Auto merge of #54976 - davidtwco:issue-52663-special-case-closures, r…

…=nikomatsakis

NLL lacks various special case handling of closures

Part of #52663.

Firstly, this PR extends existing handling of closures to also support generators.

Second, this PR adds the note found in the AST when a closure is invoked twice and captures a variable by-value:

```text
note: closure cannot be invoked more than once because it moves the variable `dict` out of its environment
  --> $DIR/issue-42065.rs:16:29
   |
LL |         for (key, value) in dict {
   |                             ^^^^
```

r? @nikomatsakis
cc @pnkfelix

src/librustc_borrowck/borrowck/check_loans.rs

@@ -609,12 +609,12 @@ impl<'a, 'tcx> CheckLoanCtxt<'a, 'tcx> {
                         new_loan.span, &nl, old_loan.span, previous_end_span, Origin::Ast),
                 (ty::UniqueImmBorrow, _) =>
                     self.bccx.cannot_uniquely_borrow_by_one_closure(
-                        new_loan.span, &nl, &new_loan_msg,
+                        new_loan.span, "closure", &nl, &new_loan_msg,
                         old_loan.span, &ol_pronoun, &old_loan_msg, previous_end_span, Origin::Ast),
                 (_, ty::UniqueImmBorrow) => {
                     let new_loan_str = &new_loan.kind.to_user_str();
                     self.bccx.cannot_reborrow_already_uniquely_borrowed(
-                        new_loan.span, &nl, &new_loan_msg, new_loan_str,
+                        new_loan.span, "closure", &nl, &new_loan_msg, new_loan_str,
                         old_loan.span, &old_loan_msg, previous_end_span, Origin::Ast)
                 }
                 (..) =>

src/librustc_mir/util/borrowck_errors.rs

@@ -221,6 +221,7 @@ pub trait BorrowckErrors<'cx>: Sized + Copy {
     fn cannot_uniquely_borrow_by_one_closure(
         self,
         new_loan_span: Span,
+        container_name: &str,
         desc_new: &str,
         opt_via: &str,
         old_loan_span: Span,
@@ -241,7 +242,7 @@ pub trait BorrowckErrors<'cx>: Sized + Copy {
         );
         err.span_label(
             new_loan_span,
-            format!("closure construction occurs here{}", opt_via),
+            format!("{} construction occurs here{}", container_name, opt_via),
         );
         err.span_label(old_loan_span, format!("borrow occurs here{}", old_opt_via));
         if let Some(previous_end_span) = previous_end_span {
@@ -253,6 +254,7 @@ pub trait BorrowckErrors<'cx>: Sized + Copy {
     fn cannot_reborrow_already_uniquely_borrowed(
         self,
         new_loan_span: Span,
+        container_name: &str,
         desc_new: &str,
         opt_via: &str,
         kind_new: &str,
@@ -275,7 +277,7 @@ pub trait BorrowckErrors<'cx>: Sized + Copy {
         err.span_label(new_loan_span, format!("borrow occurs here{}", opt_via));
         err.span_label(
             old_loan_span,
-            format!("closure construction occurs here{}", old_opt_via),
+            format!("{} construction occurs here{}", container_name, old_opt_via),
         );
         if let Some(previous_end_span) = previous_end_span {
             err.span_label(previous_end_span, "borrow from closure ends here");

src/test/ui/generator/borrowing.nll.stderr

@@ -1,10 +1,11 @@
 error[E0597]: `a` does not live long enough
-  --> $DIR/borrowing.rs:18:18
+  --> $DIR/borrowing.rs:18:29
    |
 LL |         unsafe { (|| yield &a).resume() }
-   |                  ^^^^^^^^^^^^^
-   |                  |
-   |                  borrowed value does not live long enough
+   |                  -----------^-
+   |                  ||         |
+   |                  ||         borrowed value does not live long enough
+   |                  |value captured here by generator
    |                  a temporary with access to the borrow is created here ...
 LL |         //~^ ERROR: `a` does not live long enough
 LL |     };
@@ -15,18 +16,18 @@ LL |     };
    = note: The temporary is part of an expression at the end of a block. Consider forcing this temporary to be dropped sooner, before the block's local variables are dropped. For example, you could save the expression's value in a new local variable `x` and then make `x` be the expression at the end of the block.
 
 error[E0597]: `a` does not live long enough
-  --> $DIR/borrowing.rs:24:9
+  --> $DIR/borrowing.rs:25:20
    |
-LL |       let _b = {
-   |           -- borrow later stored here
-LL |           let a = 3;
-LL | /         || {
-LL | |             yield &a
-LL | |             //~^ ERROR: `a` does not live long enough
-LL | |         }
-   | |_________^ borrowed value does not live long enough
-LL |       };
-   |       - `a` dropped here while still borrowed
+LL |     let _b = {
+   |         -- borrow later stored here
+LL |         let a = 3;
+LL |         || {
+   |         -- value captured here by generator
+LL |             yield &a
+   |                    ^ borrowed value does not live long enough
+...
+LL |     };
+   |     - `a` dropped here while still borrowed
 
 error: aborting due to 2 previous errors
 

src/test/ui/generator/dropck.nll.stderr

@@ -13,21 +13,19 @@ LL | }
    = note: values in a scope are dropped in the opposite order they are defined
 
 error[E0597]: `ref_` does not live long enough
-  --> $DIR/dropck.rs:22:11
+  --> $DIR/dropck.rs:24:18
    |
-LL |       gen = || {
-   |  ___________^
-LL | |         // but the generator can use it to drop a `Ref<'a, i32>`.
-LL | |         let _d = ref_.take(); //~ ERROR `ref_` does not live long enough
-LL | |         yield;
-LL | |     };
-   | |_____^ borrowed value does not live long enough
+LL |     gen = || {
+   |           -- value captured here by generator
+LL |         // but the generator can use it to drop a `Ref<'a, i32>`.
+LL |         let _d = ref_.take(); //~ ERROR `ref_` does not live long enough
+   |                  ^^^^ borrowed value does not live long enough
 ...
-LL |   }
-   |   -
-   |   |
-   |   `ref_` dropped here while still borrowed
-   |   borrow might be used here, when `gen` is dropped and runs the destructor for generator
+LL | }
+   | -
+   | |
+   | `ref_` dropped here while still borrowed
+   | borrow might be used here, when `gen` is dropped and runs the destructor for generator
    |
    = note: values in a scope are dropped in the opposite order they are defined
 

src/test/ui/generator/yield-while-iterating.nll.stderr

@@ -9,17 +9,15 @@ LL |             yield();
 error[E0502]: cannot borrow `x` as immutable because it is also borrowed as mutable
   --> $DIR/yield-while-iterating.rs:67:20
    |
-LL |       let mut b = || {
-   |  _________________-
-LL | |         for p in &mut x {
-LL | |             yield p;
-LL | |         }
-LL | |     };
-   | |_____- mutable borrow occurs here
-LL |       println!("{}", x[0]); //~ ERROR
-   |                      ^ immutable borrow occurs here
-LL |       b.resume();
-   |       - mutable borrow later used here
+LL |     let mut b = || {
+   |                 -- mutable borrow occurs here
+LL |         for p in &mut x {
+   |                       - first borrow occurs due to use of `x` in generator
+...
+LL |     println!("{}", x[0]); //~ ERROR
+   |                    ^ immutable borrow occurs here
+LL |     b.resume();
+   |     - mutable borrow later used here
 
 error: aborting due to 2 previous errors
 

src/test/ui/generator/yield-while-ref-reborrowed.nll.stderr

@@ -1,17 +1,15 @@
 error[E0501]: cannot borrow `x` as immutable because previous closure requires unique access
   --> $DIR/yield-while-ref-reborrowed.rs:45:20
    |
-LL |       let mut b = || {
-   |  _________________-
-LL | |         let a = &mut *x;
-LL | |         yield();
-LL | |         println!("{}", a);
-LL | |     };
-   | |_____- closure construction occurs here
-LL |       println!("{}", x); //~ ERROR
-   |                      ^ borrow occurs here
-LL |       b.resume();
-   |       - first borrow later used here
+LL |     let mut b = || {
+   |                 -- generator construction occurs here
+LL |         let a = &mut *x;
+   |                       - first borrow occurs due to use of `x` in generator
+...
+LL |     println!("{}", x); //~ ERROR
+   |                    ^ borrow occurs here
+LL |     b.resume();
+   |     - first borrow later used here
 
 error: aborting due to previous error