[lang] Add escape syntax for field names

Related to rcoh#99

Currently field names containing a space or period, e.g. `date received`
or `grpc.method`, cannot be parsed. This could be worked around using
`jq` or similar tools to rewrite the field name, but that's a pain.

This commit adds an escaped field name syntax of `["<FIELD>"]` which is
based on the Object Identifier-Index syntax[0] used by `jq`, so it
should be somewhat familiar to many people who parse JSON on the
command line.

The more obvious option of delimiting with just quotes, e.g.
"date received", creates an ambiguity between string literals and
escaped field names. For example, does `where foo == "date received"`
mean field `foo` matches field `date received`, or field `foo` matches
the string "date received"?

Example query:

```
* | json | where ["grpc.method"] == "Foo" | count by ["date received"]
```

[0]
https://stedolan.github.io/jq/manual/#ObjectIdentifier-Index:.foo,.foo.bar

src/lang.rs

@@ -366,6 +366,14 @@ fn is_ident(c: char) -> bool {
     is_alphanumeric(c as u8) || c == '_'
 }
 
+fn is_escaped_ident(c: char) -> bool {
+    match c {
+        space if is_space(space as u8) => true,
+        '.' => true,
+        _ => is_ident(c),
+    }
+}
+
 fn starts_ident(c: char) -> bool {
     is_alphabetic(c as u8) || c == '_'
 }
@@ -431,12 +439,20 @@ named!(column_ref<Span, Expr>, do_parse!(
     (Expr::Column { head: DataAccessAtom::Key(head), rest: rest })
 ));
 
-named!(ident<Span, String>, do_parse!(
+named!(ident<Span, String>, alt!(bare_ident | escaped_ident));
+
+named!(bare_ident<Span, String>, do_parse!(
     start: take_while1!(starts_ident) >>
     rest: take_while!(is_ident) >>
     (start.fragment.0.to_owned() + rest.fragment.0)
 ));
 
+named!(escaped_ident<Span, String>, do_parse!(
+    start: preceded!(tag!("[\""), take_while1!(starts_ident)) >>
+    rest: terminated!(take_while!(is_escaped_ident), tag!("\"]")) >>
+    (start.fragment.0.to_owned() + rest.fragment.0)
+));
+
 named!(arguments<Span, Vec<Expr>>, add_return_error!(SyntaxErrors::StartOfError.into(), delimited!(
    tag!("("),
    separated_list!(tag!(","), expr),
@@ -1166,6 +1182,13 @@ mod tests {
         expect_fail!(ident, "5x");
     }
 
+    #[test]
+    fn parse_escaped_ident() {
+        expect!(ident, "[\"hello world\"]", "hello world".to_string());
+        expect!(ident, "[\"hello.world\"]", "hello.world".to_string());
+        expect_fail!(ident, "\"\"");
+    }
+
     #[test]
     fn parse_var_list() {
         expect!(

tests/structured_tests/escaped_ident.toml

@@ -0,0 +1,13 @@
+query = """
+* | json | count by ["grpc.method"], ["start time"]
+"""
+input = """
+{"start time": "today", "grpc.method": "Foo"}
+{"start time": "today", "grpc.method": "Bar"}
+"""
+output = """
+["grpc.method"]        ["start time"]        _count
+-----------------------------------------------------------
+Bar                    today                 1
+Foo                    today                 1
+"""