add "temporary value dropped while borrowed" error

Issue #54131

Author: mikhail-m1

src/librustc_mir/borrow_check/error_reporting.rs

@@ -628,9 +628,9 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
 
         let tcx = self.tcx;
         let mut err =
-            tcx.path_does_not_live_long_enough(proper_span, "borrowed value", Origin::Mir);
-        err.span_label(proper_span, "temporary value does not live long enough");
-        err.span_label(drop_span, "temporary value only lives until here");
+            tcx.temporary_value_borrowed_for_too_long(proper_span, Origin::Mir);
+        err.span_label(proper_span, "creates a temporary which is freed while still in use");
+        err.span_label(drop_span, "temporary value is freed at the end of this statement");
 
         // Only give this note and suggestion if they could be relevant
         match reason {

src/librustc_mir/diagnostics.rs

@@ -2241,7 +2241,92 @@ that after `demo` finishes excuting, something else (such as the
 destructor!) could access `s.data` after the end of that shorter
 lifetime, which would again violate the `&mut`-borrow's exclusive
 access.
+"##,
+
+E0714: r##"
+This error indicates that a temporary value is being dropped
+while a borrow is still in active use.
+
+Erroneous code example:
+
+```compile_fail,E0714
+# #![feature(nll)]
+fn foo() -> i32 { 22 }
+fn bar(x: &i32) -> &i32 { x }
+let p = bar(&foo());
+         // ------ creates a temporary
+let q = *p;
+```
+
+Here, the expression `&foo()` is borrowing the expression
+`foo()`. As `foo()` is call to a function, and not the name of
+a variable, this creates a **temporary** -- that temporary stores
+the return value from `foo()` so that it can be borrowed.
+So you might imagine that `let p = bar(&foo())` is equivalent
+to this:
+
+```compile_fail,E0597
+# fn foo() -> i32 { 22 }
+# fn bar(x: &i32) -> &i32 { x }
+let p = {
+  let tmp = foo(); // the temporary
+  bar(&tmp)
+}; // <-- tmp is freed as we exit this block
+let q = p;
+```
+
+Whenever a temporary is created, it is automatically dropped (freed)
+according to fixed rules. Ordinarily, the temporary is dropped
+at the end of the enclosing statement -- in this case, after the `let`.
+This is illustrated in the example above by showing that `tmp` would
+be freed as we exit the block.
+
+To fix this problem, you need to create a local variable
+to store the value in rather than relying on a temporary.
+For example, you might change the original program to
+the following:
 
+```
+fn foo() -> i32 { 22 }
+fn bar(x: &i32) -> &i32 { x }
+let value = foo(); // dropped at the end of the enclosing block
+let p = bar(&value);
+let q = *p;
+```
+
+By introducing the explicit `let value`, we allocate storage
+that will last until the end of the enclosing block (when `value`
+goes out of scope). When we borrow `&value`, we are borrowing a
+local variable that already exists, and hence no temporary is created.
+
+Temporaries are not always dropped at the end of the enclosing
+statement. In simple cases where the `&` expression is immediately
+stored into a variable, the compiler will automatically extend
+the lifetime of the temporary until the end of the enclosinb
+block. Therefore, an alternative way to fix the original
+program is to write `let tmp = &foo()` and not `let tmp = foo()`:
+
+```
+fn foo() -> i32 { 22 }
+fn bar(x: &i32) -> &i32 { x }
+let value = &foo();
+let p = bar(value);
+let q = *p;
+```
+
+Here, we are still borrowing `foo()`, but as the borrow is assigned
+directly into a variable, the temporary will not be dropped until
+the end of the enclosing block. Similar rules apply when temporaries
+are stored into aggregate structures like a tuple or struct:
+
+```
+// Here, two temporaries are created, but
+// as they are stored directly into `value`,
+// they are not dropped until the end of the
+// enclosing block.
+fn foo() -> i32 { 22 }
+let value = (&foo(), &foo());
+```
 "##,
 
 }

src/librustc_mir/util/borrowck_errors.rs

@@ -707,6 +707,22 @@ pub trait BorrowckErrors<'cx>: Sized + Copy {
 
         self.cancel_if_wrong_origin(err, o)
     }
+
+    fn temporary_value_borrowed_for_too_long(
+        self,
+        span: Span,
+        o: Origin,
+    ) -> DiagnosticBuilder<'cx> {
+        let err = struct_span_err!(
+            self,
+            span,
+            E0714,
+            "temporary value dropped while borrowed{OGN}",
+            OGN = o
+        );
+
+        self.cancel_if_wrong_origin(err, o)
+    }
 }
 
 impl<'cx, 'gcx, 'tcx> BorrowckErrors<'cx> for TyCtxt<'cx, 'gcx, 'tcx> {

src/test/ui/borrowck/borrowck-borrow-from-temporary.nll.stderr

@@ -0,0 +1,18 @@
+error[E0714]: temporary value dropped while borrowed
+  --> $DIR/borrowck-borrow-from-temporary.rs:19:24
+   |
+LL |     let &Foo(ref x) = &id(Foo(3)); //~ ERROR borrowed value does not live long enough
+   |                        ^^^^^^^^^^ creates a temporary which is freed while still in use
+LL |     x
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+note: borrowed value must be valid for the lifetime 'a as defined on the function body at 18:8...
+  --> $DIR/borrowck-borrow-from-temporary.rs:18:8
+   |
+LL | fn foo<'a>() -> &'a isize {
+   |        ^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0714`.

src/test/ui/borrowck/borrowck-borrowed-uniq-rvalue-2.nll.stderr

@@ -1,10 +1,10 @@
-error[E0597]: borrowed value does not live long enough
+error[E0714]: temporary value dropped while borrowed
   --> $DIR/borrowck-borrowed-uniq-rvalue-2.rs:32:20
    |
 LL |     let x = defer(&vec!["Goodbye", "world!"]);
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^ - temporary value only lives until here
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^ - temporary value is freed at the end of this statement
    |                    |
-   |                    temporary value does not live long enough
+   |                    creates a temporary which is freed while still in use
 LL |     x.x[0];
    |     ------ borrow later used here
    |
@@ -13,4 +13,4 @@ LL |     x.x[0];
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0597`.
+For more information about this error, try `rustc --explain E0714`.

src/test/ui/borrowck/borrowck-borrowed-uniq-rvalue.nll.stderr

@@ -1,10 +1,10 @@
-error[E0597]: borrowed value does not live long enough
+error[E0714]: temporary value dropped while borrowed
   --> $DIR/borrowck-borrowed-uniq-rvalue.rs:20:28
    |
 LL |     buggy_map.insert(42, &*Box::new(1)); //~ ERROR borrowed value does not live long enough
-   |                            ^^^^^^^^^^^ - temporary value only lives until here
+   |                            ^^^^^^^^^^^ - temporary value is freed at the end of this statement
    |                            |
-   |                            temporary value does not live long enough
+   |                            creates a temporary which is freed while still in use
 ...
 LL |     buggy_map.insert(43, &*tmp);
    |     --------- borrow later used here
@@ -13,4 +13,4 @@ LL |     buggy_map.insert(43, &*tmp);
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0597`.
+For more information about this error, try `rustc --explain E0714`.

src/test/ui/borrowck/promote-ref-mut-in-let-issue-46557.nll.stderr

@@ -1,57 +1,57 @@
-error[E0597]: borrowed value does not live long enough
+error[E0714]: temporary value dropped while borrowed
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:15:21
    |
 LL |     let ref mut x = 1234543; //~ ERROR
-   |                     ^^^^^^^ temporary value does not live long enough
+   |                     ^^^^^^^ creates a temporary which is freed while still in use
 LL |     x
 LL | }
-   | - temporary value only lives until here
+   | - temporary value is freed at the end of this statement
    |
    = note: borrowed value must be valid for the static lifetime...
 
-error[E0597]: borrowed value does not live long enough
+error[E0714]: temporary value dropped while borrowed
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:20:25
    |
 LL |     let (ref mut x, ) = (1234543, ); //~ ERROR
-   |                         ^^^^^^^^^^^ temporary value does not live long enough
+   |                         ^^^^^^^^^^^ creates a temporary which is freed while still in use
 LL |     x
 LL | }
-   | - temporary value only lives until here
+   | - temporary value is freed at the end of this statement
    |
    = note: borrowed value must be valid for the static lifetime...
 
-error[E0597]: borrowed value does not live long enough
+error[E0714]: temporary value dropped while borrowed
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:25:11
    |
 LL |     match 1234543 {
-   |           ^^^^^^^ temporary value does not live long enough
+   |           ^^^^^^^ creates a temporary which is freed while still in use
 ...
 LL | }
-   | - temporary value only lives until here
+   | - temporary value is freed at the end of this statement
    |
    = note: borrowed value must be valid for the static lifetime...
 
-error[E0597]: borrowed value does not live long enough
+error[E0714]: temporary value dropped while borrowed
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:31:11
    |
 LL |     match (123443,) {
-   |           ^^^^^^^^^ temporary value does not live long enough
+   |           ^^^^^^^^^ creates a temporary which is freed while still in use
 ...
 LL | }
-   | - temporary value only lives until here
+   | - temporary value is freed at the end of this statement
    |
    = note: borrowed value must be valid for the static lifetime...
 
-error[E0597]: borrowed value does not live long enough
+error[E0714]: temporary value dropped while borrowed
   --> $DIR/promote-ref-mut-in-let-issue-46557.rs:37:10
    |
 LL |     &mut 1234543 //~ ERROR
-   |          ^^^^^^^ temporary value does not live long enough
+   |          ^^^^^^^ creates a temporary which is freed while still in use
 LL | }
-   | - temporary value only lives until here
+   | - temporary value is freed at the end of this statement
    |
    = note: borrowed value must be valid for the static lifetime...
 
 error: aborting due to 5 previous errors
 
-For more information about this error, try `rustc --explain E0597`.
+For more information about this error, try `rustc --explain E0714`.

src/test/ui/consts/const-eval/dont_promote_unstable_const_fn.nll.stderr

@@ -0,0 +1,43 @@
+error: `foo` is not yet stable as a const fn
+  --> $DIR/dont_promote_unstable_const_fn.rs:25:25
+   |
+LL | const fn bar() -> u32 { foo() } //~ ERROR `foo` is not yet stable as a const fn
+   |                         ^^^^^
+   |
+   = help: in Nightly builds, add `#![feature(foo)]` to the crate attributes to enable
+
+error[E0714]: temporary value dropped while borrowed
+  --> $DIR/dont_promote_unstable_const_fn.rs:28:28
+   |
+LL |     let _: &'static u32 = &foo(); //~ ERROR does not live long enough
+   |                            ^^^^^ creates a temporary which is freed while still in use
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0714]: temporary value dropped while borrowed
+  --> $DIR/dont_promote_unstable_const_fn.rs:32:28
+   |
+LL |     let _: &'static u32 = &meh(); //~ ERROR does not live long enough
+   |                            ^^^^^ creates a temporary which is freed while still in use
+...
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0714]: temporary value dropped while borrowed
+  --> $DIR/dont_promote_unstable_const_fn.rs:33:26
+   |
+LL |     let x: &'static _ = &std::time::Duration::from_millis(42).subsec_millis();
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
+LL |     //~^ does not live long enough
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0714`.

src/test/ui/consts/const-eval/dont_promote_unstable_const_fn_cross_crate.nll.stderr

@@ -0,0 +1,24 @@
+error[E0714]: temporary value dropped while borrowed
+  --> $DIR/dont_promote_unstable_const_fn_cross_crate.rs:18:28
+   |
+LL |     let _: &'static u32 = &foo(); //~ ERROR does not live long enough
+   |                            ^^^^^ creates a temporary which is freed while still in use
+LL |     let _x: &'static u32 = &foo(); //~ ERROR does not live long enough
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0714]: temporary value dropped while borrowed
+  --> $DIR/dont_promote_unstable_const_fn_cross_crate.rs:19:29
+   |
+LL |     let _x: &'static u32 = &foo(); //~ ERROR does not live long enough
+   |                             ^^^^^ creates a temporary which is freed while still in use
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0714`.

src/test/ui/consts/const-eval/promoted_raw_ptr_ops.nll.stderr

@@ -0,0 +1,35 @@
+error[E0714]: temporary value dropped while borrowed
+  --> $DIR/promoted_raw_ptr_ops.rs:14:29
+   |
+LL |     let x: &'static bool = &(42 as *const i32 == 43 as *const i32);
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
+...
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0714]: temporary value dropped while borrowed
+  --> $DIR/promoted_raw_ptr_ops.rs:16:30
+   |
+LL |     let y: &'static usize = &(&1 as *const i32 as usize + 1); //~ ERROR does not live long enough
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
+LL |     let z: &'static i32 = &(unsafe { *(42 as *const i32) }); //~ ERROR does not live long enough
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error[E0714]: temporary value dropped while borrowed
+  --> $DIR/promoted_raw_ptr_ops.rs:17:28
+   |
+LL |     let z: &'static i32 = &(unsafe { *(42 as *const i32) }); //~ ERROR does not live long enough
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
+LL | }
+   | - temporary value is freed at the end of this statement
+   |
+   = note: borrowed value must be valid for the static lifetime...
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0714`.