Disallow struct literals in ambiguous positions.

Do not identify struct literals by searching for `:`. Instead define a sub-category of expressions which excludes struct literals and re-define `for`, `if`, and other expressions which take an expression followed by a block (or non-terminal which can be replaced by a block) to take this sub-category, instead of all expressions.

0000-struct-grammar.md

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+- Start Date: 
+- RFC PR #: 
+- Rust Issue #: 
+
+# Summary
+
+Do not identify struct literals by searching for `:`. Instead define a sub-
+category of expressions which excludes struct literals and re-define `for`,
+`if`, and other expressions which take an expression followed by a block (or
+non-terminal which can be replaced by a block) to take this sub-category,
+instead of all expressions.
+
+# Motivation
+
+Parsing by looking ahead is fragile - it could easily be broken if we allow `:`
+to appear elsewhere in types (e.g., type ascription) or if we change struct
+literals to not require the `:` (e.g., if we allow empty structs to be written
+with braces, or if we allow struct literals to unify field names to local
+variable names, as has been suggested in the past and which we currently do for
+struct literal patterns). We should also be able to give better error messages
+today if users make these mistakes. More worringly, we might come up with some
+language feature in the future which is not predictable now and which breaks
+with the current system.
+
+Hopefully, it is pretty rare to use struct literals in these positions, so there
+should not be much fallout. Any problems can be easily fixed by assigning the
+struct literal into a variable. However, this is a backwards incompatible
+change, so it should block 1.0.
+
+# Detailed design
+
+Here is a simplified version of a subset of Rust's abstract syntax:
+
+```
+e      ::= x
+         | e `.` f
+         | name `{` (x `:` e)+ `}`
+         | block
+         | `for` e `in` e block
+         | `if` e block (`else` block)?
+         | `|` pattern* `|` e
+         | ...
+block  ::=  `{` (e;)* e? `}`
+```
+
+Parsing this grammar is ambiguous since `x` cannot be distinguished from `name`,
+so `e block` in the for expression is ambiguous with the struct literal
+expression. We currently solve this by using lookahead to find a `:` token in
+the struct literal.
+
+I propose the following adjustment:
+
+```
+e      ::= e'
+         | name `{` (x `:` e)+ `}`
+         | `|` pattern* `|` e
+         | ...
+e'     ::= x
+         | e `.` f
+         | block
+         | `for` e `in` e' block
+         | `if` e' block (`else` block)?
+         | `|` pattern* `|` e'
+         | ...
+block  ::=  `{` (e;)* e? `}`
+```
+
+`e' is just e without struct literal expressions. We use e' instead of e
+`wherever e is followed directly by block or any other non-terminal which may
+`have block as its first terminal (after any possible expansions).
+
+For any expressions where a sub-expression is the final lexical element
+(closures in the subset above, but also unary and binary operations), we require
+two versions of the meta-expression - the normal one in `e` and a version with
+`e'` for the final element in `e'`.
+
+Implementation would be simpler, we just add a flag to `parser::restriction`
+called `RESTRICT_BLOCK` or something, which puts us into a mode which reflects
+`e'`. We would drop in to this mode when parsing `e'` position expressions and
+drop out of it for all but the last sub-expression of an expression.
+
+# Drawbacks
+
+It makes the formal grammar and parsing a little more complicated (although it
+is simpler in terms of needing less lookahead and avoiding a special case).
+
+# Alternatives
+
+Don't do this.
+
+Allow all expressions but greedily parse non-terminals in these positions, e.g.,
+`for N {} {}` would be parsed as `for (N {}) {}`. This seems worse because I
+believe it will be much rarer to have structs in these positions than to have an
+identifier in the first position, followed by two blocks (i.e., parse as `(for N
+{}) {}`).
+
+# Unresolved questions
+
+Do we need to expose this distinction anywhere outside of the parser? E.g.,
+macros?