RFC: Partial Types (v3)

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+- Feature Name: `partial_types`
+- Start Date: 2024-12-06
+- RFC PR: [rust-lang/rfcs#3736](https://github.com/rust-lang/rfcs/pull/3736)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+
+# Summary
+[summary]: #summary
+
+This proposal is universal flexible tool to work **safe** and **zero cost binary** with partial Structs and Tuples in parameters, arguments, references and  borrows.
+
+Advantages: maximum type safety, maximum type control guarantee, no ambiguities, zero-cost-binary, flexibility, usability and universality.
+
+
+# Motivation
+[motivation]: #motivation
+
+Partial Types proposal is a generalization on "partial borrowing"-like proposals. Safe, Flexible controllable partial parameters for functions and partial consumption (including partial borrowing) are highly needed.
+
+Partial Types extension gives to Product Types (`PT = T1 and T2 and T3 and ..`), Structs and Tuples first of all, a good **mathematical guarantee** to borrow-checker that borrowing the whole variable with partial type and pretending to borrow just permitted fields is **fully safe** (without using `unsafe`).
+```rust
+struct StructABC { a: u32, b: i64, c: f32, }
+
+// function with partial parameter Struct
+fn ref_a (s : & StructABC.{a}) -> &u32 {
+    &s.a
+}
+
+let s = StructABC {a: 4, b: 7, c: 0.0};
+
+// partial expression at argument
+let sa = ref_a(& s.{a});
+```
+
+And since it is a guarantee by **type**, not by **values**, it has _zero cost_ in binary! Any type error is a compiler error, so no errors in the runtime.
+
+This extension is not only fully backward-compatible, but is fully forward-compatible! Forward-compatibility is an ability to use updated functions old way.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Partiality of type (or partial type access) is written as `Path.{fld1, fld2, fld3}` after Path (Type name), where `fld1`, `fld2`, .. are only permitted read (and maybe to write if variable is `mut`) fields of this type, the rest of fields are forbidden to read and write and unset.
+
+But the same time fields that are forbidden to read and write it is totally Ok to borrow, re-borrow, move, re-move, without any consequences - because the Compiler guarantee that in safe mode it is impossible to use such fields. It is a compile error if someone try to access it.
+
+## Partial Structs and Tuples
+
+For Product Types `PT = T1 and T2 and T3 and ..`), for structs, tuples we need not only partiality of a type, but also "partial access" expression: `Expr .{fld1, fld2, fld3}`, where `fld1`, `fld2`, .. are permitted fields of this type, the rest of fields are forbidden.
+
+One step to partial borrows Structs and Tuples.
+```rust
+struct Point {
+    x: f64,
+    y: f64, 
+    was_x: f64, 
+    was_y: f64,
+    state : f64,
+}
+let mut p1 = Point {x:1.0, y:2.0, was_x: 4.0, was_y: 5.0, state: 12.0};
+    // p1 : Point
+
+let ref_p1was = &mut p1.{wax_x, was_y};
+    // ref_p1was : &mut Point.{was_x, was_y}
+
+let ref_p1now = &mut p1.{x, y};
+    // ref_p1now : &mut Point.{x, y}
+```
+It is simple and will be possible. 
+
+It is easy to write functions, which consume partial parameters:
+```rust
+impl Point {
+    fn ref_x (self : & Self.{x}) -> &f64 {
+        &self.x
+    }
+
+    fn refmut_y (self : &mut Self.{y}) -> &mut f64 {
+        &mut self.y
+    }
+}
+let ref_p1x = p1.ref_x();
+let refmut_p1y = p1.refmut_y();
+```
+It is expected, that `self` is **always** cut partiality of argument by same partiality as self-parameter by partial expression before use (even if implicit rules are off)!
+
+Pseudo-rust:
+```rust
+fn ref_xy (self : & Self.{ x, y}) -> &f64 {
+    /*  */
+}
+
+p1.ref_xy();
+// which "desugar"
+Point::ref_xy(& p1.{x, y});
+```
+
+Product-Typed argument type must match with function parameter type or argument type could has **more** permitted partiality then parameter type.
+```rust
+// Struct ~ Product Type
+struct S4 {a : i32, b : i32, c : i32, d : i32}
+
+fn do_sab(s : S4.{a, b}) { /* .. */ }
+
+let s = S4 {a: 6, b: 7, c: 8, d: 9};
+
+do_sab(s.{a});       // s.{a} - error
+do_sab(s.{b});       // s.{b} - error
+do_sab(s.{a, b});    // s.{a, b} - Ok
+do_sab(s.{a, b, c}); // s.{a, b, c} - Ok
+do_sab(s);           // s.{*} - Ok
+```
+
+
+# Reference-level explanation
+
+The core Idea of this proposal is "Proxy Borrowing" - we borrow the whole variable, but borrow-checker pretends it borrow just permitted/allowed fields.
+
+Automatically Type-checker gives a mathematical guarantee, because all denied/forbidden fields remain intact! 
+
+And this mean, that Proxy Borrowing borrowing is fully **safe** and _zero cost_ in binary.
+
+## Proxy Borrowing
+
+Borrowing rules for partial types:
+
+`PermittedField` field borrowing rules are ordinary Rust rules. New variable borrows the whole variable (with partial type), but checker pretends it borrows just permitted fields of this variable.
+
+Not-`PermittedField` filed is always is ready to borrow regardless if origin field is denied(by move, by reference, by borrow).
+
+When we write a code for full or partial borrow, the link of object itself returns, but borrow-checker checks to borrow of permitted fields only.
+
+This new mechanism of is simple and universal.
+
+```rust
+struct S4 {a : i32, b : i32, c : i32, d : i32}
+let s = S4 {a : 5, b: 6, c: 7, d: 8};
+    // s : S4
+
+let r_sd = & s.{d};
+    // r_sd : & S4.{d}
+    //
+    // borrow-checker check just for &s.d
+
+let mut mr_sabc = &mut s.{a, b, c};
+    // mr_sabc : &mut S4.{a, b, c}
+    //
+    // borrow-checkercheck just for &mut s.a, &mut s.b, &mut s.c
+
+let rr_sbc = & mr_sabc.{b, c};
+    // rr_sbc : && S4.{b, c}
+    //
+    // borrow-checker check just for &mr_sabc.b, &mr_sabc.c
+
+let mut mrr_sa = &mut mr_sabc.{a};
+    // mrr_sa : &&mut S4.{a}
+    //
+    // borrow-checker check just for &mut mr_sabc.a
+```
+
+## Syntax
+
+Second, but still important - syntax.
+
+### Partiality Syntax
+
+Minimal Partiality we could write:
+```
+Partiality:      .{ PartialFields* }
+PartialFields:   PermittedField (, PermittedField )* ,?
+PermittedField:  IDENTIFIER | TUPLE_INDEX
+```
+
+### Partial Struct syntax
+
+Syntax is needed to Struct Type - is update `TypePath`
+```
+TypePath:   ::? TypePathSegment (:: TypePathSegment)* Partiality?
+```
+
+### Partial Tuple syntax
+
+For Tuple Type we need to update `TupleType`
+```
+TupleType:  ( ) | ( ( Type , )+ Type? ) Partiality?
+```
+
+### Partial Expression syntax
+
+For Expression we need create new kind of Expression:
+```
+PartialExpression:   Expression Partiality
+```
+
+and include it into `ExpressionWithoutBdeny`:
+```
+ExpressionWithoutBdeny:   ... | FieldExpression | PartialExpression | ...
+```
+
+
+## Logic Scheme
+
+Third, but still important - Logic Scheme.
+
+For pseudo-rust we suppose, partiality is a `HashSet` of permitted field-names.
+
+Common rules:
+```rust
+fn bar(v : SomeType.{'type_prtlty}) 
+{ /* .. */ }
+
+let v : SomeType.{'var_prtlty}; 
+```
+Then:
+
+(1) If `SomeType` is not supported type (neither Struct nor Tuple) then Error.
+
+(2) If partiality has no extra field-names `type_prtlty.is_subset(full_prtlty)` it compiles, otherwise Error.
+
+(3) If `var_prtlty.is_subset(full_prtlty)` it compiles, otherwise Error.
+
+(4) If `type_prtlty.is_empty()` or `var_prtlty.is_empty()` (if they are explicitly written as '`.{}`') then Error
+
+### Partial Struct and Tuples Logic Scheme
+
+
+Let we have (pseudo-rust) and `st_param_prtlty` and `st_arg_prtlty` are `HashSet` of permitted field-names: 
+```rust
+fn bar(s : SomeStructOrTuple.{'st_param_prtlty}) 
+{ /* .. */ }
+
+let s : SomeStructOrTuple.{'st_arg_prtlty}; 
+bar(s);
+
+let rsp = & s.{'expr_prtlty};
+
+impl SomeStructOrTuple.{'st_impl_prtlty} {
+    fn foo(self : Self.{'st_slf_prtlty}) 
+	{ /* .. */ }
+}
+
+s.foo();
+// (4) desugars into:
+SomeStructOrTuple.{'st_impl_prtlty}::foo(s.{'st_slf_prtlty});
+```
+Then:
+
+(1) If `st_arg_prtlty.is_superset(st_param_prtlty)` it compiles, otherwise Error.
+
+(2) If `expr_prtlty.is_subset(st_arg_prtlty)` it compiles, otherwise Error.
+
+(3) If `st_slf_prtlty.is_subset(st_impl_prtlty)` it compiles, otherwise Error.
+
+(4) Updating desugaring for `self` (and `Rhs`) variables.
+
+Desugaring `s.foo()` into `SomeStructOrTuple.{'st_impl_prtlty}::foo(s.{'st_slf_prtlty})` .
+
+(5) It has **no sense** to have several implementation of same product-type and different partiality. 
+
+(6) Anyway let we have several implementations for same type, but different partiality. And `all_st_impl_prtlty` is an `array` of each `st_impl_prtlty`.
+
+If `all_st_impl_prtlty.iter().any(|&sip| st_arg_prtlty.is_subset(sip))` it compiles, otherwise Error.
+
+(8) If `1 == all_st_impl_prtlty.iter().fold(0, |acc, &sip| if st_arg_prtlty.is_subset(sip) {acc+1} else {acc})` it compiles, otherwise ?Error.
+
+We expect that just one "implementation" partiality is match and we choose it for calling a method.
+
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- it is definitely not a minor change
+- type system became much more complicated
+
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+ A lot of proposals that are alternatives to Partial Product Types in a whole:
+ - Partial Types (v2) [#3426](https://github.com/rust-lang/rfcs/pull/3426)
+ - Partial Mutability [#3428](https://github.com/rust-lang/rfcs/pull/3428)
+ - Partial Types [#3420](https://github.com/rust-lang/rfcs/pull/3420)
+ - Partial borrowing [issue#1215](https://github.com/rust-lang/rfcs/issues/1215)
+ - View patterns [internals#16879](https://internals.rust-lang.org/t/view-types-based-on-pattern-matching/16879)
+ - Permissions [#3380](https://github.com/rust-lang/rfcs/pull/3380)
+ - Fields in Traits [#1546](https://github.com/rust-lang/rfcs/pull/1546)
+ - ImplFields [issue#3269](https://github.com/rust-lang/rfcs/issues/3269)
+
+
+# Prior art
+[prior-art]: #prior-art
+
+Most languages don't have such strict rules for references and links as Rust, so this feature is almost unnecessary for them.
+
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+It is better to have 2 ways to write partiality:  a list with just "allowed" fields and just "forbidden" fields for better ergonomics
+
+```rust
+let t1 = s10.{fld1, fld2, fld3, fld4, fld5, fld6, fld7, fld8};
+//  just "allowed" fields
+//  t1 : S10.{fld1, fld2, fld3, fld4, fld5, fld6, fld7, fld8}
+
+let t2 = S10.{off fld9, fld10};
+//  just "forbidden" fields
+//  t2 : S10.{fld1, fld2, fld3, fld4, fld5, fld6, fld7, fld8}
+```
+
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+Adding "partiality" opens wide variety of future possibilities.
+
+
+## Partial Mutability
+
+*partly independent sub-proposal*.
+
+For full flexibility of using partial borrowing partial mutability is needed!
+
+For Product Partial Types (structs, tuples) we use "partial mutability" expression: `mut .{fld1, fld2, ..}`, where `fld1`, `fld2`, .. are mutable fields of this type, the rest of fields are immutable(constant).
+
+Partly mutable variables become possible for Product Partial Types:
+```rust
+struct S4 {a : i32, b : i32, c : i32, d : i32}
+
+let mut.{a}       s_ma   = S4 {a: 6, b: 7, c: 8, d: 9};
+let mut.{b, c}    s_mbc  = S4 {a: 6, b: 7, c: 8, d: 9};
+let mut.{a, c, d} s_macd = S4 {a: 6, b: 7, c: 8, d: 9};
+```
+
+It is also possible to make partial-mutable references.
+
+Not-`PermittedField` filed is always is ready to mutable and immutable borrow regardless if origin field is denied(by move, by reference, by borrow), is visible, is mutable:
+```rust
+   fn mab_s(s : &mut.{a,b} S4) 
+   { /* ... */ }
+
+   mab_s(&mut.{a,b} s_macd);
+```
+It is expected, that `&mut.{..}` is a third type of borrowing!
+
+Example with full flexibility of using partial borrowing together with partial mutability
+
+```rust
+impl Point {
+   pub fn mx_rstate(self : &mut.{x} Self.{x, state}) 
+   { /* ... */ }
+
+   pub fn my_rstate(self : &mut.{y} Self.{y, state}) 
+   { /* ... */ }
+
+   pub fn mxy_rstate(self : &mut.{x,y} Self.{x, y, state}) { 
+    /* ... */
+    self.{x, state}.mx_rstate(); // explicit
+    self.mx_rstate(); // same implicit
+    /* ... */
+    self.{y, state}.my_rstate(); // explicit
+    self.my_rstate(); // same implicit
+    /* ... */
+   }
+}
+```
+
+
+## Explicit Off Fields
+
+This extension is not a mandatory. Tuple type has "naked" structure, so it would be handy have more pretty visuals, instead of mark all permitted fields in "partiality", write `off` before denied field.
+```rust
+let t :: (i32, &u64, f64, u8).{1,3};
+// same as
+let t :: (off i32, &u64, off f64, u8);
+```
+
+This extension is not just pretty, but useful for Tuples.
+
+
+## Partial Types to Sum Types (Enums)
+
+Partial Types extension gives to Sum Types (`ST = T1 or T2 or T3 or ..`), Enums first of all, a good tool for "partial functions".
+```rust
+enum EnumABC { A(u32), B(i64), C(f32), }
+
+// function with partial parameter Enum
+fn print_A(a: EnumABC.{A}) {
+    println!("a is {}", a.0);
+}
+
+let ea = EnumABC::A(7);
+//  ea : EnumABC.{A} inferred
+
+print_A(ea);
+```