rust 2021, this closure will no longer implement `Clone` as `f.1` does not implement `Clone`
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `f` to be fully captured
         let f_1 = f.1;

src/test/ui/closures/2229_closure_analysis/migrations/auto_traits.stderr

@@ -2,7 +2,10 @@ error: changes to closure capture in Rust 2021 will affect `Send` closure trait
   --> $DIR/auto_traits.rs:14:19
    |
 LL |       thread::spawn(move || unsafe {
-   |  ___________________^
+   |                     ^       - in Rust 2018, this closure would implement `Send` as `fptr` implements `Send`, but in Rust 2021, this closure will no longer implement `Send` as `fptr.0` does not implement `Send`
+   |  ___________________|
+   | |
+LL | |
 LL | |
 LL | |
 LL | |
@@ -24,15 +27,18 @@ LL |     thread::spawn(move || { let _ = &fptr; unsafe {
 LL |
 LL |
 LL |
-LL |         *fptr.0 = 20;
 LL |
+LL |         *fptr.0 = 20;
  ...
 
 error: changes to closure capture in Rust 2021 will affect `Sync`, `Send` closure trait implementation
-  --> $DIR/auto_traits.rs:33:19
+  --> $DIR/auto_traits.rs:34:19
    |
 LL |       thread::spawn(move || unsafe {
-   |  ___________________^
+   |                     ^       - in Rust 2018, this closure would implement `Sync`, `Send` as `fptr` implements `Sync`, `Send`, but in Rust 2021, this closure will no longer implement `Sync`, `Send` as `fptr.0.0` does not implement `Sync`, `Send`
+   |  ___________________|
+   | |
+LL | |
 LL | |
 LL | |
 LL | |
@@ -49,15 +55,18 @@ LL |     thread::spawn(move || { let _ = &fptr; unsafe {
 LL |
 LL |
 LL |
-LL |         *fptr.0.0 = 20;
 LL |
+LL |         *fptr.0.0 = 20;
  ...
 
 error: changes to closure capture in Rust 2021 will affect `Clone` closure trait implementation, and drop order
-  --> $DIR/auto_traits.rs:56:13
+  --> $DIR/auto_traits.rs:58:13
    |
 LL |       let c = || {
-   |  _____________^
+   |               ^  - in Rust 2018, this closure would implement `Clone` as `f` implements `Clone`, but in Rust 2021, this closure will no longer implement `Clone` as `f.1` does not implement `Clone`
+   |  _____________|
+   | |
+LL | |
 LL | |
 LL | |
 LL | |
@@ -78,8 +87,8 @@ LL |     let c = || { let _ = &f;
 LL |
 LL |
 LL |
-LL |         let f_1 = f.1;
 LL |
+LL |         let f_1 = f.1;
  ...
 
 error: aborting due to 3 previous errors

src/test/ui/closures/2229_closure_analysis/migrations/mir_calls_to_shims.fixed

@@ -19,6 +19,7 @@ where
     let f = panic::AssertUnwindSafe(f);
     let result = panic::catch_unwind(move || { let _ = &f; 
         //~^ ERROR: `UnwindSafe`, `RefUnwindSafe` closure trait implementation
+        //~| NOTE: in Rust 2018, this closure would implement `UnwindSafe`, `RefUnwindSafe` as `f` implements `UnwindSafe`, `RefUnwindSafe`, but in Rust 2021, this closure will no longer implement `UnwindSafe`, `RefUnwindSafe` as `f.0` does not implement `UnwindSafe`, `RefUnwindSafe`
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `f` to be fully captured
         f.0()

src/test/ui/closures/2229_closure_analysis/migrations/mir_calls_to_shims.rs

@@ -19,6 +19,7 @@ where
     let f = panic::AssertUnwindSafe(f);
     let result = panic::catch_unwind(move || {
         //~^ ERROR: `UnwindSafe`, `RefUnwindSafe` closure trait implementation
+        //~| NOTE: in Rust 2018, this closure would implement `UnwindSafe`, `RefUnwindSafe` as `f` implements `UnwindSafe`, `RefUnwindSafe`, but in Rust 2021, this closure will no longer implement `UnwindSafe`, `RefUnwindSafe` as `f.0` does not implement `UnwindSafe`, `RefUnwindSafe`
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `f` to be fully captured
         f.0()

src/test/ui/closures/2229_closure_analysis/migrations/mir_calls_to_shims.stderr

@@ -2,7 +2,10 @@ error: changes to closure capture in Rust 2021 will affect `UnwindSafe`, `RefUnw
   --> $DIR/mir_calls_to_shims.rs:20:38
    |
 LL |       let result = panic::catch_unwind(move || {
-   |  ______________________________________^
+   |                                        ^       - in Rust 2018, this closure would implement `UnwindSafe`, `RefUnwindSafe` as `f` implements `UnwindSafe`, `RefUnwindSafe`, but in Rust 2021, this closure will no longer implement `UnwindSafe`, `RefUnwindSafe` as `f.0` does not implement `UnwindSafe`, `RefUnwindSafe`
+   |  ______________________________________|
+   | |
+LL | |
 LL | |
 LL | |
 LL | |
@@ -24,8 +27,8 @@ LL |     let result = panic::catch_unwind(move || { let _ = &f;
 LL |
 LL |
 LL |
-LL |         f.0()
 LL |
+LL |         f.0()
  ...
 
 error: aborting due to previous error