Merge pull request #2073 from dianne/deref-desugar-scoping-difference

Note scoping differences between `*x` and `*x.deref()` , and `a[b]` and `*a.index(b)`

Author: ehuss

src/expressions/array-expr.md

@@ -84,7 +84,22 @@ r[expr.array.index.array]
 [Array] and [slice]-typed values can be indexed by writing a square-bracket-enclosed expression of type `usize` (the index) after them. When the array is mutable, the resulting [memory location] can be assigned to.
 
 r[expr.array.index.trait]
-For other types an index expression `a[b]` is equivalent to `*std::ops::Index::index(&a, b)`, or `*std::ops::IndexMut::index_mut(&mut a, b)` in a mutable place expression context. Just as with methods, Rust will also insert dereference operations on `a` repeatedly to find an implementation.
+For other types an index expression `a[b]` is equivalent to `*std::ops::Index::index(&a, b)`, or `*std::ops::IndexMut::index_mut(&mut a, b)` in a mutable place expression context, except that when the index expression undergoes [temporary lifetime extension], the indexed expression `a` also has its [temporary scope] extended. Just as with methods, Rust will also insert dereference operations on `a` repeatedly to find an implementation.
+
+```rust
+// The temporary holding the result of `vec![()]` is extended to
+// live to the end of the block, so `x` may be used in subsequent
+// statements.
+let x = &vec![()][0];
+# x;
+```
+
+```rust,compile_fail,E0716
+// The temporary holding the result of `vec![()]` is dropped at the
+// end of the statement, so it's an error to use `y` after.
+let y = &*std::ops::Index::index(&vec![()], 0); // ERROR
+# y;
+```
 
 r[expr.array.index.zero-index]
 Indices are zero-based for arrays and slices.
@@ -127,3 +142,5 @@ The array index expression can be implemented for types other than arrays and sl
 [panic]: ../panic.md
 [path]: path-expr.md
 [slice]: ../types/slice.md
+[temporary lifetime extension]: destructors.scope.lifetime-extension
+[temporary scope]: destructors.scope.temporary

src/expressions/operator-expr.md

@@ -176,7 +176,7 @@ r[expr.deref.safety]
 Dereferencing a raw pointer requires `unsafe`.
 
 r[expr.deref.traits]
-On non-pointer types `*x` is equivalent to `*std::ops::Deref::deref(&x)` in an [immutable place expression context](../expressions.md#mutability) and `*std::ops::DerefMut::deref_mut(&mut x)` in a mutable place expression context.
+On non-pointer types `*x` is equivalent to `*std::ops::Deref::deref(&x)` in an [immutable place expression context](../expressions.md#mutability) and `*std::ops::DerefMut::deref_mut(&mut x)` in a mutable place expression context, except that when `*x` undergoes [temporary lifetime extension], the dereferenced expression `x` also has its [temporary scope] extended.
 
 ```rust
 # struct NoCopy;
@@ -189,6 +189,21 @@ let c = Box::new(NoCopy);
 let d: NoCopy = *c;
 ```
 
+```rust
+// The temporary holding the result of `String::new()` is extended
+// to live to the end of the block, so `x` may be used in subsequent
+// statements.
+let x = &*String::new();
+# x;
+```
+
+```rust,compile_fail,E0716
+// The temporary holding the result of `String::new()` is dropped at
+// the end of the statement, so it's an error to use `y` after.
+let y = &*std::ops::Deref::deref(&String::new()); // ERROR
+# y;
+```
+
 r[expr.try]
 ## The try propagation expression