Merge pull request #2250 from petrochenkov/impldyn

mini-RFC: Finalize syntax of `impl Trait` and `dyn Trait` with multiple bounds

text/2250-finalize-impl-dyn-syntax.md

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+- Feature Name: N/A
+- Start Date: 2017-12-16
+- RFC PR: [rust-lang/rfcs#2250](https://github.com/rust-lang/rfcs/pull/2250)
+- Rust Issue: [rust-lang/rust#34511](https://github.com/rust-lang/rust/issues/34511)
+
+# Summary
+[summary]: #summary
+
+Finalize syntax of `impl Trait` and `dyn Trait` with multiple bounds before
+stabilization of these features.
+
+# Motivation
+[motivation]: #motivation
+
+Current priority of `+` in `impl Trait1 + Trait2` / `dyn Trait1 + Trait2` brings
+inconsistency in the type grammar.  
+This RFC outlines possible syntactic
+alternatives and suggests one of them for stabilization.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+"Alternative 2" (see reference-level explanation) is selected for stabilization.
+
+`impl Trait1 + Trait2` / `dyn Trait1 + Trait2` now require parentheses in all
+contexts where they are used inside of unary operators `&(impl Trait1 + Trait2)`
+/ `&(dyn Trait1 + Trait2)`, similarly to trait object types without
+prefix, e.g. `&(Trait1 + Trait2)`.
+
+Additionally, parentheses are required in all cases where `+` in `impl` or `dyn`
+is ambiguous.  
+For example, `Fn() -> impl A + B` can be interpreted as both
+`(Fn() -> impl A) + B` (low priority plus) or `Fn() -> (impl A + B)` (high
+priority plus), so we are refusing to disambiguate and require explicit
+parentheses.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+## Current situation
+
+In the current implementation when we see `impl` or `dyn` we start parsing
+following bounds separated by `+`s greedily regardless of context, so `+`
+effectively gets the strongest priority.
+
+So, for example:
+- `&dyn A + B` is parsed as `&(dyn A + B)`
+- `Fn() -> impl A + B` is parsed as `Fn() -> (impl A + B)`
+- `x as &dyn A + y` is parsed as `x as &(dyn A + y)`.
+
+Compare this with parsing of trait object types without prefixes
+([RFC 438](https://github.com/rust-lang/rfcs/pull/438)):
+- `&A + B` is parsed as `(&A) + B` and is an error
+- `Fn() -> A + B` is parsed as `(Fn() -> A) + B`
+- `x as &A + y` is parsed as `(x as &A) + y`
+
+Also compare with unary operators in bounds themselves:
+- `for<'a> A<'a> + B` is parsed as `(for<'a> A<'a>) + B`,
+not `for<'a> (A<'a> + B)`
+- `?A + B` is parsed as `(?A) + B`, not `?(A + B)`
+
+In general, binary operations like `+` have lower priority than unary operations
+in all contexts - expressions, patterns, types. So the priorities as implemented
+bring inconsistency and may break intuition.
+
+## Alternative 1: high priority `+` (status quo)
+
+Pros:
+- The greedy parsing with high priority of `+` after `impl` / `dyn`
+has one benefit - it requires the least amout of parentheses from all the
+alternatives.  
+Parentheses are needed only when the greedy behaviour needs to be prevented,
+e.g. `Fn() -> &(dyn Write) + Send`, this doesn't happen often.
+
+Cons:
+- Inconsistent and possibly surprising operator priorities.
+- `impl` / `dyn` is a somewhat weird syntactic construction, it's not an usual
+unary operator, its a prefix describing how to interpret the following tokens.
+In particular, if the `impl A + B` needs to be parenthesized for some reason,
+it needs to be done like this `(impl A + B)`, and not `impl (A + B)`. The second
+variant is a parsing error, but some people find it surprising and expect it to
+work, as if `impl` were an unary operator.
+
+## Alternative 2: low priority `+`
+
+Basically, `impl A + B` is parsed using same rules as `A + B`.
+
+If `impl A + B` is located inside a higher priority operator like `&` it has
+to be parenthesized.  
+If it is located at intersection of type and expressions
+grammars like `expr1 as Type + expr2`, it has to be parenthesized as well.
+
+`&dyn A + B` / `Fn() -> impl A + B` / `x as &dyn A + y` has to be rewritten as
+`&(dyn A + B)` / `Fn() -> (impl A + B)` / `x as &(dyn A + y)` respectively.
+
+One location must be mentioned specially, the location in a function return
+type:
+```rust
+fn f() -> impl A + B
+{
+    // Do things
+}
+```
+This is probably the most common location for `impl Trait` types.
+In theory, it doesn't require parentheses in any way - it's not inside of an
+unary operator and it doesn't cross expression boundaries.  
+However, it creates a bit of percieved inconsistency with function-like traits
+and function pointers that do require parentheses for `impl Trait` in return
+types (`Fn() -> (impl A + B)` / `fn() -> (impl A + B)`) because they, in their
+turn, can appear inside of unary operators and casts.  
+So, if avoiding this is considered more important than ergonomics, then
+we can require parentheses in function definitions as well.
+```rust
+fn f() -> (impl A + B)
+{
+    // Do things
+}
+```
+
+Pros:
+- Consistent priorities of binary and unary operators.
+- Parentheses are required relatively rarely (unless we require them in
+function definitions as well).
+
+Cons:
+- More parentheses than in the "Alternative 1".
+- `impl` / `dyn` is still a somewhat weird prefix construction and `dyn (A + B)`
+is not a valid syntax.
+
+## Alternative 3: Unary operator
+
+`impl` and `dyn` can become usual unary operators in type grammar like `&` or
+`*const`.  
+Their application to any other types except for (possibly parenthesized) paths
+(single `A`) or "legacy trait objects" (`A + B`) becomes an error, but this
+could be changed in the future if some other use is found.
+
+`&dyn A + B` / `Fn() -> impl A + B` / `x as &dyn A + y` has to be rewritten as
+`&dyn(A + B)` / `Fn() -> impl(A + B)` / `x as &dyn(A + y)` respectively.
+
+Function definitions with `impl A + B` in return type have to be rewritten too.
+```rust
+fn f() -> impl(A + B)
+{
+    // Do things
+}
+```
+
+Pros:
+- Consistent priorities of binary and unary operators.
+- `impl` / `dyn` are usual unary operators, `dyn (A + B)` is a valid syntax.
+
+Cons:
+- The largest amount of parentheses, parentheses are always required.  
+Parentheses are noise, there may be even less desire to use `dyn` in trait
+objects now, if something like `Box<Write + Send>` turns into
+`Box<dyn(Write + Send)>`.
+
+## Other alternatives
+
+Two separate grammars can be used depending on context
+(https://github.com/rust-lang/rfcs/pull/2250#issuecomment-352435687) -
+Alternative 1/2 in lists of arguments like `Box<dyn A + B>` or
+`Fn(impl A + B, impl A + B)`, and Alternative 3 otherwise (`&dyn (A + B)`).
+
+## Compatibility
+
+The alternatives are ordered by strictness from the most relaxed Alternative 1
+to the strictest Alternative 3, but switching from more strict alternatives to
+less strict is not exactly backward-compatible.
+
+Switching from 2/3 to 1 can change meaning of legal code in rare cases.
+Switching from 3 to 2/1 requires keeping around the syntax with parentheses
+after `impl` / `dyn`.
+
+Alternative 2 can be backward-compatibly extended to "relaxed 3" in which
+parentheses like `dyn (A + B)` are permitted, but technically unnecessary.
+Such parens may keep people expecting `dyn (A + B)` to work happy, but
+complicate parsing by introducing more ambiguities to the grammar.
+
+While unary operators like `&` "obviously" have higher priority than `+`,
+cases like `Fn() -> impl A + B` are not so obvious.  
+The Alternative 2 considers "low priority plus" to have lower priority than `Fn`
+, so `Fn() -> impl A + B` can be treated as `(Fn() -> impl A) + B`, however
+it may be more intuitive and consistent with `fn` items to make `+` have higher
+priority than `Fn` (but still lower priority than `&`).  
+As an immediate solution we refuse to disambiguate this case and treat
+`Fn() -> impl A + B` as an error, so we can change the rules in the future and
+interpret `Fn() -> impl A + B` (and maybe even `Fn() -> A + B` after long
+deprecation period) as `Fn() -> (impl A + B)` (and `Fn() -> (A + B)`,
+respectively).
+
+## Experimental check
+
+An application of all the alternatives to rustc and libstd codebase can be found
+in [this branch](https://github.com/petrochenkov/rust/commits/impldyntest).
+The first commit is the baseline (Alternative 1) and the next commits show
+changes required to move to Alternatives 2 and 3. Alternative 2 requires fewer
+changes compared to Alternative 3.
+
+As the RFC author interprets it, the Alternative 3 turns out to be impractical
+due to common use of `Box`es and other contexts where the parens are technically
+unnecessary, but required by Alternative 3.  
+The number of parens required by Alternative 2 is limited and they seem
+appropriate because they follow "normal" priorities for unary and binary
+operators.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+See above.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+See above.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.