Auto merge of rust-lang#52959 - matthewjasper:closure-spans, r=pnkfelix

[NLL] Use smaller spans for errors involving closure captures

Closes rust-lang#51170
Closes rust-lang#46599

Error messages involving closures now point to the captured variable/closure args.

r? @pnkfelix

src/librustc_mir/borrow_check/mod.rs

@@ -1542,7 +1542,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         if borrow_of_local_data(&borrow.borrowed_place) {
             let err = self.tcx
                 .cannot_borrow_across_generator_yield(
-                    self.retrieve_borrow_span(borrow),
+                    self.retrieve_borrow_spans(borrow).var_or_use(),
                     yield_span,
                     Origin::Mir,
                 );

src/librustc_mir/borrow_check/mutability_errors.rs

@@ -160,7 +160,6 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
         let act;
         let acted_on;
 
-
         let span = match error_access {
             AccessKind::Move => {
                 err = self.tcx
@@ -180,31 +179,23 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
                 act = "borrow as mutable";
                 acted_on = "borrowed as mutable";
 
-                let closure_span = self.find_closure_span(span, location);
-                if let Some((args, var)) = closure_span {
-                    err = self.tcx.cannot_borrow_path_as_mutable_because(
-                        args,
-                        &item_msg,
-                        &reason,
-                        Origin::Mir,
-                    );
-                    err.span_label(
-                        var,
-                        format!(
-                            "mutable borrow occurs due to use of `{}` in closure",
-                            self.describe_place(access_place).unwrap(),
-                        ),
-                    );
-                    args
-                } else {
-                    err = self.tcx.cannot_borrow_path_as_mutable_because(
-                        span,
-                        &item_msg,
-                        &reason,
-                        Origin::Mir,
-                    );
-                    span
-                }
+                let borrow_spans = self.borrow_spans(span, location);
+                let borrow_span = borrow_spans.args_or_use();
+                err = self.tcx.cannot_borrow_path_as_mutable_because(
+                    borrow_span,
+                    &item_msg,
+                    &reason,
+                    Origin::Mir,
+                );
+                borrow_spans.var_span_label(
+                    &mut err,
+                    format!(
+                        "mutable borrow occurs due to use of `{}` in closure",
+                        // always Some() if the message is printed.
+                        self.describe_place(access_place).unwrap_or(String::new()),
+                    )
+                );
+                borrow_span
             }
         };
 

src/librustc_mir/borrow_check/nll/explain_borrow/mod.rs

@@ -62,18 +62,24 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         );
 
         match find_use::find(mir, regioncx, tcx, region_sub, context.loc) {
-            Some(Cause::LiveVar(_local, location)) => {
-                if self.is_borrow_location_in_loop(context.loc) {
-                    err.span_label(
-                        mir.source_info(location).span,
-                        "borrow used here in later iteration of loop".to_string(),
-                    );
+            Some(Cause::LiveVar(local, location)) => {
+                let span = mir.source_info(location).span;
+                let spans = self.move_spans(&Place::Local(local), location)
+                    .or_else(|| self.borrow_spans(span, location));
+                let message = if self.is_borrow_location_in_loop(context.loc) {
+                    if spans.for_closure() {
+                        "borrow captured here by closure in later iteration of loop"
+                    } else {
+                        "borrow used here in later iteration of loop"
+                    }
                 } else {
-                    err.span_label(
-                        mir.source_info(location).span,
-                        "borrow later used here".to_string(),
-                    );
-                }
+                    if spans.for_closure() {
+                        "borrow later captured here by closure"
+                    } else {
+                        "borrow later used here"
+                    }
+                };
+                err.span_label(spans.var_or_use(), message);
             }
 
             Some(Cause::DropVar(local, location)) => match &mir.local_decls[local].name {

src/librustc_mir/util/borrowck_errors.rs

@@ -203,8 +203,15 @@ pub trait BorrowckErrors<'cx>: Sized + Copy {
             desc,
             OGN = o
         );
-        err.span_label(old_loan_span, "first closure is constructed here");
-        err.span_label(new_loan_span, "second closure is constructed here");
+        if old_loan_span == new_loan_span {
+            err.span_label(
+                old_loan_span,
+                "closures are constructed here in different iterations of loop"
+            );
+        } else {
+            err.span_label(old_loan_span, "first closure is constructed here");
+            err.span_label(new_loan_span, "second closure is constructed here");
+        }
         if let Some(old_load_end_span) = old_load_end_span {
             err.span_label(old_load_end_span, "borrow from first closure ends here");
         }

src/test/ui/borrowck/borrowck-closures-two-mut.stderr

@@ -77,11 +77,11 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
   --> $DIR/borrowck-closures-two-mut.rs:24:24
    |
 LL |     let c1 = to_fn_mut(|| x = 4);
-   |                        -- - previous borrow occurs due to use of `x` in closure
+   |                        -- - first borrow occurs due to use of `x` in closure
    |                        |
    |                        first mutable borrow occurs here
 LL |     let c2 = to_fn_mut(|| x = 5); //~ ERROR cannot borrow `x` as mutable more than once
-   |                        ^^ - borrow occurs due to use of `x` in closure
+   |                        ^^ - second borrow occurs due to use of `x` in closure
    |                        |
    |                        second mutable borrow occurs here
 LL |     //~| ERROR cannot borrow `x` as mutable more than once
@@ -92,11 +92,11 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
   --> $DIR/borrowck-closures-two-mut.rs:36:24
    |
 LL |     let c1 = to_fn_mut(|| set(&mut x));
-   |                        --          - previous borrow occurs due to use of `x` in closure
+   |                        --          - first borrow occurs due to use of `x` in closure
    |                        |
    |                        first mutable borrow occurs here
 LL |     let c2 = to_fn_mut(|| set(&mut x)); //~ ERROR cannot borrow `x` as mutable more than once
-   |                        ^^          - borrow occurs due to use of `x` in closure
+   |                        ^^          - second borrow occurs due to use of `x` in closure
    |                        |
    |                        second mutable borrow occurs here
 LL |     //~| ERROR cannot borrow `x` as mutable more than once
@@ -107,11 +107,11 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
   --> $DIR/borrowck-closures-two-mut.rs:44:24
    |
 LL |     let c1 = to_fn_mut(|| x = 5);
-   |                        -- - previous borrow occurs due to use of `x` in closure
+   |                        -- - first borrow occurs due to use of `x` in closure
    |                        |
    |                        first mutable borrow occurs here
 LL |     let c2 = to_fn_mut(|| set(&mut x)); //~ ERROR cannot borrow `x` as mutable more than once
-   |                        ^^          - borrow occurs due to use of `x` in closure
+   |                        ^^          - second borrow occurs due to use of `x` in closure
    |                        |
    |                        second mutable borrow occurs here
 LL |     //~| ERROR cannot borrow `x` as mutable more than once
@@ -122,11 +122,11 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
   --> $DIR/borrowck-closures-two-mut.rs:52:24
    |
 LL |     let c1 = to_fn_mut(|| x = 5);
-   |                        -- - previous borrow occurs due to use of `x` in closure
+   |                        -- - first borrow occurs due to use of `x` in closure
    |                        |
    |                        first mutable borrow occurs here
 LL |     let c2 = to_fn_mut(|| { let _y = to_fn_mut(|| set(&mut x)); }); // (nested closure)
-   |                        ^^                                  - borrow occurs due to use of `x` in closure
+   |                        ^^                                  - second borrow occurs due to use of `x` in closure
    |                        |
    |                        second mutable borrow occurs here
 ...
@@ -137,11 +137,11 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
   --> $DIR/borrowck-closures-two-mut.rs:65:24
    |
 LL |     let c1 = to_fn_mut(|| set(&mut *x.f));
-   |                        --           - previous borrow occurs due to use of `x` in closure
+   |                        --           - first borrow occurs due to use of `x` in closure
    |                        |
    |                        first mutable borrow occurs here
 LL |     let c2 = to_fn_mut(|| set(&mut *x.f));
-   |                        ^^           - borrow occurs due to use of `x` in closure
+   |                        ^^           - second borrow occurs due to use of `x` in closure
    |                        |
    |                        second mutable borrow occurs here
 ...

src/test/ui/borrowck/borrowck-escaping-closure-error-1.nll.stderr

@@ -1,8 +1,10 @@
 error[E0597]: `books` does not live long enough
-  --> $DIR/borrowck-escaping-closure-error-1.rs:23:11
+  --> $DIR/borrowck-escaping-closure-error-1.rs:23:14
    |
 LL |     spawn(|| books.push(4));
-   |           ^^^^^^^^^^^^^^^^ borrowed value does not live long enough
+   |           -- ^^^^^ borrowed value does not live long enough
+   |           |
+   |           value captured here
 LL |     //~^ ERROR E0373
 LL | }
    | - `books` dropped here while still borrowed

src/test/ui/borrowck/borrowck-escaping-closure-error-2.nll.stderr

@@ -1,8 +1,10 @@
 error[E0597]: `books` does not live long enough
-  --> $DIR/borrowck-escaping-closure-error-2.rs:21:14
+  --> $DIR/borrowck-escaping-closure-error-2.rs:21:17
    |
 LL |     Box::new(|| books.push(4))
-   |              ^^^^^^^^^^^^^^^^ borrowed value does not live long enough
+   |              -- ^^^^^ borrowed value does not live long enough
+   |              |
+   |              value captured here
 LL |     //~^ ERROR E0373
 LL | }
    | - `books` dropped here while still borrowed

src/test/ui/error-codes/E0504.nll.stderr

@@ -1,17 +1,16 @@
 error[E0505]: cannot move out of `fancy_num` because it is borrowed
   --> $DIR/E0504.rs:19:13
    |
-LL |       let fancy_ref = &fancy_num;
-   |                       ---------- borrow of `fancy_num` occurs here
+LL |     let fancy_ref = &fancy_num;
+   |                     ---------- borrow of `fancy_num` occurs here
 LL | 
-LL |       let x = move || {
-   |  _____________^
-LL | |         println!("child function: {}", fancy_num.num); //~ ERROR E0504
-LL | |     };
-   | |_____^ move out of `fancy_num` occurs here
+LL |     let x = move || {
+   |             ^^^^^^^ move out of `fancy_num` occurs here
+LL |         println!("child function: {}", fancy_num.num); //~ ERROR E0504
+   |                                        --------- move occurs due to use in closure
 ...
-LL |       println!("main function: {}", fancy_ref.num);
-   |                                     ------------- borrow later used here
+LL |     println!("main function: {}", fancy_ref.num);
+   |                                   ------------- borrow later used here
 
 error: aborting due to previous error
 

src/test/ui/issue-11192.nll.stderr

@@ -5,7 +5,7 @@ LL |     let mut test = |foo: &Foo| {
    |                    ----------- mutable borrow occurs here
 LL |         println!("access {}", foo.x);
 LL |         ptr = box Foo { x: ptr.x + 1 };
-   |         --- previous borrow occurs due to use of `ptr` in closure
+   |         --- first borrow occurs due to use of `ptr` in closure
 ...
 LL |     test(&*ptr);
    |     -----^^^^^-

src/test/ui/issue-11873.nll.stderr

@@ -2,7 +2,9 @@ error[E0505]: cannot move out of `v` because it is borrowed
   --> $DIR/issue-11873.rs:14:14
    |
 LL |     let mut f = || v.push(2);
-   |                 ------------ borrow of `v` occurs here
+   |                 -- - borrow occurs due to use in closure
+   |                 |
+   |                 borrow of `v` occurs here
 LL |     let _w = v; //~ ERROR: cannot move out of `v`
    |              ^ move out of `v` occurs here
 LL | 

src/test/ui/issue-18783.nll.stderr

@@ -2,11 +2,11 @@ error[E0499]: cannot borrow `y` as mutable more than once at a time
   --> $DIR/issue-18783.rs:17:21
    |
 LL |     c.push(Box::new(|| y = 0));
-   |                     -- - previous borrow occurs due to use of `y` in closure
+   |                     -- - first borrow occurs due to use of `y` in closure
    |                     |
    |                     first mutable borrow occurs here
 LL |     c.push(Box::new(|| y = 0));
-   |                     ^^ - borrow occurs due to use of `y` in closure
+   |                     ^^ - second borrow occurs due to use of `y` in closure
    |                     |
    |                     second mutable borrow occurs here
 LL | //~^ ERROR cannot borrow `y` as mutable more than once at a time
@@ -17,11 +17,11 @@ error[E0499]: cannot borrow `y` as mutable more than once at a time
   --> $DIR/issue-18783.rs:26:29
    |
 LL |     Push::push(&c, Box::new(|| y = 0));
-   |                             -- - previous borrow occurs due to use of `y` in closure
+   |                             -- - first borrow occurs due to use of `y` in closure
    |                             |
    |                             first mutable borrow occurs here
 LL |     Push::push(&c, Box::new(|| y = 0));
-   |                             ^^ - borrow occurs due to use of `y` in closure
+   |                             ^^ - second borrow occurs due to use of `y` in closure
    |                             |
    |                             second mutable borrow occurs here
 LL | //~^ ERROR cannot borrow `y` as mutable more than once at a time

src/test/ui/issue-24357.nll.stderr

@@ -2,7 +2,9 @@ error[E0382]: use of moved value: `x`
   --> $DIR/issue-24357.rs:16:12
    |
 LL |    let f = move || { let y = x; };
-   |            ---------------------- value moved here
+   |            -------           - variable moved due to use in closure
+   |            |
+   |            value moved into closure here
 LL |    //~^ NOTE value moved (into closure) here
 LL |    let z = x;
    |            ^ value used here after move

src/test/ui/issue-27282-move-match-input-into-guard.stderr

@@ -1,11 +1,13 @@
 error[E0505]: cannot move out of `b` because it is borrowed
-  --> $DIR/issue-27282-move-match-input-into-guard.rs:26:16
+  --> $DIR/issue-27282-move-match-input-into-guard.rs:26:17
    |
 LL |     match b {
    |           - borrow of `b` occurs here
 LL |         &mut false => {},
 LL |         _ if { (|| { let bar = b; *bar = false; })();
-   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ move out of `b` occurs here
+   |                 ^^             - move occurs due to use in closure
+   |                 |
+   |                 move out of `b` occurs here
 ...
 LL |         &mut true => { println!("You might think we should get here"); },
    |         --------- borrow later used here
@@ -14,7 +16,9 @@ error[E0382]: use of moved value: `*b`
   --> $DIR/issue-27282-move-match-input-into-guard.rs:29:14
    |
 LL |         _ if { (|| { let bar = b; *bar = false; })();
-   |                ----------------------------------- value moved here
+   |                 --             - variable moved due to use in closure
+   |                 |
+   |                 value moved into closure here
 ...
 LL |         &mut true => { println!("You might think we should get here"); },
    |              ^^^^ value used here after move

src/test/ui/issue-27282-mutate-before-diverging-arm-1.stderr

@@ -5,7 +5,7 @@ LL |     match x {
    |           - borrow occurs here
 ...
 LL |             (|| { *x = None; drop(force_fn_once); })();
-   |              ^^    - borrow occurs due to use of `x` in closure
+   |              ^^    - second borrow occurs due to use of `x` in closure
    |              |
    |              closure construction occurs here
 ...

src/test/ui/issue-27282-mutate-before-diverging-arm-2.stderr

@@ -5,7 +5,7 @@ LL |     match x {
    |           - borrow occurs here
 ...
 LL |                 (|| { *x = None; drop(force_fn_once); })();
-   |                  ^^    - borrow occurs due to use of `x` in closure
+   |                  ^^    - second borrow occurs due to use of `x` in closure
    |                  |
    |                  closure construction occurs here
 ...

src/test/ui/issue-4335.nll.stderr

@@ -5,10 +5,12 @@ LL |     id(Box::new(|| *v))
    |                    ^^ cannot move out of captured variable in an `FnMut` closure
 
 error[E0597]: `v` does not live long enough
-  --> $DIR/issue-4335.rs:16:17
+  --> $DIR/issue-4335.rs:16:21
    |
 LL |     id(Box::new(|| *v))
-   |                 ^^^^^ borrowed value does not live long enough
+   |                 --  ^ borrowed value does not live long enough
+   |                 |
+   |                 value captured here
 ...
 LL | }
    | - `v` dropped here while still borrowed

src/test/ui/issue-6801.nll.stderr

@@ -2,7 +2,9 @@ error[E0505]: cannot move out of `x` because it is borrowed
   --> $DIR/issue-6801.rs:29:13
    |
 LL |       let sq =  || { *x * *x };
-   |                 -------------- borrow of `x` occurs here
+   |                 --    - borrow occurs due to use in closure
+   |                 |
+   |                 borrow of `x` occurs here
 LL | 
 LL |       twice(x); //~ ERROR: cannot move out of
    |             ^ move out of `x` occurs here