Clarify what access struct updates do

Author: matthewjasper

src/expressions.md

@@ -98,11 +98,16 @@ expressions are value expressions.
 
 A *value expression* is an expression that represents an actual value.
 
-The left operand of an [assignment][assign] or [compound assignment] expression
-is a place expression context, as is the single operand of a unary [borrow], and
-the operand of any [implicit borrow]. The discriminant or subject of a
-[match expression][match] and right side of a [let statement] is also a place
-expression context. All other expression contexts are value expression contexts.
+The following contexts are *place expression* contexts:
+
+* The left operand of an [assignment][assign] or [compound assignment] expression.
+* The operand of a unary [borrow] or [dereference] operator.
+* The operand of a field expression.
+* The indexed operand of an array indexing expression.
+* The operand of any [implicit borrow].
+* The initializer of a [let statement].
+* The [scrutinee] of a [match expression][match].
+* The base of a [functional update] struct expression.
 
 > Note: Historically, place expressions were called *lvalues* and value
 > expressions were called *rvalues*.
@@ -279,6 +284,7 @@ They are never allowed before:
 [closure expressions]:  expressions/closure-expr.html
 [enum variant]:         expressions/enum-variant-expr.html
 [field]:                expressions/field-expr.html
+[functional update]:    expressions/struct-expr.html#functional-update-syntax
 [grouped]:              expressions/grouped-expr.html
 [literals]:             expressions/literal-expr.html
 [match]:                expressions/match-expr.html
@@ -316,6 +322,7 @@ They are never allowed before:
 [let statement]:        statements.html#let-statements
 [Mutable `static` items]: items/static-items.html#mutable-statics
 [const contexts]:       const_eval.html
+[scrutinee]:            glossary.html#scrutinee
 [slice]:                types/slice.html
 [statement]:            statements.html
 [static variables]:     items/static-items.html

src/expressions/struct-expr.md

@@ -56,19 +56,23 @@ colon.
 A value of a [union] type can also be created using this syntax, except that it must
 specify exactly one field.
 
+## Functional update syntax
+
 A struct expression can terminate with the syntax `..` followed by an
 expression to denote a functional update. The expression following `..` (the
-base) must have the same struct type as the new struct type being formed. The
-entire expression denotes the result of constructing a new struct (with the
-same type as the base expression) with the given values for the fields that
-were explicitly specified and the values in the base expression for all other
-fields. Just as with all struct expressions, all of the fields of the struct
-must be [visible], even those not explicitly named.
+base) must have the same struct type as the new struct type being formed.
+
+The entire expression uses the given values for the fields that were specified
+and moves or copies the remaining fields from the base expression. Just as with
+all struct expressions, all of the fields of the struct must be [visible], even
+those not explicitly named.
 
 ```rust
 # struct Point3d { x: i32, y: i32, z: i32 }
-let base = Point3d {x: 1, y: 2, z: 3};
-Point3d {y: 0, z: 10, .. base};
+let mut base = Point3d {x: 1, y: 2, z: 3};
+let y_ref = &mut base.y;
+Point3d {y: 0, z: 10, .. base}; // OK, only base.x is accessed
+drop(y_ref);
 ```
 
 Struct expressions with curly braces can't be used directly in a [loop] or [if]