Add EBNF for AST to book

book/src/SUMMARY.md

@@ -20,6 +20,7 @@
        - [The include flag: which items should be extracted, and how?](faq/include-flags.md)
 - [Contributing]()
     - [Architecture](contributing/architecture.md)
+    - [AST/EBNF](contributing/ast_ebnf.md)
     - [Hax Cargo subcommand]()
     - [Frontend: the Rustc driver]()
     - [Frontend: the exporter]()

book/src/contributing/ast_ebnf.md

@@ -0,0 +1,243 @@
+We currently take inputs from the following AST (visualization can be done
+using <https://rr.red-dove.com/ui>). Literals are strings, numbers and
+booleans
+
+``` ebnf
+char ::= [a-zA-Z]
+string ::= char*
+digit ::= [0-9]
+uint ::= digit+
+int ::= ("-")? uint
+float ::= int (".")? uint
+bool ::= "true" | "false"
+
+local_var ::= ident
+global_var ::= rust-path-identifier
+
+literal ::=
+| "\"" string "\""
+| "'" char "'"
+| int
+| float
+| bool
+```
+
+We support a number of simple types characters, strings, booleans and
+numbers. Number types for integers (8,16,32,64,128 bit or machine sized)
+and floats (16,32, or 64 bit). Composite types are tuples, fixed length
+lists (arrays), variable length lists (vectors/slices), ptr types, and
+function types. Lastly we have named types defined by items, e.g. enums
+and structs.
+
+``` ebnf
+ty ::=
+| "bool"
+| "char"
+| "u8" | "u16" | "u32" | "u64"
+| "u128" | "usize"
+| "i8" | "i16" | "i32" | "i64"
+| "i128" | "isize"
+| "f16" | "f32" | "f64"
+| "str"
+| (ty ",")*
+| "[" ty ";" int "]"
+| "[" ty "]"
+| "*const" ty | "*mut" ty
+| "*" expr | "*mut" expr
+| ident
+| (ty "->")* ty
+| dyn (goal)+
+```
+
+The patterns allowed reflect these types. Wildcard patterns, literal
+types, typed patterns, list patterns, record or tuple patterns.
+
+``` ebnf
+pat ::=
+| "_"
+| ident "{" (ident ":" pat ";")* "}"
+| ident "(" (pat ",")* ")"
+| (pat "|")* pat
+| "[" (pat ",")* "]"
+| "&" pat
+| literal
+| ("&")? ("mut")? ident ("@" pat)?
+```
+
+The simple expressions are literals, local or global variables, type
+casts, assignments and lists. Control flow expressions, if statements,
+match statements, loops, return, break and continue. The rest is blocks,
+macro calls, lambda functions and borrowing.
+
+``` ebnf
+expr ::=
+| "if" expr "{" expr "}" ("else" "{" expr "}")?
+| "if" "let" pat (":" ty)? "=" expr "{" expr "}" ("else" "{" expr "}")?
+| expr "(" (expr ",")* ")"
+| literal
+| "[" (expr ",")* "]" | "[" expr ";" int "]"
+| ident "{" (ident ":"expr ";")* "}"
+| ident "{" (ident ":"expr ";")* ".." expr "}"
+| "match" expr guard "{"
+(("|" pat)* "=>" (expr "," | "{" expr "}"))*
+"}"
+| "let" pat (":" ty)? "=" expr ";" expr
+| "let" pat (":" ty)? "=" expr "else" "{" expr "}" ";" expr
+| modifiers "{" expr "}"
+| local_var
+| global_var
+| expr "as" ty
+| "loop" "{" expr "}"$~[\subref{fig:ebnf-iterators}]$
+| "while" "(" expr ")" "{" expr "}"$~[\subref{fig:ebnf-iterators}]$
+| "for" "(" pat "in" expr ")" "{" expr "}"$~[\subref{fig:ebnf-iterators}]$
+| "for" "(" "let" ident "in" expr ".." expr ")" "{" expr "}"$~[\subref{fig:ebnf-iterators}]$
+| "break" expr
+| "continue"
+| pat "=" expr
+| "return" expr
+| expr "?"
+| "&" ("mut")? expr
+| "&" expr "as" "&const _"
+| "&mut" expr "as" "&mut _"
+| "|" pat "|" expr
+```
+
+The items supported are functions, type aliasing, enums, structs, trait
+definitions and implementations, and imports.
+
+``` ebnf
+item ::=
+| "const" ident "=" expr
+| "static" ident "=" expr
+| modifiers "fn" ident ("<" (generics ",")* ">")? "(" (pat ":" ty ",")* ")" (":" ty)? "{" expr "}"
+| "type" ident "=" ty
+| "enum" ident ("<" (generics ",")* ">")? "{" (ident ("(" (ty)* ")")? ",")* "}"
+| "struct" ident ("<" (generics ",")* ">")? "{" (ident ":" ty ",")* "}"
+| "trait" ident ("<" (generics ",")* ">")? "{" (trait_item)* "}"
+| "impl" ("<" (generics ",")* ">")? ident "for" ty "{" (impl_item)* "}"
+| "mod" ident "{" (item)* "}"
+| "use" path ";"
+```
+
+The full AST description
+
+``` ebnf
+char ::= [a-zA-Z]
+string ::= char*
+digit ::= [0-9]
+uint ::= digit+
+int ::= ("-")? uint
+float ::= int (".")? uint
+bool ::= "true" | "false"
+
+local_var ::= ident
+global_var ::= rust-path-identifier
+
+literal ::=
+| "\"" string "\""
+| "'" char "'"
+| int
+| float  [a]
+| bool
+
+generic_value ::=
+| "'" ident
+| ty
+| expr
+
+goal ::=
+| ident "<" (generic_value ",")* ">"
+
+ty ::=
+| "bool"
+| "char"
+| "u8" | "u16" | "u32" | "u64"
+| "u128" | "usize"
+| "i8" | "i16" | "i32" | "i64"
+| "i128" | "isize"
+| "f16" | "f32" | "f64"  [a]
+| "str"
+| (ty ",")*
+| "[" ty ";" int "]"
+| "[" ty "]"
+| "*const" ty | "*mut" ty  [b]
+| "*" expr | "*mut" expr  [b]
+| ident
+| (ty "->")* ty
+| dyn (goal)+  [c]
+
+pat ::=
+| "_"
+| ident "{" (ident ":" pat ";")* "}"
+| ident "(" (pat ",")* ")"
+| (pat "|")* pat
+| "[" (pat ",")* "]"  [d]
+| "&" pat
+| literal
+| ("&")? ("mut")? ident ("@" pat)?  [e]
+
+modifiers ::=
+| ""
+| "unsafe" modifiers
+| "const" modifiers
+| "async" modifiers  [b]
+
+guard ::=
+| "if" "let" pat (":" ty)? "=" expr
+
+expr ::=
+| "if" expr "{" expr "}" ("else" "{" expr "}")?
+| "if" "let" pat (":" ty)? "=" expr "{" expr "}" ("else" "{" expr "}")?
+| expr "(" (expr ",")* ")"
+| literal
+| "[" (expr ",")* "]" | "[" expr ";" int "]"
+| ident "{" (ident ":"expr ";")* "}"
+| ident "{" (ident ":"expr ";")* ".." expr "}"
+| "match" expr guard "{"
+(("|" pat)* "=>" (expr "," | "{" expr "}"))*
+"}"
+| "let" pat (":" ty)? "=" expr ";" expr
+| "let" pat (":" ty)? "=" expr "else" "{" expr "}" ";" expr
+| modifiers "{" expr "}"
+| local_var
+| global_var
+| expr "as" ty
+| "loop" "{" expr "}"$~[\subref{fig:ebnf-iterators}]$
+| "while" "(" expr ")" "{" expr "}"$~[\subref{fig:ebnf-iterators}]$
+| "for" "(" pat "in" expr ")" "{" expr "}"$~[\subref{fig:ebnf-iterators}]$
+| "for" "(" "let" ident "in" expr ".." expr ")" "{" expr "}"$~[\subref{fig:ebnf-iterators}]$
+| "break" expr
+| "continue"
+| pat "=" expr
+| "return" expr
+| expr "?"
+| "&" ("mut")? expr  [e]
+| "&" expr "as" "&const _"  [b]
+| "&mut" expr "as" "&mut _"
+| "|" pat "|" expr
+
+impl_item ::=
+| "type" ident "=" ty ";"
+| modifiers "fn" ident ("<" (generics ",")* ">")? "(" (pat ":" ty ",")* ")" (":" ty)? "{" expr "}"
+
+trait_item ::=
+| "type" ident ";"
+| modifiers "fn" ident ("<" (generics ",")* ">")? "(" (pat ":" ty ",")* ")" (":" ty)? ("{" expr "}" | ";")
+
+item ::=
+| "const" ident "=" expr
+| "static" ident "=" expr  [b]
+| modifiers "fn" ident ("<" (generics ",")* ">")? "(" (pat ":" ty ",")* ")" (":" ty)? "{" expr "}"
+| "type" ident "=" ty
+| "enum" ident ("<" (generics ",")* ">")? "{" (ident ("(" (ty)* ")")? ",")* "}"
+| "struct" ident ("<" (generics ",")* ">")? "{" (ident ":" ty ",")* "}"
+| "trait" ident ("<" (generics ",")* ">")? "{" (trait_item)* "}"
+| "impl" ("<" (generics ",")* ">")? ident "for" ty "{" (impl_item)* "}"
+| "mod" ident "{" (item)* "}"
+| "use" path ";"
+```
+(a) no support yet for raw pointers, async/await, static, extern, or union types
+(b) partial support for nested matching and range patterns
+(c) partial support for mutable borrows
+(d) most backends lack support for dynamic dispatch, floating point operations
+(e) some backends only handle specific forms of iterators