Fix crashes due to lexing ambiguity of string delimiters (#394)

There are some lexing ambituities in primes vs cmd delimiters. We break
these with a simple rule in the lexer but there's edge cases of invalid
or extremely strange syntax where this can be inconsistent with the
parser. The following were some such cases which caused an assertion
error in the parser.

    "var\"#\"``\$"
    "x in'``\$"

This change avoids crashing in those cases, emitting an error instead.

See also #25

src/parser.jl

@@ -301,6 +301,10 @@ function is_both_unary_and_binary(t)
     )
 end
 
+function is_string_macro_suffix(k)
+    k == K"Identifier" || is_keyword(k) || is_word_operator(k) || is_number(k)
+end
+
 # flisp: invalid-identifier?
 function is_valid_identifier(k)
     k = kind(k)
@@ -1707,7 +1711,7 @@ function parse_call_chain(ps::ParseState, mark, is_macrocall=false)
             parse_string(ps, true)
             t = peek_token(ps)
             k = kind(t)
-            if !preceding_whitespace(t) && (k == K"Identifier" || is_keyword(k) || is_word_operator(k) || is_number(k))
+            if !preceding_whitespace(t) && is_string_macro_suffix(k)
                 # Macro sufficies can include keywords and numbers
                 # x"s"y    ==> (macrocall @x_str (string-r "s") "y")
                 # x"s"end  ==> (macrocall @x_str (string-r "s") "end")
@@ -2344,7 +2348,7 @@ function parse_macro_name(ps::ParseState)
     # @! x   ==>  (macrocall @! x)
     # @.. x  ==>  (macrocall @.. x)
     # @$ x   ==>  (macrocall @$ x)
-    # @var"#" x   ==>  (macrocall (var #) @$ x)
+    # @var"#" x   ==>  (macrocall (var @#) x)
     bump_disallowed_space(ps)
     mark = position(ps)
     parse_atom(ps, false)
@@ -3182,7 +3186,13 @@ function parse_string(ps::ParseState, raw::Bool)
         t = peek_full_token(ps)
         k = kind(t)
         if k == K"$"
-            @assert !raw  # The lexer detects raw strings separately
+            if raw
+                # FIXME: This case is actually a tokenization error:
+                # The `K"$"` token should not occur when a raw string
+                # is being parsed, but this would require the lexer to know
+                # about the parse state. (see also parse_atom)
+                break
+            end
             if prev_chunk_newline
                 # """\n$x\n a"""  ==>  (string-s x "\n" " a")
                 indent_ref_i = first_byte(t)
@@ -3526,11 +3536,16 @@ function parse_atom(ps::ParseState, check_identifiers=true)
                 # var"x"+  ==>  x
                 # var"x")  ==>  x
                 # var"x"(  ==>  x
-            else
+            elseif is_string_macro_suffix(k)
                 # var"x"end  ==>  (var x (error-t))
                 # var"x"1    ==>  (var x (error-t))
                 # var"x"y    ==>  (var x (error-t))
-                bump(ps, TRIVIA_FLAG, error="suffix not allowed after var\"...\" syntax")
+                bump(ps, TRIVIA_FLAG, error="suffix not allowed after `var\"...\"` syntax")
+            elseif k == K"`" || k == K"\"" || k == K"\"\"\"" || k == K"```"
+                # Disallow `var"#""str". To allow this we'd need to fix `raw`
+                # detection in lex_quote to be consistent with the parser.
+                bump_invisible(ps, K"error", TRIVIA_FLAG,
+                               error="`var\"...\"` syntax not supported as string macro name")
             end
             emit(ps, mark, K"var")
         elseif check_identifiers && is_closing_token(ps, leading_kind)

test/parser.jl

@@ -1012,9 +1012,13 @@ parsestmt_test_specs = [
     "(abstract\ntype X end)" => "(wrapper (parens abstract (error-t type X)) (error-t end ✘))"
     "(mutable\nstruct X end)" => "(wrapper (parens mutable (error-t struct X)) (error-t end ✘))"
 
-    # The following is currently broken but at least the parser shouldn't
-    # crash.
+    # Lexer vs parser: issues detecting which tokens are string delimiters and
+    # detecting raw vs non-raw strings. The old parser was tightly coupled to
+    # the lexer and the parser state was used to disambiguate these cases.
     "x in' '" => "(call-i x in (char (error)))"
+    "x in'``\$" => "(call-i x in (call-i (juxtapose (char '`' (error-t)) (macrocall core_@cmd (cmdstring-r (error-t)))) \$ (error)))"
+    "var\"#\"`str`" => "(juxtapose (var # (error-t)) (macrocall core_@cmd (cmdstring-r \"str\")))"
+    "var\"#\"\"str\"" => "(juxtapose (var # (error-t)) (error-t) (string \"str\"))"
 ]
 
 @testset "Parser does not crash on broken code" begin