update coerce docs and unify relevant tests

src/librustc_typeck/check/coercion.rs

@@ -10,45 +10,30 @@
 //!
 //! Note that if we are expecting a reference, we will *reborrow*
 //! even if the argument provided was already a reference. This is
-//! useful for freezing mut/const things (that is, when the expected is &T
-//! but you have &const T or &mut T) and also for avoiding the linearity
+//! useful for freezing mut things (that is, when the expected type is &T
+//! but you have &mut T) and also for avoiding the linearity
 //! of mut things (when the expected is &mut T and you have &mut T). See
-//! the various `src/test/ui/coerce-reborrow-*.rs` tests for
+//! the various `src/test/ui/coerce/*.rs` tests for
 //! examples of where this is useful.
 //!
 //! ## Subtle note
 //!
-//! When deciding what type coercions to consider, we do not attempt to
-//! resolve any type variables we may encounter. This is because `b`
-//! represents the expected type "as the user wrote it", meaning that if
-//! the user defined a generic function like
+//! When infering the generic arguments of functions, the argument
+//! order is relevant, which can lead to the following edge case:
 //!
-//!    fn foo<A>(a: A, b: A) { ... }
+//! ```rust
+//! fn foo<T>(a: T, b: T) {
+//!     // ...
+//! }
 //!
-//! and then we wrote `foo(&1, @2)`, we will not auto-borrow
-//! either argument. In older code we went to some lengths to
-//! resolve the `b` variable, which could mean that we'd
-//! auto-borrow later arguments but not earlier ones, which
-//! seems very confusing.
+//! foo(&7i32, &mut 7i32);
+//! // This compiles, as we first infer `T` to be `&i32`,
+//! // and then coerce `&mut 7i32` to `&7i32`.
 //!
-//! ## Subtler note
-//!
-//! However, right now, if the user manually specifies the
-//! values for the type variables, as so:
-//!
-//!    foo::<&int>(@1, @2)
-//!
-//! then we *will* auto-borrow, because we can't distinguish this from a
-//! function that declared `&int`. This is inconsistent but it's easiest
-//! at the moment. The right thing to do, I think, is to consider the
-//! *unsubstituted* type when deciding whether to auto-borrow, but the
-//! *substituted* type when considering the bounds and so forth. But most
-//! of our methods don't give access to the unsubstituted type, and
-//! rightly so because they'd be error-prone. So maybe the thing to do is
-//! to actually determine the kind of coercions that should occur
-//! separately and pass them in. Or maybe it's ok as is. Anyway, it's
-//! sort of a minor point so I've opted to leave it for later -- after all,
-//! we may want to adjust precisely when coercions occur.
+//! foo(&mut 7i32, &7i32);
+//! // This does not compile, as we first infer `T` to be `&mut i32`
+//! // and are then unable to coerce `&7i32` to `&mut i32`.
+//! ```
 
 use crate::astconv::AstConv;
 use crate::check::{FnCtxt, Needs};
@@ -96,6 +81,8 @@ impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
 
 type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
 
+/// Coercing a mutable reference to an immutable works, while
+/// coercing `&T` to `&mut T` should be forbidden.
 fn coerce_mutbls<'tcx>(
     from_mutbl: hir::Mutability,
     to_mutbl: hir::Mutability,

src/test/ui/coercion/coerce-overloaded-autoderef-fail.rs

@@ -0,0 +1,32 @@
+fn borrow_mut<T>(x: &mut T) -> &mut T { x }
+fn borrow<T>(x: &T) -> &T { x }
+
+fn borrow_mut2<T>(_: &mut T, _: &mut T) {}
+fn borrow2<T>(_: &mut T, _: &T) {}
+
+fn double_mut_borrow<T>(x: &mut Box<T>) {
+    let y = borrow_mut(x);
+    let z = borrow_mut(x);
+    //~^ ERROR cannot borrow `*x` as mutable more than once at a time
+    drop((y, z));
+}
+
+fn double_imm_borrow(x: &mut Box<i32>) {
+    let y = borrow(x);
+    let z = borrow(x);
+    **x += 1;
+    //~^ ERROR cannot assign to `**x` because it is borrowed
+    drop((y, z));
+}
+
+fn double_mut_borrow2<T>(x: &mut Box<T>) {
+    borrow_mut2(x, x);
+    //~^ ERROR cannot borrow `*x` as mutable more than once at a time
+}
+
+fn double_borrow2<T>(x: &mut Box<T>) {
+    borrow2(x, x);
+    //~^ ERROR cannot borrow `*x` as mutable because it is also borrowed as immutable
+}
+
+pub fn main() {}

src/test/ui/coercion/coerce-overloaded-autoderef.stderr → src/test/ui/coercion/coerce-overloaded-autoderef-fail.stderr

@@ -1,5 +1,5 @@
 error[E0499]: cannot borrow `*x` as mutable more than once at a time
-  --> $DIR/coerce-overloaded-autoderef.rs:9:24
+  --> $DIR/coerce-overloaded-autoderef-fail.rs:9:24
    |
 LL |     let y = borrow_mut(x);
    |                        - first mutable borrow occurs here
@@ -10,7 +10,7 @@ LL |     drop((y, z));
    |           - first borrow later used here
 
 error[E0506]: cannot assign to `**x` because it is borrowed
-  --> $DIR/coerce-overloaded-autoderef.rs:17:5
+  --> $DIR/coerce-overloaded-autoderef-fail.rs:17:5
    |
 LL |     let y = borrow(x);
    |                    - borrow of `**x` occurs here
@@ -22,7 +22,7 @@ LL |     drop((y, z));
    |           - borrow later used here
 
 error[E0499]: cannot borrow `*x` as mutable more than once at a time
-  --> $DIR/coerce-overloaded-autoderef.rs:23:20
+  --> $DIR/coerce-overloaded-autoderef-fail.rs:23:20
    |
 LL |     borrow_mut2(x, x);
    |     ----------- -  ^ second mutable borrow occurs here
@@ -31,7 +31,7 @@ LL |     borrow_mut2(x, x);
    |     first borrow later used by call
 
 error[E0502]: cannot borrow `*x` as mutable because it is also borrowed as immutable
-  --> $DIR/coerce-overloaded-autoderef.rs:28:5
+  --> $DIR/coerce-overloaded-autoderef-fail.rs:28:5
    |
 LL |     borrow2(x, x);
    |     -------^^^^-^

src/test/ui/coercion/coerce-overloaded-autoderef.rs

@@ -1,32 +1,68 @@
-fn borrow_mut<T>(x: &mut T) -> &mut T { x }
-fn borrow<T>(x: &T) -> &T { x }
+// run-pass
+#![allow(unused_braces)]
+#![allow(dead_code)]
+// pretty-expanded FIXME #23616
 
-fn borrow_mut2<T>(_: &mut T, _: &mut T) {}
-fn borrow2<T>(_: &mut T, _: &T) {}
+use std::rc::Rc;
 
-fn double_mut_borrow<T>(x: &mut Box<T>) {
-    let y = borrow_mut(x);
-    let z = borrow_mut(x);
-    //~^ ERROR cannot borrow `*x` as mutable more than once at a time
-    drop((y, z));
+// Examples from the "deref coercions" RFC, at rust-lang/rfcs#241.
+
+fn use_ref<T>(_: &T) {}
+fn use_mut<T>(_: &mut T) {}
+
+fn use_rc<T>(t: Rc<T>) {
+    use_ref(&*t);  // what you have to write today
+    use_ref(&t);   // what you'd be able to write
+    use_ref(&&&&&&t);
+    use_ref(&mut &&&&&t);
+    use_ref(&&&mut &&&t);
+}
+
+fn use_mut_box<T>(mut t: &mut Box<T>) {
+    use_mut(&mut *t); // what you have to write today
+    use_mut(t);       // what you'd be able to write
+    use_mut(&mut &mut &mut t);
+
+    use_ref(&*t);      // what you have to write today
+    use_ref(t);        // what you'd be able to write
+    use_ref(&&&&&&t);
+    use_ref(&mut &&&&&t);
+    use_ref(&&&mut &&&t);
 }
 
-fn double_imm_borrow(x: &mut Box<i32>) {
-    let y = borrow(x);
-    let z = borrow(x);
-    **x += 1;
-    //~^ ERROR cannot assign to `**x` because it is borrowed
-    drop((y, z));
+fn use_nested<T>(t: &Box<T>) {
+    use_ref(&**t);  // what you have to write today
+    use_ref(t);     // what you'd be able to write (note: recursive deref)
+    use_ref(&&&&&&t);
+    use_ref(&mut &&&&&t);
+    use_ref(&&&mut &&&t);
+}
+
+fn use_slice(_: &[u8]) {}
+fn use_slice_mut(_: &mut [u8]) {}
+
+fn use_vec(mut v: Vec<u8>) {
+    use_slice_mut(&mut v[..]); // what you have to write today
+    use_slice_mut(&mut v);     // what you'd be able to write
+    use_slice_mut(&mut &mut &mut v);
+
+    use_slice(&v[..]);  // what you have to write today
+    use_slice(&v);      // what you'd be able to write
+    use_slice(&&&&&&v);
+    use_slice(&mut &&&&&v);
+    use_slice(&&&mut &&&v);
 }
 
-fn double_mut_borrow2<T>(x: &mut Box<T>) {
-    borrow_mut2(x, x);
-    //~^ ERROR cannot borrow `*x` as mutable more than once at a time
+fn use_vec_ref(v: &Vec<u8>) {
+    use_slice(&v[..]);  // what you have to write today
+    use_slice(v);       // what you'd be able to write
+    use_slice(&&&&&&v);
+    use_slice(&mut &&&&&v);
+    use_slice(&&&mut &&&v);
 }
 
-fn double_borrow2<T>(x: &mut Box<T>) {
-    borrow2(x, x);
-    //~^ ERROR cannot borrow `*x` as mutable because it is also borrowed as immutable
+fn use_op_rhs(s: &mut String) {
+    *s += {&String::from(" ")};
 }
 
 pub fn main() {}

src/test/ui/coercion/coerce-reborrow-multi-arg-fail.rs

@@ -0,0 +1,6 @@
+fn test<T>(_a: T, _b: T) {}
+
+fn main() {
+    test(&mut 7, &7);
+    //~^ mismatched types
+}

src/test/ui/coercion/coerce-reborrow-multi-arg-fail.stderr

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/coerce-reborrow-multi-arg-fail.rs:4:18
+   |
+LL |     test(&mut 7, &7);
+   |                  ^^ types differ in mutability
+   |
+   = note: expected mutable reference `&mut {integer}`
+                      found reference `&{integer}`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/coercion/coerce-reborrow-multi-arg.rs

@@ -0,0 +1,9 @@
+// build-pass
+fn test<T>(_a: T, _b: T) {}
+
+fn main() {
+    test(&7, &7);
+    test(&7, &mut 7);
+    test::<&i32>(&mut 7, &7);
+    test::<&i32>(&mut 7, &mut 7);
+}

src/test/ui/coercion/coerce-to-bang-cast.rs

@@ -1,7 +1,5 @@
 #![feature(never_type)]
 
-fn foo(x: usize, y: !, z: usize) { }
-
 fn cast_a() {
     let y = {return; 22} as !;
     //~^ ERROR non-primitive cast

src/test/ui/coercion/coerce-to-bang-cast.stderr

@@ -1,11 +1,11 @@
 error[E0605]: non-primitive cast: `i32` as `!`
-  --> $DIR/coerce-to-bang-cast.rs:6:13
+  --> $DIR/coerce-to-bang-cast.rs:4:13
    |
 LL |     let y = {return; 22} as !;
    |             ^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
 
 error[E0605]: non-primitive cast: `i32` as `!`
-  --> $DIR/coerce-to-bang-cast.rs:11:13
+  --> $DIR/coerce-to-bang-cast.rs:9:13
    |
 LL |     let y = 22 as !;
    |             ^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object