Destructuring assignment

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+- Feature Name: `destructuring_assignment`
+- Start Date: 2020-04-17
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#71126](https://github.com/rust-lang/rust/issues/71126)
+- Proof-of-concept: [rust-lang/rust#71156](https://github.com/rust-lang/rust/pull/71156)
+
+# Summary
+[summary]: #summary
+
+We allow destructuring on assignment, as in `let` declarations. For instance, the following are now
+accepted:
+
+```rust
+(a, b) = (0, 1);
+(x, y, .., z) = (1.0, 2.0, 3.0, 4.0, 5.0);
+[_, f] = foo();
+[g, _, h, ..] = ['a', 'w', 'e', 's', 'o', 'm', 'e', '!'];
+Struct { x: a, y: b } = bar();
+```
+
+This brings assignment in line with `let` declaration, in which destructuring is permitted. This
+will simplify and improve idiomatic code involving mutability.
+
+# Motivation
+[motivation]: #motivation
+
+Destructuring assignment increases the consistency of the language, in which assignment is typically
+expected to behave similarly to variable declations. The aim is that this feature will increase the
+clarity and concision of idiomatic Rust, primarily in code that makes use of mutability. This
+feature is
+[highly desired among Rust developers](https://github.com/rust-lang/rfcs/issues/372).
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+You may destructure a value when making an assignment, just as when you declare variables. See the
+[Summary](#Summary) for examples. The following structures may be destructured:
+
+- Tuples.
+- Slices.
+- (Tuple) structs.
+
+You may use `_` and `..` as in a normal declaration pattern to ignore certain values.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The feature as described here has been implemented as a proof-of-concept (https://github.com/rust-lang/rust/pull/71156). It follows essentially the
+[suggestions of @Kimundi](https://github.com/rust-lang/rfcs/issues/372#issuecomment-214022963) and
+[of @drunwald](https://github.com/rust-lang/rfcs/issues/372#issuecomment-262519146).
+
+The Rust compiler already parses complex expressions on the left-hand side of an assignment, but
+does not handle them other than emitting an error later in compilation. We propose to add
+special-casing for several classes of expressions on the left-hand side of an assignment, which act
+in accordance with destructuring assignment: i.e. as if the left-hand side were actually a pattern.
+Actually supporting patterns directly on the left-hand side of an assignment significantly
+complicates Rust's grammar and it is not clear that it is even technically feasible. Conversely,
+handling some classes of expressions is much simpler, and is indistinguishable to users, who will
+receive pattern-oriented diagnostics due to the desugaring of expressions into patterns.
+
+The general idea is that we will desugar the following complex assignments as demonstrated.
+
+```rust
+(a, b) = (3, 4);
+
+[a, b] = [3, 4];
+
+Struct { x: a, y: b } = Struct { x: 3, y: 4};
+
+// desugars to:
+
+{
+    let (_a, _b) = (3, 4);
+    a = _a;
+    b = _b;
+}
+
+{
+    let [_a, _b] = [3, 4];
+    a = _a;
+    b = _b;
+}
+
+{
+    let Struct { x: _a, y: _b } = Struct { x: 3, y: 4};
+    a = _a;
+    b = _b;
+}
+```
+
+We support the following classes of expressions:
+
+- Tuples.
+- Slices.
+- (Tuple) structs.
+
+In the desugaring, we convert the expression `(a, b)` into an analogous pattern `(_a, _b)` (whose
+identifiers are fresh and thus do not conflict with existing variables). A nice side-effect is that
+we inherit the diagnostics for normal pattern-matching, so users benefit from existing diagnostics for destructuring declarations.
+
+## Diagnostics
+
+It is worth being explicit that, in the implementation, the diagnostics that are reported are
+pattern diagnostics: that is, because the desugaring occurs regardless, the messages will imply that
+the left-hand side of an assignment is a true pattern (the one the expression has been converted
+to). For example:
+
+```rust
+[*a] = [1, 2]; // error: pattern requires 1 element but array has 2
+```
+
+Whilst `[*a]` is not strictly speaking a pattern, it behaves similarly to one in this context. We
+think that this results in a better user experience, as intuitively the left-hand side of a
+destructuring assignment acts like a pattern "in spirit", but this is technically false: we should
+be careful that this does not result in misleading diagnostics.
+
+## Underscores and ellipses
+
+In patterns, we may use `_` and `..` to ignore certain values, without binding them. While range
+patterns already have analogues in terms of range expressions, the underscore wildcard pattern
+currently has no analogous expression. We thus add one, which is only permitted in the left-hand side
+of an assignment: any other use results in the same "reserved identifier" error that currently
+occurs for invalid uses of `_` as an expression. A consequence is that the following becomes valid:
+
+```rust
+_ = 5;
+```
+
+Functional record update syntax (i.e. `..x`) is forbidden in destructuring assignment, as we believe
+there is no sensible and clear semantics for it in this setting. This restriction could be relaxed
+in the future if a use-case is found.
+
+The desugaring treats the `_` expression as an `_` pattern and the fully empty range `..` as a `..` pattern. No corresponding assignments are generated. For example:
+
+```rust
+let mut a;
+(a, _) = (3, 4);
+(.., a) = (1, 2, 3, 4);
+
+// desugars to:
+
+{
+    let (_a, _) = (3, 4);
+    a = _a;
+}
+
+{
+    let (.., _a) = (1, 2, 3, 4);
+    a = _a;
+}
+```
+
+and similarly for slices and structs.
+
+## Unsupported patterns
+
+We do not support the following "patterns" in destructuring assignment:
+
+- `&x = foo();`.
+- `&mut x = foo();`.
+- `ref x = foo();`.
+
+This is primarily for learnability: the behaviour of `&` can already be slightly confusing to
+newcomers, as it has different meanings depending on whether it is used in an expression or pattern.
+In destructuring assignment, the left-hand side of an assignment consists of sub*expressions*, but
+which act intuitively like patterns, so it is not clear what `&` and friends should mean. We feel it is more
+confusing than helpful to allow these cases. Conversely, destructuring tuples, slices or structs is
+very natural and we do not foresee confusion with allowing these.
+
+Our implementation is forwards-compatible with allowing these patterns in destructuring assigmnent, in any case, so we lose nothing by not allowing them from the start.
+
+## Compound destructuring assignment
+
+We forbid destructuring compound assignment, i.e. destructuring for operators like `+=`, `*=` and so
+on. This is both for the sake of simplicity and since there are relevant design questions that do not have obvious answers,
+e.g. how this could interact with custom implementations of the operators.
+
+## Order-of-assignment
+
+The right-hand side of the assignment is always evaluated first. Then, assignments are performed
+left-to-right. Note that component expressions in the left-hand side may be complex, and not simply
+identifiers.
+
+In a declaration, each identifier may be bound at most once. That is, the following is invalid:
+
+```rust
+let (a, a) = (1, 2);
+```
+
+For destructuring assignments, we currently permit assignments containing identical identifiers. However, these trigger an "unused assignment"
+warning.
+
+```rust
+(a, a) = (1, 2); // warning: value assigned to `a` is never read
+assert_eq!(a, 2);
+```
+
+We could try to explicitly forbid this. However, the chosen behaviour is justified in two ways:
+- A destructuring
+assignment can always be written as a series of assignments, so this behaviour matches its
+expansion.
+- In general, we are not able to tell when overlapping
+assignments are made, so the error would be fallible. This is illustrated by the following example:
+
+```rust
+fn foo<'a>(x: &'a mut u32) -> &'a mut u32 {
+    x
+}
+
+fn main() {
+    let mut x: u32 = 10;
+    // We cannot tell that the same variable is being assigned to
+    // in this instance.
+    (*foo(&mut x), *foo(&mut x)) = (5, 6);
+    assert_eq!(x, 6);
+}
+```
+
+We thus feel that a lint is more appropriate.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- It could be argued that this feature increases the surface area of the language and thus complexity. However, we feel that by decreasing surprise, it actually makes the language less complex for users.
+- It is possible that these changes could result in some confusing diagnostics. However, we have not found any during testing, and these could in any case be ironed out before stabilisation.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+As we argue above, we believe this change increases the perceived consistency of Rust and improves
+idiomatic code in the presence of mutability, and that the
+implementation is simple and intuitive.
+
+One potential alternative that has been put forth in the past is to allow arbitrary patterns on the left-hand side of an assignment,
+but as discussed above and [extensively in this
+thread](https://github.com/rust-lang/rfcs/issues/372), it is difficult to see how this could work in
+practice (especially with complex left-hand sides that do not simply involve identifiers) and it is not clear that this would have any advantages.
+
+# Prior art
+[prior-art]: #prior-art
+
+The most persuasive prior art is Rust itself, which already permits destructuring
+declarations. Intuitively, a declaration is an assignment that also introduces a new binding.
+Therefore, it seems clear that assignments should act similarly to declarations where possible.
+However, it is also the case that destructuring assignments are present in many languages that permit destructuring
+declarations.
+
+- JavaScript
+[supports destructuring assignment](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment).
+- Python [supports destructuring assignment](https://blog.tecladocode.com/destructuring-in-python/).
+- Perl
+[supports destructuring assignment](https://perl6advent.wordpress.com/2017/12/05/day-5-destructure-your-arguments-with-perl-6-signatures/).
+- And so on...
+
+It is a general pattern that languages support destructuring assignment when they support
+destructuring declarations.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+- The implementation already supports destructuring of every class of expressions that currently make
+sense in Rust. This feature naturally should be extended to any new class of expressions for which
+it makes sense.
+- It could make sense to permit
+[destructuring compound assignments](#Compound-destructuring-assignment) in the future, though we
+defer this question for later discussions.
+- It could make sense to permit [`ref` and `&`](#Unsupported-patterns) in the future.
+- It [has been suggested](https://github.com/rust-lang/rfcs/issues/372#issuecomment-365606878) that
+mixed declarations and assignments could be permitted, as in the following:
+
+```rust
+let a;
+(a, let b) = (1, 2);
+assert_eq!((a, b), (1, 2));
+```
+
+We do not pursue this here, but note that it would be compatible with our desugaring.