Add dropck documentation (#1767)

src/SUMMARY.md

@@ -139,6 +139,7 @@
     - [Tracking moves and initialization](./borrow_check/moves_and_initialization.md)
         - [Move paths](./borrow_check/moves_and_initialization/move_paths.md)
     - [MIR type checker](./borrow_check/type_check.md)
+    - [Drop check](./borrow_check/drop_check.md)
     - [Region inference](./borrow_check/region_inference.md)
         - [Constraint propagation](./borrow_check/region_inference/constraint_propagation.md)
         - [Lifetime parameters](./borrow_check/region_inference/lifetime_parameters.md)

src/borrow_check/drop_check.md

@@ -0,0 +1,154 @@
+# Drop Check
+
+We generally require the type of locals to be well-formed whenever the
+local is used. This includes proving the where-bounds of the local and
+also requires all regions used by it to be live.
+
+The only exception to this is when implicitly dropping values when they
+go out of scope. This does not necessarily require the value to be live:
+
+```rust
+fn main() {
+    let x = vec![];
+    {
+        let y = String::from("I am temporary");
+        x.push(&y);
+    }
+    // `x` goes out of scope here, after the reference to `y`
+    // is invalidated. This means that while dropping `x` its type
+    // is not well-formed as it contain regions which are not live.
+}
+```
+
+This is only sound if dropping the value does not try to access any dead
+region. We check this by requiring the type of the value to be
+drop-live.
+The requirements for which are computed in `fn dropck_outlives`.
+
+The rest of this section uses the following type definition for a type
+which requires its region parameter to be live:
+
+```rust
+struct PrintOnDrop<'a>(&'a str);
+impl<'a> Drop for PrintOnDrop<'_> {
+    fn drop(&mut self) {
+        println!("{}", self.0);
+    }
+}
+```
+
+## How values are dropped
+
+At its core, a value of type `T` is dropped by executing its "drop
+glue". Drop glue is compiler generated and first calls `<T as
+Drop>::drop` and then recursively calls the drop glue of any recursively
+owned values.
+
+- If `T` has an explicit `Drop` impl, call `<T as Drop>::drop`.
+- Regardless of whether `T` implements `Drop`, recurse into all values
+  *owned* by `T`:
+    - references, raw pointers, function pointers, function items, trait
+      objects[^traitobj], and scalars do not own anything.
+    - tuples, slices, and arrays consider their elements to be owned.
+      For arrays of length zero we do not own any value of the element
+      type.
+    - all fields (of all variants) of ADTs are considered owned. We
+      consider all variants for enums. The exception here is
+      `ManuallyDrop<U>` which is not considered to own `U`.
+      `PhantomData<U>` also does not own anything.
+      closures and generators own their captured upvars.
+
+Whether a type has drop glue is returned by [`fn
+Ty::needs_drop`](https://github.com/rust-lang/rust/blob/320b412f9c55bf480d26276ff0ab480e4ecb29c0/compiler/rustc_middle/src/ty/util.rs#L1086-L1108).
+
+### Partially dropping a local
+
+For types which do not implement `Drop` themselves, we can also
+partially move parts of the value before dropping the rest. In this case
+only the drop glue for the not-yet moved values is called, e.g.
+
+```rust
+fn main() {
+    let mut x = (PrintOnDrop("third"), PrintOnDrop("first"));
+    drop(x.1);
+    println!("second")
+}
+```
+
+During MIR building we assume that a local may get dropped whenever it
+goes out of scope *as long as its type needs drop*. Computing the exact
+drop glue for a variable happens **after** borrowck in the
+`ElaborateDrops` pass. This means that even if some part of the local
+have been dropped previously, dropck still requires this value to be
+live. This is the case even if we completely moved a local.
+
+```rust
+fn main() {
+    let mut x;
+    {
+        let temp = String::from("I am temporary");
+        x = PrintOnDrop(&temp);
+        drop(x);
+    }
+} //~ ERROR `temp` does not live long enough.
+```
+
+It should be possible to add some amount of drop elaboration before
+borrowck, allowing this example to compile. There is an unstable feature
+to move drop elaboration before const checking:
+[#73255](https://github.com/rust-lang/rust/issues/73255). Such a feature
+gate does not exist for doing some drop elaboration before borrowck,
+although there's a [relevant
+MCP](https://github.com/rust-lang/compiler-team/issues/558).
+
+[^traitobj]: you can consider trait objects to have a builtin `Drop`
+implementation which directly uses the `drop_in_place` provided by the
+vtable. This `Drop` implementation requires all its generic parameters
+to be live.
+
+### `dropck_outlives`
+
+There are two distinct "liveness" computations that we perform:
+
+* a value `v` is *use-live* at location `L` if it may be "used" later; a
+  *use* here is basically anything that is not a *drop*
+* a value `v` is *drop-live* at location `L` if it maybe dropped later
+
+When things are *use-live*, their entire type must be valid at `L`. When
+they are *drop-live*, all regions that are required by dropck must be
+valid at `L`.  The values dropped in the MIR are *places*.
+
+The constraints computed by `dropck_outlives` for a type closely match
+the generated drop glue for that type. Unlike drop glue,
+`dropck_outlives` cares about the types of owned values, not the values
+itself. For a value of type `T`
+
+- if `T` has an explicit `Drop`, require all generic arguments to be
+  live, unless they are marked with `#[may_dangle]` in which case they
+  are fully ignored
+- regardless of whether `T` has an explicit `Drop`, recurse into all
+  types *owned* by `T`
+    - references, raw pointers, function pointers, function items, trait
+      objects[^traitobj], and scalars do not own anything.
+    - tuples, slices and arrays consider their element type to be owned.
+      **For arrays we currently do not check whether their length is
+      zero**.
+    - all fields (of all variants) of ADTs are considered owned. The
+      exception here is `ManuallyDrop<U>` which is not considered to own
+      `U`. **We consider `PhantomData<U>` to own `U`**.
+    - closures and generators own their captured upvars.
+
+The sections marked in bold are cases where `dropck_outlives` considers
+types to be owned which are ignored by `Ty::needs_drop`. We only rely on
+`dropck_outlives` if `Ty::needs_drop` for the containing local returned
+`true`.This means liveness requirements can change depending on whether
+a type is contained in a larger local. **This is inconsistent, and
+should be fixed: an example [for
+arrays](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8b5f5f005a03971b22edb1c20c5e6cbe)
+and [for
+`PhantomData`](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=44c6e2b1fae826329fd54c347603b6c8).**[^core]
+
+One possible way these inconsistencies can be fixed is by MIR building
+to be more pessimistic, probably by making `Ty::needs_drop` weaker, or
+alternatively, changing `dropck_outlives` to be more precise, requiring
+fewer regions to be live.