8000 Unify while/while let and if/if let expressions. · rust-lang/reference@b83d8e6 · GitHub
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Unify while/while let and if/if let expressions.
This updates the `if` and `while` expressions so that they each are presented as a single expression kind with multiple condition operators instead of being logically separated from their `let` counterparts. This also includes various fixes and additions.
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‎src/SUMMARY.md

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@@ -62,7 +62,7 @@
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- [Closure expressions](expressions/closure-expr.md)
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- [Loop expressions](expressions/loop-expr.md)
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- [Range expressions](expressions/range-expr.md)
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- [If and if let expressions](expressions/if-expr.md)
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- [If expressions](expressions/if-expr.md)
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- [Match expressions](expressions/match-expr.md)
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- [Return expressions](expressions/return-expr.md)
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- [Await expressions](expressions/await-expr.md)

‎src/attributes/type_system.md

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@@ -133,7 +133,6 @@ Non-exhaustive types are always considered inhabited in downstream crates.
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[_StructExpression_]: ../expressions/struct-expr.md
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[_StructPattern_]: ../patterns.md#struct-patterns
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[_TupleStructPattern_]: ../patterns.md#tuple-struct-patterns
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[`if let`]: ../expressions/if-expr.md#if-let-expressions
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[`match`]: ../expressions/match-expr.md
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[attributes]: ../attributes.md
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[enum]: ../items/enumerations.md

‎src/const_eval.md

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@@ -45,8 +45,8 @@ to be run.
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* pointer to address casts and
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* function pointer to address casts.
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* Calls of [const functions] and const methods.
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* [loop], [while] and [`while let`] expressions.
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* [if], [`if let`] and [match] expressions.
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* [loop] and [while] expressions.
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* [if] and [match] expressions.
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## Const context
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@@ -121,7 +121,6 @@ Conversely, the following are possible in a const function, but not in a const c
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[grouped]: expressions/grouped-expr.md
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[interior mutability]: interior-mutability.md
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[if]: expressions/if-expr.md#if-expressions
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[`if let`]: expressions/if-expr.md#if-let-expressions
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[lazy boolean]: A3E2 expressions/operator-expr.md#lazy-boolean-operators
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[let statements]: statements.md#let-statements
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[literals]: expressions/literal-expr.md
@@ -138,4 +137,3 @@ Conversely, the following are possible in a const function, but not in a const c
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[struct]: expressions/struct-expr.md
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[tuple expressions]: expressions/tuple-expr.md
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[while]: expressions/loop-expr.md#predicate-loops
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[`while let`]: expressions/loop-expr.md#predicate-pattern-loops

‎src/destructors.md

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@@ -388,12 +388,12 @@ variable or field from being dropped automatically.
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[tuple expression]: expressions/tuple-expr.md#tuple-expressions
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[`for`]: expressions/loop-expr.md#iterator-loops
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[`if let`]: expressions/if-expr.md#if-let-expressions
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[`if let`]: expressions/if-expr.md#if-let-patterns
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[`if`]: expressions/if-expr.md#if-expressions
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[`let` statement]: statements.md#let-statements
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[`loop`]: expressions/loop-expr.md#infinite-loops
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[`match`]: expressions/match-expr.md
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[`while let`]: expressions/loop-expr.md#predicate-pattern-loops
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[`while let`]: expressions/loop-expr.md#while-let-patterns
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[`while`]: expressions/loop-expr.md#predicate-loops
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[`<T as std::ops::Drop>::drop`]: ../std/ops/trait.Drop.html#tymethod.drop

‎src/expressions.md

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@@ -38,7 +38,6 @@
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> &nbsp;&nbsp; &nbsp;&nbsp; | [_UnsafeBlockExpression_]\
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> &nbsp;&nbsp; &nbsp;&nbsp; | [_LoopExpression_]\
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> &nbsp;&nbsp; &nbsp;&nbsp; | [_IfExpression_]\
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> &nbsp;&nbsp; &nbsp;&nbsp; | [_IfLetExpression_]\
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> &nbsp;&nbsp; &nbsp;&nbsp; | [_MatchExpression_]\
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> &nbsp;&nbsp; )
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@@ -295,13 +294,13 @@ They are never allowed before:
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[call expressions]: expressions/call-expr.md
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[field]: expressions/field-expr.md
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[functional update]: expressions/struct-expr.md#functional-update-syntax
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[`if let`]: expressions/if-expr.md#if-let-expressions
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[`if let`]: expressions/if-expr.md#if-let-patterns
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[match]: expressions/match-expr.md
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[method-call]: expressions/method-call-expr.md
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[paths]: expressions/path-expr.md
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[struct]: expressions/struct-expr.md
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[tuple expressions]: expressions/tuple-expr.md
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[`while let`]: expressions/loop-expr.md#predicate-pattern-loops
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[`while let`]: expressions/loop-expr.md#while-let-patterns
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[array expressions]: expressions/array-expr.md
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[array indexing]: expressions/array-expr.md#array-and-slice-indexing-expressions
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[_FieldExpression_]: expressions/field-expr.md
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[_GroupedExpression_]: expressions/grouped-expr.md
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[_IfExpression_]: expressions/if-expr.md#if-expressions
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[_IfLetExpression_]: expressions/if-expr.md#if-let-expressions
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[_IndexExpression_]: expressions/array-expr.md#array-and-slice-indexing-expressions
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[_LazyBooleanExpression_]: expressions/operator-expr.md#lazy-boolean-operators
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[_LiteralExpression_]: expressions/literal-expr.md

‎src/expressions/block-expr.md

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@@ -145,7 +145,7 @@ let a = unsafe { an_unsafe_fn() };
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[Inner attributes] are allowed directly after the opening brace of a block expression in the following situations:
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* [Function] and [method] bodies.
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* Loop bodies ([`loop`], [`while`], [`while let`], and [`for`]).
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* Loop bodies ([`loop`], [`while`], and [`for`]).
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* Block expressions used as a [statement].
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* Block expressions as elements of [array expressions], [tuple expressions],
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[call expressions], and tuple-style [struct] expressions.
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[`loop`]: loop-expr.md#infinite-loops
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[`std::ops::Fn`]: ../../std/ops/trait.Fn.html
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[`std::future::Future`]: ../../std/future/trait.Future.html
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[`while let`]: loop-expr.md#predicate-pattern-loops
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[`while`]: loop-expr.md#predicate-loops
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[array expressions]: array-expr.md
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[call expressions]: call-expr.md

‎src/expressions/if-expr.md

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@@ -2,29 +2,30 @@
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## Syntax
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The same syntax is used by `if`, `if let` and chains of expressions.
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> **<sup>Syntax</sup>**\
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> _IfExpression_ :\
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> &nbsp;&nbsp; `if` _IfLetList_ [_BlockExpression_]\
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> &nbsp;&nbsp; ( `else` _IfLetList_ [_BlockExpression_] )<sup>\?</sup>
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> &nbsp;&nbsp; `if` _IfConditions_ [_BlockExpression_]\
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> &nbsp;&nbsp; (`else` ( [_BlockExpression_] | _IfExpression_ ) )<sup>\?</sup>
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>
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> _IfLet_ :\
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> &nbsp;&nbsp; [_Expression_]<sub>_except struct expression_</sub>
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> | `let` [_Pattern_] `=` [_Expression_]<sub>_except struct expression_</sub>
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> _IfConditions_ :\
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> &nbsp;&nbsp; _IfCondition_ ( && _IfCondition_ )*
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>
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> _IfLetList_ :\
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> &nbsp;&nbsp; _IfLet_ ( && _IfLet_ )*
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## `if`
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> _IfCondition_ :\
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> &nbsp;&nbsp; &nbsp;&nbsp; [_Expression_]<sub>_except struct expression_</sub>\
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> &nbsp;&nbsp; | `let` [_Pattern_] `=` [_Scrutinee_]
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An `if` expression is a conditional branch in program control.
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The syntax of an `if` expression is a condition operand, followed by a consequent block, any number of `else if` conditions and blocks, and an optional trailing `else` block.
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The condition operands must have the [boolean type].
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If a condition operand evaluates to `true`, the consequent block is executed and any subsequent `else if` or `else` block is skipped.
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If a condition operand evaluates to `false`, the consequent block is skipped and any subsequent `else if` condition is evaluated.
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The syntax of an `if` expression is a sequence of one or more condition operands separated by `&&`,
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followed by a consequent block, any number of `else if` conditions and blocks, and an optional trailing `else` block.
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Condition operands must be either an [_Expression_] with a [boolean type] or a conditional `let` match.
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If all of the condition operands evaluate to `true` and all of the `let` patterns successfully match their [scrutinee]s,
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the consequent block is executed and any subsequent `else if` or `else` block is skipped.
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If any condition operand evaluates to `false` or any `let` pattern does not match its scrutinee,
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the consequent block is skipped and any subsequent `else if` condition is evaluated.
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If all `if` and `else if` conditions evaluate to `false` then any `else` block is executed.
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An if expression evaluates to the same value as the executed block, or `()` if no block is evaluated.
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An `if` expression evaluates to the same value as the executed block, or `()` if no block is evaluated.
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An `if` expression must have the same type in all situations.
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```rust
@@ -37,6 +38,7 @@ if x == 4 {
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println!("x is something else");
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}
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// `if` can be used as an expression.
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let y = if 12 * 15 > 150 {
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"Bigger"
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} else {
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assert_eq!(y, "Bigger");
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```
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## `if let`
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## `if let` patterns
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An `if let` expression is semantically similar to an `if` expression but in place of a condition operand it expects the keyword `let` followed by a pattern, an `=` and a [scrutinee] operand.
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If the value of the scrutinee matches the pattern, the corresponding block will execute.
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Otherwise, flow proceeds to the following `else` block if it exists.
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Like `if` expressions, `if let` expressions have a value determined by the block that is evaluated.
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`let` patterns in an `if` condition allow binding new variables into scope when the pattern matches successfully.
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The following examples illustrate bindings using `let` patterns:
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```rust
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let dish = ("Ham", "Eggs");
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// this body will be skipped because the pattern is refuted
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// This body will be skipped because the pattern is refuted.
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if let ("Bacon", b) = dish {
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println!("Bacon is served with {}", b);
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} else {
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// This block is evaluated instead.
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println!("No bacon will be served");
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}
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// this body will execute
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// This body will execute.
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if let ("Ham", b) = dish {
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println!("Ham is served with {}", b);
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}
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}
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```
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`if` and `if let` expressions can be intermixed:
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```rust
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let x = Some(3);
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let a = if let Some(1) = x {
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1
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} else if x == Some(2) {
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2
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} else if let Some(y) = x {
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y
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} else {
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-1
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};
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assert_eq!(a, 3);
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```
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Multiple patterns may be specified with the `|` operator. This has the same semantics as with `|` in `match` expressions:
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Multiple patterns may be specified with the `|` operator.
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This has the same semantics as with `|` in [`match` expressions]:
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```rust
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enum E {
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}
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```
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The expression cannot be a [lazy boolean operator expression][_LazyBooleanOperatorExpression_].
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Scrutinee expressions also cannot be a [lazy boolean operator expression][_LazyBooleanOperatorExpression_] due to ambiguity and precedence with [chains of expressions][_ChainsOfExpressions_].
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## Chains of conditions
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## Chains of expressions
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Multiple condition operands can be separated with `&&`.
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Similar to a `&&` [_LazyBooleanOperatorExpression_], each operand is evaluated from left-to-right until an operand evaluates as `false` or a `let` match fails,
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in which case the subsequent operands are not evaluated.
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The bindings of each pattern are put into scope to be available for the next condition operand and the consequent block.
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The following is an example of chaining multiple expressions, mixing `let` bindings and boolean expressions, and with expressions able to reference pattern bindings from previous expressions:
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println!("Peek a boo");
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}
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}
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```
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The above is equivalent to the following without using expression chains:
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The above is equivalent to the following without using chains of conditions:
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```rust
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fn nested() {
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let outer_opt = Some(Some(1i32));
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}
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```
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In other words, chains are equivalent to nested [`if let` expressions]:
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If any condition operand is a `let` pattern, then none of the condition operands can be a `||` [lazy boolean operator expression][_LazyBooleanOperatorExpression_] due to ambiguity and precedence with the `let` scrutinee.
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If a `||` expression is needed, then parentheses can be used. For example:
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<!-- ignore: expansion example -->
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```rust,ignore
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if let PAT0 = EXPR0 && let PAT1 = EXPR1 {
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/* body */
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} else {
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/* else */
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}
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```rust
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# let foo = Some(123);
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# let condition1 = true;
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# let condition2 = false;
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// Parentheses are required here.
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if let Some(x) = foo && (condition1 || condition2) { /*...*/ }
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```
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is equivalent to
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<!-- ignore: expansion example -->
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```rust,ignore
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if let PAT0 = EXPR0 {
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if let PAT1 = EXPR1 {
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/* body */
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}
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else {
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/* else */
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}
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} else {
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/* else */
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}
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```
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[_BlockExpression_]: block-expr.md
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[_ChainsOfExpressions_]: #chains-of-expressions
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[_Expression_]: ../expressions.md
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[_LazyBooleanOperatorExpression_]: operator-expr.md#lazy-boolean-operators
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[_Pattern_]: ../patterns.md
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[_Scrutinee_]: match-expr.md
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[`match` expression]: match-expr.md
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[`match` expressions]: match-expr.md
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[boolean type]: ../types/boolean.md
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[scrutinee]: ../glossary.md#scrutinee

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