Improve formatting of closure capture migration suggestion for multi-line closures.

compiler/rustc_typeck/src/check/upvar.rs

@@ -649,11 +649,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         match self.tcx.sess.source_map().span_to_snippet(closure_body_span) {
                             Ok(s) => {
                                 let trimmed = s.trim_start();
+                                let mut lines = trimmed.lines();
+                                let line1 = lines.next().unwrap_or_default();
 
                                 // If the closure contains a block then replace the opening brace
                                 // with "{ let _ = (..); "
-                                let sugg = if let Some('{') = trimmed.chars().next() {
-                                    format!("{{ {}; {}", migration_string, &trimmed[1..])
+                                let sugg = if line1.trim_end() == "{" {
+                                    // This is a multi-line closure with just a `{` on the first line,
+                                    // so we put the `let` on its own line.
+                                    // We take the indentation from the next non-empty line.
+                                    let line2 = lines.filter(|line| !line.is_empty()).next().unwrap_or_default();
+                                    let indent = line2.split_once(|c: char| !c.is_whitespace()).unwrap_or_default().0;
+                                    format!("{{\n{}{};{}", indent, migration_string, &trimmed[line1.len()..])
+                                } else if line1.starts_with('{') {
+                                    format!("{{ {}; {}", migration_string, &trimmed[1..].trim_start())
                                 } else {
                                     format!("{{ {}; {} }}", migration_string, s)
                                 };

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

@@ -55,7 +55,8 @@ impl Clone for U {
 
 fn test_clone_trait() {
     let f = U(S(String::from("Hello World")), T(0));
-    let c = || { let _ = &f; 
+    let c = || {
+        let _ = &f;
         //~^ ERROR: `Clone` trait implementation for closure and drop order
         //~| NOTE: in Rust 2018, this closure would implement `Clone` as `f` implements `Clone`, but in Rust 2021, this closure would no longer implement `Clone` as `f.1` does not implement `Clone`
         //~| NOTE: for more information, see

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

@@ -58,12 +58,12 @@ LL | }
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `f` to be fully captured
    |
-LL ~     let c = || { let _ = &f; 
+LL ~     let c = || {
+LL +         let _ = &f;
 LL +
 LL +
 LL +
 LL +
-LL +         let f_1 = f.1;
  ...
 
 error: aborting due to 3 previous errors

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

@@ -12,7 +12,8 @@ fn test1_all_need_migration() {
     let t1 = (String::new(), String::new());
     let t2 = (String::new(), String::new());
 
-    let c = || { let _ = (&t, &t1, &t2); 
+    let c = || {
+        let _ = (&t, &t1, &t2);
         //~^ ERROR: drop order
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `t`, `t1`, `t2` to be fully captured
@@ -38,7 +39,8 @@ fn test2_only_precise_paths_need_migration() {
     let t1 = (String::new(), String::new());
     let t2 = (String::new(), String::new());
 
-    let c = || { let _ = (&t, &t1); 
+    let c = || {
+        let _ = (&t, &t1);
         //~^ ERROR: drop order
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `t`, `t1` to be fully captured
@@ -59,7 +61,8 @@ fn test2_only_precise_paths_need_migration() {
 fn test3_only_by_value_need_migration() {
     let t = (String::new(), String::new());
     let t1 = (String::new(), String::new());
-    let c = || { let _ = &t; 
+    let c = || {
+        let _ = &t;
         //~^ ERROR: drop order
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `t` to be fully captured
@@ -80,7 +83,8 @@ fn test4_only_non_copy_types_need_migration() {
     // `t1` is Copy because all of its elements are Copy
     let t1 = (0i32, 0i32);
 
-    let c = || { let _ = &t; 
+    let c = || {
+        let _ = &t;
         //~^ ERROR: drop order
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `t` to be fully captured
@@ -101,7 +105,8 @@ fn test5_only_drop_types_need_migration() {
     // `s` doesn't implement Drop or any elements within it, and doesn't need migration
     let s = S(0i32, 0i32);
 
-    let c = || { let _ = &t; 
+    let c = || {
+        let _ = &t;
         //~^ ERROR: drop order
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `t` to be fully captured
@@ -119,7 +124,8 @@ fn test5_only_drop_types_need_migration() {
 fn test6_move_closures_non_copy_types_might_need_migration() {
     let t = (String::new(), String::new());
     let t1 = (String::new(), String::new());
-    let c = move || { let _ = (&t1, &t); 
+    let c = move || {
+        let _ = (&t1, &t);
         //~^ ERROR: drop order
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `t1`, `t` to be fully captured
@@ -139,7 +145,8 @@ fn test6_move_closures_non_copy_types_might_need_migration() {
 fn test7_drop_non_drop_aggregate_need_migration() {
     let t = (String::new(), String::new(), 0i32);
 
-    let c = || { let _ = &t; 
+    let c = || {
+        let _ = &t;
         //~^ ERROR: drop order
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `t` to be fully captured

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

@@ -28,12 +28,12 @@ LL | #![deny(rust_2021_incompatible_closure_captures)]
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `t`, `t1`, `t2` to be fully captured
    |
-LL ~     let c = || { let _ = (&t, &t1, &t2); 
+LL ~     let c = || {
+LL +         let _ = (&t, &t1, &t2);
 LL +
 LL +
 LL +
 LL + 
-LL +         let _t = t.0;
  ...
 
 error: changes to closure capture in Rust 2021 will affect drop order
@@ -57,12 +57,12 @@ LL | }
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `t`, `t1` to be fully captured
    |
-LL ~     let c = || { let _ = (&t, &t1); 
+LL ~     let c = || {
+LL +         let _ = (&t, &t1);
 LL +
 LL +
 LL +
 LL +         let _t = t.0;
-LL +
  ...
 
 error: changes to closure capture in Rust 2021 will affect drop order
@@ -80,12 +80,12 @@ LL | }
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `t` to be fully captured
    |
-LL ~     let c = || { let _ = &t; 
+LL ~     let c = || {
+LL +         let _ = &t;
 LL +
 LL +
 LL +
 LL +         let _t = t.0;
-LL +
  ...
 
 error: changes to closure capture in Rust 2021 will affect drop order
@@ -103,12 +103,12 @@ LL | }
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `t` to be fully captured
    |
-LL ~     let c = || { let _ = &t; 
+LL ~     let c = || {
+LL +         let _ = &t;
 LL +
 LL +
 LL +
 LL +         let _t = t.0;
-LL +
  ...
 
 error: changes to closure capture in Rust 2021 will affect drop order
@@ -126,12 +126,12 @@ LL | }
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `t` to be fully captured
    |
-LL ~     let c = || { let _ = &t; 
+LL ~     let c = || {
+LL +         let _ = &t;
 LL +
 LL +
 LL +
 LL +         let _t = t.0;
-LL +
  ...
 
 error: changes to closure capture in Rust 2021 will affect drop order
@@ -154,12 +154,12 @@ LL | }
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `t1`, `t` to be fully captured
    |
-LL ~     let c = move || { let _ = (&t1, &t); 
+LL ~     let c = move || {
+LL +         let _ = (&t1, &t);
 LL +
 LL +
 LL +
 LL +         println!("{} {}", t1.1, t.1);
-LL +
  ...
 
 error: changes to closure capture in Rust 2021 will affect drop order
@@ -177,12 +177,12 @@ LL | }
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `t` to be fully captured
    |
-LL ~     let c = || { let _ = &t; 
+LL ~     let c = || {
+LL +         let _ = &t;
 LL +
 LL +
 LL +
 LL +         let _t = t.0;
-LL +
  ...
 
 error: aborting due to 7 previous errors

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

@@ -34,7 +34,8 @@ impl<T> Drop for GenericStruct<T> {
 fn significant_drop_needs_migration() {
     let t = (SigDrop {}, SigDrop {});
 
-    let c = || { let _ = &t; 
+    let c = || {
+        let _ = &t;
         //~^ ERROR: drop order
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `t` to be fully captured
@@ -54,7 +55,8 @@ fn generic_struct_with_significant_drop_needs_migration() {
     let t = Wrapper(GenericStruct(SigDrop {}, SigDrop {}), 5);
 
     // move is used to force i32 to be copied instead of being a ref
-    let c = move || { let _ = &t; 
+    let c = move || {
+        let _ = &t;
         //~^ ERROR: drop order
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `t` to be fully captured

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

@@ -18,12 +18,12 @@ LL | #![deny(rust_2021_incompatible_closure_captures)]
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `t` to be fully captured
    |
-LL ~     let c = || { let _ = &t; 
+LL ~     let c = || {
+LL +         let _ = &t;
 LL +
 LL +
 LL +
 LL +         let _t = t.0;
-LL +
  ...
 
 error: changes to closure capture in Rust 2021 will affect drop order
@@ -41,12 +41,12 @@ LL | }
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `t` to be fully captured
    |
-LL ~     let c = move || { let _ = &t; 
+LL ~     let c = move || {
+LL +         let _ = &t;
 LL +
 LL +
 LL +
 LL +         let _t = t.1;
-LL +
  ...
 
 error: aborting due to 2 previous errors

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

@@ -16,7 +16,8 @@ impl Drop for Foo {
 
 fn closure_contains_block() {
     let t = (Foo(0), Foo(0));
-    let c = || { let _ = &t; 
+    let c = || {
+        let _ = &t;
         //~^ ERROR: drop order
         //~| NOTE: for more information, see
         //~| HELP: add a dummy let to cause `t` to be fully captured

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

@@ -18,12 +18,12 @@ LL | #![deny(rust_2021_incompatible_closure_captures)]
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `t` to be fully captured
    |
-LL ~     let c = || { let _ = &t; 
+LL ~     let c = || {
+LL +         let _ = &t;
 LL +
 LL +
 LL +
 LL +         let _t = t.0;
-LL +
  ...
 
 error: changes to closure capture in Rust 2021 will affect drop order

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

@@ -17,7 +17,8 @@ where
     F: FnOnce(),
 {
     let f = panic::AssertUnwindSafe(f);
-    let result = panic::catch_unwind(move || { let _ = &f; 
+    let result = panic::catch_unwind(move || {
+        let _ = &f;
         //~^ ERROR: `UnwindSafe`, `RefUnwindSafe` trait implementation for closure
         //~| NOTE: in Rust 2018, this closure would implement `UnwindSafe`, `RefUnwindSafe` as `f` implements `UnwindSafe`, `RefUnwindSafe`, but in Rust 2021, this closure would no longer implement `UnwindSafe`, `RefUnwindSafe` as `f.0` does not implement `UnwindSafe`, `RefUnwindSafe`
         //~| NOTE: for more information, see

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

@@ -15,12 +15,12 @@ LL | #![deny(rust_2021_incompatible_closure_captures)]
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
 help: add a dummy let to cause `f` to be fully captured
    |
-LL ~     let result = panic::catch_unwind(move || { let _ = &f; 
+LL ~     let result = panic::catch_unwind(move || {
+LL +         let _ = &f;
 LL +
 LL +
 LL +
 LL +
-LL +         f.0()
  ...
 
 error: aborting due to previous error

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

@@ -20,7 +20,8 @@ impl Clone for U {
 fn test_multi_issues() {
     let f1 = U(S(String::from("foo")), T(0));
     let f2 = U(S(String::from("bar")), T(0));
-    let c = || { let _ = (&f1, &f2); 
+    let c = || {
+        let _ = (&f1, &f2);
         //~^ ERROR: `Clone` trait implementation for closure and drop order
         //~| NOTE: in Rust 2018, this closure would implement `Clone` as `f1` implements `Clone`, but in Rust 2021, this closure would no longer implement `Clone` as `f1.0` does not implement `Clone`
         //~| NOTE: for more information, see
@@ -39,7 +40,8 @@ fn test_multi_issues() {
 
 fn test_capturing_all_disjoint_fields_individually() {
     let f1 = U(S(String::from("foo")), T(0));
-    let c = || { let _ = &f1; 
+    let c = || {
+        let _ = &f1;
         //~^ ERROR: `Clone` trait implementation for closure
         //~| NOTE: in Rust 2018, this closure would implement `Clone` as `f1` implements `Clone`, but in Rust 2021, this closure would no longer implement `Clone` as `f1.0` does not implement `Clone`
         //~| NOTE: for more information, see
@@ -64,7 +66,8 @@ impl Clone for U1 {
 
 fn test_capturing_several_disjoint_fields_individually_1() {
     let f1 = U1(S(String::from("foo")), T(0), S(String::from("bar")));
-    let c = || { let _ = &f1; 
+    let c = || {
+        let _ = &f1;
         //~^ ERROR: `Clone` trait implementation for closure
         //~| NOTE: in Rust 2018, this closure would implement `Clone` as `f1` implements `Clone`, but in Rust 2021, this closure would no longer implement `Clone` as `f1.0` does not implement `Clone`
         //~| NOTE: in Rust 2018, this closure would implement `Clone` as `f1` implements `Clone`, but in Rust 2021, this closure would no longer implement `Clone` as `f1.2` does not implement `Clone`
@@ -83,7 +86,8 @@ fn test_capturing_several_disjoint_fields_individually_1() {
 
 fn test_capturing_several_disjoint_fields_individually_2() {
     let f1 = U1(S(String::from("foo")), T(0), S(String::from("bar")));
-    let c = || { let _ = &f1; 
+    let c = || {
+        let _ = &f1;
         //~^ ERROR: `Clone` trait implementation for closure and drop order
         //~| NOTE: in Rust 2018, this closure would implement `Clone` as `f1` implements `Clone`, but in Rust 2021, this closure would no longer implement `Clone` as `f1.0` does not implement `Clone`
         //~| NOTE: for more information, see