Merge pull request #1156 from est31/let_else

Document let else statements

Author: ehuss

src/items/functions.md

@@ -59,8 +59,8 @@ fn answer_to_life_the_universe_and_everything() -> i32 {
 
 ## Function parameters
 
-As with `let` bindings, function parameters are irrefutable [patterns], so any
-pattern that is valid in a let binding is also valid as a parameter:
+Function parameters are irrefutable [patterns], so any pattern that is valid in
+an else-less `let` binding is also valid as a parameter:
 
 ```rust
 fn first((value, _): (i32, i32)) -> i32 { value }

src/statements.md

@@ -54,17 +54,38 @@ fn outer() {
 > **<sup>Syntax</sup>**\
 > _LetStatement_ :\
 > &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup> `let` [_PatternNoTopAlt_]
->     ( `:` [_Type_] )<sup>?</sup> (`=` [_Expression_] )<sup>?</sup> `;`
-
-A *`let` statement* introduces a new set of [variables], given by an
-irrefutable [pattern]. The pattern is followed optionally by a type
-annotation and then optionally by an initializer expression. When no
-type annotation is given, the compiler will infer the type, or signal
+>     ( `:` [_Type_] )<sup>?</sup> (`=` [_Expression_] [†](#let-else-restriction)
+>     ( `else` [_BlockExpression_]) <sup>?</sup> ) <sup>?</sup> `;`
+>
+> <span id="let-else-restriction">† When an `else` block is specified, the
+> _Expression_ must not be a [_LazyBooleanExpression_], or end with a `}`.</span>
+
+A *`let` statement* introduces a new set of [variables], given by a [pattern].
+The pattern is followed optionally by a type annotation and then either ends,
+or is followed by an initializer expression plus an optional `else` block.
+When no type annotation is given, the compiler will infer the type, or signal
 an error if insufficient type information is available for definite
 inference. Any variables introduced by a variable declaration are visible
 from the point of declaration until the end of the enclosing block scope,
 except when they are shadowed by another variable declaration.
 
+If an `else` block is not present, the pattern must be irrefutable.
+If an `else` block is present, the pattern may be refutable.
+If the pattern does not match (this requires it to be refutable), the `else`
+block is executed.
+The `else` block must always diverge (evaluate to the [never type]).
+
+```rust
+let (mut v, w) = (vec![1, 2, 3], 42); // The bindings may be mut or const
+let Some(t) = v.pop() else { // Refutable patterns require an else block
+    panic!(); // The else block must diverge
+};
+let [u, v] = [v[0], v[1]] else { // This pattern is irrefutable, so the compiler
+                                 // will lint as the else block is redundant.
+    panic!();
+};
+```
+
 ## Expression statements
 
 > **<sup>Syntax</sup>**\
@@ -121,16 +142,19 @@ statement are [`cfg`], and [the lint check attributes].
 [function]: items/functions.md
 [item]: items.md
 [module]: items/modules.md
+[never type]: types/never.md
 [canonical path]: paths.md#canonical-paths
 [implementations]: items/implementations.md
 [variables]: variables.md
 [outer attributes]: attributes.md
 [`cfg`]: conditional-compilation.md
 [the lint check attributes]: attributes/diagnostics.md#lint-check-attributes
 [pattern]: patterns.md
+[_BlockExpression_]: expressions/block-expr.md
 [_ExpressionStatement_]: #expression-statements
 [_Expression_]: expressions.md
 [_Item_]: items.md
+[_LazyBooleanExpression_]: expressions/operator-expr.md#lazy-boolean-operators
 [_LetStatement_]: #let-statements
 [_MacroInvocationSemi_]: macros.md#macro-invocation
 [_OuterAttribute_]: attributes.md