You signed in with another tab or window. Reload to refresh your session.You signed out in another tab or window. Reload to refresh your session.You switched accounts on another tab or window. Reload to refresh your session.Dismiss alert
(`expr[expr]`), and [field references](#field-expressions) (`expr.f`).
2212
2212
All other expressions are rvalues.
2213
2213
2214
-
The left operand of an [assignment](#assignment-expressions),
2215
-
[binary move](#binary-move-expressions) or
2214
+
The left operand of an [assignment](#assignment-expressions) or
2216
2215
[compound-assignment](#compound-assignment-expressions) expression is an lvalue context,
2217
-
as is the single operand of a unary [borrow](#unary-operator-expressions),
2218
-
or [move](#unary-move-expressions) expression,
2219
-
and _both_ operands of a [swap](#swap-expressions) expression.
2216
+
as is the single operand of a unary [borrow](#unary-operator-expressions).
2220
2217
All other expression contexts are rvalue contexts.
2221
2218
2222
2219
When an lvalue is evaluated in an _lvalue context_, it denotes a memory location;
@@ -2229,9 +2226,8 @@ A temporary's lifetime equals the largest lifetime of any reference that points
2229
2226
2230
2227
When a [local variable](#memory-slots) is used
2231
2228
as an [rvalue](#lvalues-rvalues-and-temporaries)
2232
-
the variable will either be [moved](#move-expressions) or copied,
2233
-
depending on its type.
2234
-
For types that contain [owning pointers](#owning-pointers)
2229
+
the variable will either be moved or copied, depending on its type.
2230
+
For types that contain [owning pointers](#pointer-types)
2235
2231
or values that implement the special trait `Drop`,
2236
2232
the variable is moved.
2237
2233
All other types are copied.
@@ -2890,16 +2886,26 @@ match x {
2890
2886
2891
2887
The first pattern matches lists constructed by applying `Cons` to any head value, and a
2892
2888
tail value of `~Nil`. The second pattern matches _any_ list constructed with `Cons`,
2893
-
ignoring the values of its arguments. The difference between `_` and `*` is that the pattern `C(_)` is only type-correct if
2894
-
`C` has exactly one argument, while the pattern `C(..)` is type-correct for any enum variant `C`, regardless of how many arguments `C` has.
2895
-
2896
-
To execute an `match` expression, first the head expression is evaluated, then
2897
-
its value is sequentially compared to the patterns in the arms until a match
2889
+
ignoring the values of its arguments. The difference between `_` and `*` is that the pattern
2890
+
`C(_)` is only type-correct if `C` has exactly one argument, while the pattern `C(..)` is
2891
+
type-correct for any enum variant `C`, regardless of how many arguments `C` has.
2892
+
2893
+
A `match` behaves differently depending on whether or not the head expression
2894
+
is an [lvalue or an rvalue](#lvalues-rvalues-and-temporaries).
2895
+
If the head expression is an rvalue, it is
2896
+
first evaluated into a temporary location, and the resulting value
2897
+
is sequentially compared to the patterns in the arms until a match
2898
2898
is found. The first arm with a matching pattern is chosen as the branch target
2899
2899
of the `match`, any variables bound by the pattern are assigned to local
2900
2900
variables in the arm's block, and control enters the block.
2901
2901
2902
-
An example of an `match` expression:
2902
+
When the head expression is an lvalue, the match does not allocate a
2903
+
temporary location (however, a by-value binding may copy or move from
2904
+
the lvalue). When possible, it is preferable to match on lvalues, as the
2905
+
lifetime of these matches inherits the lifetime of the lvalue, rather
2906
+
than being restricted to the inside of the match.
2907
+
2908
+
An example of a `match` expression:
2903
2909
2904
2910
~~~~
2905
2911
# fn process_pair(a: int, b: int) { }
@@ -2929,19 +2935,31 @@ Patterns that bind variables
2929
2935
default to binding to a copy or move of the matched value
2930
2936
(depending on the matched value's type).
2931
2937
This can be changed to bind to a reference by
2932
-
using the ```ref``` keyword,
2933
-
or to a mutable reference using ```ref mut```.
2934
-
2935
-
A pattern that's just an identifier,
2936
-
like `Nil` in the previous answer,
2937
-
could either refer to an enum variant that's in scope,
2938
-
or bind a new variable.
2939
-
The compiler resolves this ambiguity by forbidding variable bindings that occur in ```match``` patterns from shadowing names of variants that are in scope.
2940
-
For example, wherever ```List``` is in scope,
2941
-
a ```match``` pattern would not be able to bind ```Nil``` as a new name.
2942
-
The compiler interprets a variable pattern `x` as a binding _only_ if there is no variant named `x` in scope.
2943
-
A convention you can use to avoid conflicts is simply to name variants with upper-case letters,
2944
-
and local variables with lower-case letters.
2938
+
using the `ref` keyword,
2939
+
or to a mutable reference using `ref mut`.
2940
+
2941
+
Patterns can also dereference pointers by using the `&`,
2942
+
`~` or `@` symbols, as appropriate. For example, these two matches
2943
+
on `x: &int` are equivalent:
2944
+
2945
+
~~~~
2946
+
# let x = &3;
2947
+
let y = match *x { 0 => "zero", _ => "some" };
2948
+
let z = match x { &0 => "zero", _ => "some" };
2949
+
2950
+
assert_eq!(y, z);
2951
+
~~~~
2952
+
2953
+
A pattern that's just an identifier, like `Nil` in the previous answer,
2954
+
could either refer to an enum variant that's in scope, or bind a new variable.
2955
+
The compiler resolves this ambiguity by forbidding variable bindings that occur
2956
+
in `match` patterns from shadowing names of variants that are in scope.
2957
+
For example, wherever `List` is in scope,
2958
+
a `match` pattern would not be able to bind `Nil` as a new name.
2959
+
The compiler interprets a variable pattern `x` as a binding _only_ if there is
2960
+
no variant named `x` in scope.
2961
+
A convention you can use to avoid conflicts is simply to name variants with
2962
+
upper-case letters, and local variables with lower-case letters.
2945
2963
2946
2964
Multiple match patterns may be joined with the `|` operator.
2947
2965
A range of values may be specified with `..`.
@@ -3122,19 +3140,20 @@ A `struct` *type* is a heterogeneous product of other types, called the *fields*
3122
3140
the *record* types of the ML family,
3123
3141
or the *structure* types of the Lisp family.]
3124
3142
3125
-
New instances of a `struct` can be constructed with a [struct expression](#struct-expressions).
3143
+
New instances of a `struct` can be constructed with a [struct expression](#structure-expressions).
3126
3144
3127
3145
The memory order of fields in a `struct` is given by the item defining it.
3128
3146
Fields may be given in any order in a corresponding struct *expression*;
3129
3147
the resulting `struct` value will always be laid out in memory in the order specified by the corresponding *item*.
3130
3148
3131
-
The fields of a `struct` may be qualified by [visibility modifiers](#visibility-modifiers),
3149
+
The fields of a `struct` may be qualified by [visibility modifiers](#re-exporting-and-visibility),
3132
3150
to restrict access to implementation-private data in a structure.
3133
3151
3134
3152
A _tuple struct_ type is just like a structure type, except that the fields are anonymous.
3135
3153
3136
3154
A _unit-like struct_ type is like a structure type, except that it has no fields.
3137
-
The one value constructed by the associated [structure expression](#structure-expression) is the only value that inhabits such a type.
3155
+
The one value constructed by the associated [structure expression](#structure-expressions)
3156
+
is the only value that inhabits such a type.
3138
3157
3139
3158
### Enumerated types
3140
3159
@@ -3805,7 +3824,7 @@ over the output format of a Rust crate.
3805
3824
### Logging system
3806
3825
3807
3826
The runtime contains a system for directing [logging
3808
-
expressions](#log-expressions) to a logging console and/or internal logging
3827
+
expressions](#logging-expressions) to a logging console and/or internal logging
3809
3828
buffers. Logging can be enabled per module.
3810
3829
3811
3830
Logging output is enabled by setting the `RUST_LOG` environment
0 commit comments