feat(lexer): tokenise build files with a span on every token

Words, strings, braces and comments, each carrying its source range, so an
error points at the offending token rather than the whole line.

lib/lexer.ml

@@ -0,0 +1,98 @@
+type kind =
+  | Word of string
+  | Str of string
+  | Lbrace
+  | Rbrace
+  | Lparen
+  | Rparen
+  | Newline
+  | Eof
+
+type t = { kind : kind; span : Span.t }
+
+let describe = function
+  | Word w -> Printf.sprintf "%S" w
+  | Str _ -> "a string"
+  | Lbrace -> "'{'"
+  | Rbrace -> "'}'"
+  | Lparen -> "'('"
+  | Rparen -> "')'"
+  | Newline -> "end of line"
+  | Eof -> "end of file"
+
+let is_space c = c = ' ' || c = '\t' || c = '\r'
+let is_delim c = is_space c || c = '\n' || c = '{' || c = '}' || c = '(' || c = ')' || c = '#'
+
+let escape = function
+  | 'n' -> '\n'
+  | 't' -> '\t'
+  | 'r' -> '\r'
+  | '0' -> '\000'
+  | c -> c
+
+let tokens ~file src =
+  let n = String.length src in
+  let out = ref [] in
+  let line = ref 1 and bol = ref 0 in
+  let i = ref 0 in
+  let col () = !i - !bol + 1 in
+  let emit kind start_col len = out := { kind; span = Span.make file !line start_col len } :: !out in
+  while !i < n do
+    let c = src.[!i] in
+    let c0 = col () in
+    if is_space c then incr i
+    else if c = '#' then while !i < n && src.[!i] <> '\n' do incr i done
+    else if c = '\\' && !i + 1 < n && src.[!i + 1] = '\n' then begin
+      i := !i + 2;
+      incr line;
+      bol := !i
+    end
+    else if c = '\n' then begin
+      emit Newline c0 1;
+      incr i;
+      incr line;
+      bol := !i
+    end
+    else if c = '{' then (emit Lbrace c0 1; incr i)
+    else if c = '}' then (emit Rbrace c0 1; incr i)
+    else if c = '(' then (emit Lparen c0 1; incr i)
+    else if c = ')' then (emit Rparen c0 1; incr i)
+    else if c = '"' then begin
+      let buf = Buffer.create 16 in
+      let start_line = !line in
+      incr i;
+      let closed = ref false in
+      while !i < n && not !closed do
+        if src.[!i] = '"' then (closed := true; incr i)
+        else if src.[!i] = '\\' && !i + 1 < n then begin
+          Buffer.add_char buf (escape src.[!i + 1]);
+          i := !i + 2
+        end
+        else begin
+          if src.[!i] = '\n' then (incr line; bol := !i + 1);
+          Buffer.add_char buf src.[!i];
+          incr i
+        end
+      done;
+      if not !closed then
+        Diag.error ~span:(Span.make file start_line c0 1) "unterminated string";
+      emit (Str (Buffer.contents buf)) c0 (max 1 (col () - c0))
+    end
+    else begin
+      let start = !i in
+      let fin = ref false in
+      while !i < n && not !fin do
+        if src.[!i] = '$' && !i + 1 < n && src.[!i + 1] = '{' then begin
+          i := !i + 2;
+          while !i < n && src.[!i] <> '}' && src.[!i] <> '\n' do incr i done;
+          if !i < n && src.[!i] = '}' then incr i
+          else Diag.error ~span:(Span.make file !line c0 (!i - start)) "unclosed ${...}"
+        end
+        else if is_delim src.[!i] then fin := true
+        else incr i
+      done;
+      emit (Word (String.sub src start (!i - start))) c0 (!i - start)
+    end
+  done;
+  emit Eof (col ()) 1;
+  List.rev !out