Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

12 files changed, 1827 insertions, 0 deletions

diff --git a/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/METADATA b/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/METADATA
new file mode 100644
index 0000000000..04346d93a0
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/METADATA
@@ -0,0 +1,42 @@
+Metadata-Version: 2.1
+Name: fluent.syntax
+Version: 0.19.0
+Summary: Localization library for expressive translations.
+Home-page: https://github.com/projectfluent/python-fluent
+Author: Mozilla
+Author-email: l10n-drivers@mozilla.org
+License: APL 2
+Keywords: fluent,localization,l10n
+Classifier: Development Status :: 3 - Alpha
+Classifier: Intended Audience :: Developers
+Classifier: License :: OSI Approved :: Apache Software License
+Classifier: Programming Language :: Python :: 3.6
+Classifier: Programming Language :: Python :: 3.7
+Classifier: Programming Language :: Python :: 3.8
+Classifier: Programming Language :: Python :: 3.9
+Classifier: Programming Language :: Python :: 3 :: Only
+Description-Content-Type: text/x-rst
+Requires-Dist: typing-extensions (<5,>=3.7)
+
+``fluent.syntax`` |fluent.syntax|
+---------------------------------
+
+Read, write, and transform `Fluent`_ files.
+
+This package includes the parser, serializer, and traversal
+utilities like Visitor and Transformer. You’re looking for this package
+if you work on tooling for Fluent in Python.
+
+.. code-block:: python
+
+   >>> from fluent.syntax import parse, ast, serialize
+   >>> resource = parse("a-key = String to localize")
+   >>> resource.body[0].value.elements[0].value = "Localized string"
+   >>> serialize(resource)
+   'a-key = Localized string\n'
+
+
+Find the full documentation on https://projectfluent.org/python-fluent/fluent.syntax/.
+
+.. _fluent: https://projectfluent.org/
+.. |fluent.syntax| image:: https://github.com/projectfluent/python-fluent/workflows/fluent.syntax/badge.svg
diff --git a/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/RECORD b/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/RECORD
new file mode 100644
index 0000000000..375fe44f84
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/RECORD
@@ -0,0 +1,12 @@
+fluent/syntax/__init__.py,sha256=hSmmHKIWanCOPgJIMAA5qEQqfZLywJRWB6KZsrWjSjA,808
+fluent/syntax/ast.py,sha256=x18U6wwdDEyB2uBFPfHV5Xy01g_TPAPx5I-16eipOXo,11689
+fluent/syntax/errors.py,sha256=CDNTnoys3yzo9z4asXtdTj2tBUODmsLDCXfkLT7Iguo,2636
+fluent/syntax/parser.py,sha256=qSehrK_dthGGn4WKenWSjHSf7O5eg8l5gGaPQ2d1amo,22521
+fluent/syntax/py.typed,sha256=47DEQpj8HBSa-_TImW-5JCeuQeRkm5NMpJWZG3hSuFU,0
+fluent/syntax/serializer.py,sha256=7cMjxLpEaXPTjoVeJM8622EJj-afPIhjyw4vLKSO9O8,8207
+fluent/syntax/stream.py,sha256=DQHMDWS6vBbCzMXWle7GOlGb41rQqQ5-EwTFlSli8Ao,8638
+fluent/syntax/visitor.py,sha256=1b7ZmfXrbf_1Pwxw__dq3bRbzi1ErBnVdlAlmryvG3M,2164
+fluent.syntax-0.19.0.dist-info/METADATA,sha256=zzu67EEeV4_6dR-AKBmzgFSSKiGFMa3_WH_EXzReCHc,1561
+fluent.syntax-0.19.0.dist-info/WHEEL,sha256=a-zpFRIJzOq5QfuhBzbhiA1eHTzNCJn8OdRvhdNX0Rk,110
+fluent.syntax-0.19.0.dist-info/top_level.txt,sha256=E6y0EXb_8ntRq2470rEss448Ec6wP_-DI3zVECukrn0,7
+fluent.syntax-0.19.0.dist-info/RECORD,,
diff --git a/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/WHEEL b/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/WHEEL
new file mode 100644
index 0000000000..f771c29b87
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/WHEEL
@@ -0,0 +1,6 @@
+Wheel-Version: 1.0
+Generator: bdist_wheel (0.40.0)
+Root-Is-Purelib: true
+Tag: py2-none-any
+Tag: py3-none-any
+
diff --git a/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/top_level.txt b/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/top_level.txt
new file mode 100644
index 0000000000..a3582d405a
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent.syntax-0.19.0.dist-info/top_level.txt
@@ -0,0 +1 @@
+fluent
diff --git a/third_party/python/fluent.syntax/fluent/syntax/__init__.py b/third_party/python/fluent.syntax/fluent/syntax/__init__.py
new file mode 100644
index 0000000000..1ff31745e6
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent/syntax/__init__.py
@@ -0,0 +1,34 @@
+from typing import Any
+
+from . import ast
+from .errors import ParseError
+from .parser import FluentParser
+from .serializer import FluentSerializer
+from .stream import FluentParserStream
+from .visitor import Transformer, Visitor
+
+__all__ = [
+    'FluentParser',
+    'FluentParserStream',
+    'FluentSerializer',
+    'ParseError',
+    'Transformer',
+    'Visitor',
+    'ast',
+    'parse',
+    'serialize'
+]
+
+
+def parse(source: str, **kwargs: Any) -> ast.Resource:
+    """Create an ast.Resource from a Fluent Syntax source.
+    """
+    parser = FluentParser(**kwargs)
+    return parser.parse(source)
+
+
+def serialize(resource: ast.Resource, **kwargs: Any) -> str:
+    """Serialize an ast.Resource to a unicode string.
+    """
+    serializer = FluentSerializer(**kwargs)
+    return serializer.serialize(resource)
diff --git a/third_party/python/fluent.syntax/fluent/syntax/ast.py b/third_party/python/fluent.syntax/fluent/syntax/ast.py
new file mode 100644
index 0000000000..d2e4849079
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent/syntax/ast.py
@@ -0,0 +1,376 @@
+import re
+import sys
+import json
+from typing import Any, Callable, Dict, List, TypeVar, Union, cast
+
+Node = TypeVar('Node', bound='BaseNode')
+ToJsonFn = Callable[[Dict[str, Any]], Any]
+
+
+def to_json(value: Any, fn: Union[ToJsonFn, None] = None) -> Any:
+    if isinstance(value, BaseNode):
+        return value.to_json(fn)
+    if isinstance(value, list):
+        return list(to_json(item, fn) for item in value)
+    if isinstance(value, tuple):
+        return list(to_json(item, fn) for item in value)
+    else:
+        return value
+
+
+def from_json(value: Any) -> Any:
+    if isinstance(value, dict):
+        cls = getattr(sys.modules[__name__], value['type'])
+        args = {
+            k: from_json(v)
+            for k, v in value.items()
+            if k != 'type'
+        }
+        return cls(**args)
+    if isinstance(value, list):
+        return list(map(from_json, value))
+    else:
+        return value
+
+
+def scalars_equal(node1: Any, node2: Any, ignored_fields: List[str]) -> bool:
+    """Compare two nodes which are not lists."""
+
+    if type(node1) != type(node2):
+        return False
+
+    if isinstance(node1, BaseNode):
+        return node1.equals(node2, ignored_fields)
+
+    return cast(bool, node1 == node2)
+
+
+class BaseNode:
+    """Base class for all Fluent AST nodes.
+
+    All productions described in the ASDL subclass BaseNode, including Span and
+    Annotation.  Implements __str__, to_json and traverse.
+    """
+
+    def clone(self: Node) -> Node:
+        """Create a deep clone of the current node."""
+        def visit(value: Any) -> Any:
+            """Clone node and its descendants."""
+            if isinstance(value, BaseNode):
+                return value.clone()
+            if isinstance(value, list):
+                return [visit(child) for child in value]
+            if isinstance(value, tuple):
+                return tuple(visit(child) for child in value)
+            return value
+
+        # Use all attributes found on the node as kwargs to the constructor.
+        return self.__class__(
+            **{name: visit(value) for name, value in vars(self).items()}
+        )
+
+    def equals(self, other: 'BaseNode', ignored_fields: List[str] = ['span']) -> bool:
+        """Compare two nodes.
+
+        Nodes are deeply compared on a field by field basis. If possible, False
+        is returned early. When comparing attributes and variants in
+        SelectExpressions, the order doesn't matter. By default, spans are not
+        taken into account.
+        """
+
+        self_keys = set(vars(self).keys())
+        other_keys = set(vars(other).keys())
+
+        if ignored_fields:
+            for key in ignored_fields:
+                self_keys.discard(key)
+                other_keys.discard(key)
+
+        if self_keys != other_keys:
+            return False
+
+        for key in self_keys:
+            field1 = getattr(self, key)
+            field2 = getattr(other, key)
+
+            # List-typed nodes are compared item-by-item.  When comparing
+            # attributes and variants, the order of items doesn't matter.
+            if isinstance(field1, list) and isinstance(field2, list):
+                if len(field1) != len(field2):
+                    return False
+
+                for elem1, elem2 in zip(field1, field2):
+                    if not scalars_equal(elem1, elem2, ignored_fields):
+                        return False
+
+            elif not scalars_equal(field1, field2, ignored_fields):
+                return False
+
+        return True
+
+    def to_json(self, fn: Union[ToJsonFn, None] = None) -> Any:
+        obj = {
+            name: to_json(value, fn)
+            for name, value in vars(self).items()
+        }
+        obj.update(
+            {'type': self.__class__.__name__}
+        )
+        return fn(obj) if fn else obj
+
+    def __str__(self) -> str:
+        return json.dumps(self.to_json())
+
+
+class SyntaxNode(BaseNode):
+    """Base class for AST nodes which can have Spans."""
+
+    def __init__(self, span: Union['Span', None] = None, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.span = span
+
+    def add_span(self, start: int, end: int) -> None:
+        self.span = Span(start, end)
+
+
+class Resource(SyntaxNode):
+    def __init__(self, body: Union[List['EntryType'], None] = None, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.body = body or []
+
+
+class Entry(SyntaxNode):
+    """An abstract base class for useful elements of Resource.body."""
+
+
+class Message(Entry):
+    def __init__(self,
+                 id: 'Identifier',
+                 value: Union['Pattern', None] = None,
+                 attributes: Union[List['Attribute'], None] = None,
+                 comment: Union['Comment', None] = None,
+                 **kwargs: Any):
+        super().__init__(**kwargs)
+        self.id = id
+        self.value = value
+        self.attributes = attributes or []
+        self.comment = comment
+
+
+class Term(Entry):
+    def __init__(self, id: 'Identifier', value: 'Pattern', attributes: Union[List['Attribute'], None] = None,
+                 comment: Union['Comment', None] = None, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.id = id
+        self.value = value
+        self.attributes = attributes or []
+        self.comment = comment
+
+
+class Pattern(SyntaxNode):
+    def __init__(self, elements: List[Union['TextElement', 'Placeable']], **kwargs: Any):
+        super().__init__(**kwargs)
+        self.elements = elements
+
+
+class PatternElement(SyntaxNode):
+    """An abstract base class for elements of Patterns."""
+
+
+class TextElement(PatternElement):
+    def __init__(self, value: str, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.value = value
+
+
+class Placeable(PatternElement):
+    def __init__(self,
+                 expression: Union['InlineExpression', 'Placeable', 'SelectExpression'],
+                 **kwargs: Any):
+        super().__init__(**kwargs)
+        self.expression = expression
+
+
+class Expression(SyntaxNode):
+    """An abstract base class for expressions."""
+
+
+class Literal(Expression):
+    """An abstract base class for literals."""
+
+    def __init__(self, value: str, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.value = value
+
+    def parse(self) -> Dict[str, Any]:
+        return {'value': self.value}
+
+
+class StringLiteral(Literal):
+    def parse(self) -> Dict[str, str]:
+        def from_escape_sequence(matchobj: Any) -> str:
+            c, codepoint4, codepoint6 = matchobj.groups()
+            if c:
+                return cast(str, c)
+            codepoint = int(codepoint4 or codepoint6, 16)
+            if codepoint <= 0xD7FF or 0xE000 <= codepoint:
+                return chr(codepoint)
+            # Escape sequences reresenting surrogate code points are
+            # well-formed but invalid in Fluent. Replace them with U+FFFD
+            # REPLACEMENT CHARACTER.
+            return '�'
+
+        value = re.sub(
+            r'\\(?:(\\|")|u([0-9a-fA-F]{4})|U([0-9a-fA-F]{6}))',
+            from_escape_sequence,
+            self.value
+        )
+        return {'value': value}
+
+
+class NumberLiteral(Literal):
+    def parse(self) -> Dict[str, Union[float, int]]:
+        value = float(self.value)
+        decimal_position = self.value.find('.')
+        precision = 0
+        if decimal_position >= 0:
+            precision = len(self.value) - decimal_position - 1
+        return {
+            'value': value,
+            'precision': precision
+        }
+
+
+class MessageReference(Expression):
+    def __init__(self, id: 'Identifier', attribute: Union['Identifier', None] = None, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.id = id
+        self.attribute = attribute
+
+
+class TermReference(Expression):
+    def __init__(self,
+                 id: 'Identifier',
+                 attribute: Union['Identifier', None] = None,
+                 arguments: Union['CallArguments', None] = None,
+                 **kwargs: Any):
+        super().__init__(**kwargs)
+        self.id = id
+        self.attribute = attribute
+        self.arguments = arguments
+
+
+class VariableReference(Expression):
+    def __init__(self, id: 'Identifier', **kwargs: Any):
+        super().__init__(**kwargs)
+        self.id = id
+
+
+class FunctionReference(Expression):
+    def __init__(self, id: 'Identifier', arguments: 'CallArguments', **kwargs: Any):
+        super().__init__(**kwargs)
+        self.id = id
+        self.arguments = arguments
+
+
+class SelectExpression(Expression):
+    def __init__(self, selector: 'InlineExpression', variants: List['Variant'], **kwargs: Any):
+        super().__init__(**kwargs)
+        self.selector = selector
+        self.variants = variants
+
+
+class CallArguments(SyntaxNode):
+    def __init__(self,
+                 positional: Union[List[Union['InlineExpression', Placeable]], None] = None,
+                 named: Union[List['NamedArgument'], None] = None,
+                 **kwargs: Any):
+        super().__init__(**kwargs)
+        self.positional = [] if positional is None else positional
+        self.named = [] if named is None else named
+
+
+class Attribute(SyntaxNode):
+    def __init__(self, id: 'Identifier', value: Pattern, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.id = id
+        self.value = value
+
+
+class Variant(SyntaxNode):
+    def __init__(self, key: Union['Identifier', NumberLiteral], value: Pattern, default: bool = False, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.key = key
+        self.value = value
+        self.default = default
+
+
+class NamedArgument(SyntaxNode):
+    def __init__(self, name: 'Identifier', value: Union[NumberLiteral, StringLiteral], **kwargs: Any):
+        super().__init__(**kwargs)
+        self.name = name
+        self.value = value
+
+
+class Identifier(SyntaxNode):
+    def __init__(self, name: str, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.name = name
+
+
+class BaseComment(Entry):
+    def __init__(self, content: Union[str, None] = None, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.content = content
+
+
+class Comment(BaseComment):
+    def __init__(self, content: Union[str, None] = None, **kwargs: Any):
+        super().__init__(content, **kwargs)
+
+
+class GroupComment(BaseComment):
+    def __init__(self, content: Union[str, None] = None, **kwargs: Any):
+        super().__init__(content, **kwargs)
+
+
+class ResourceComment(BaseComment):
+    def __init__(self, content: Union[str, None] = None, **kwargs: Any):
+        super().__init__(content, **kwargs)
+
+
+class Junk(SyntaxNode):
+    def __init__(self,
+                 content: Union[str, None] = None,
+                 annotations: Union[List['Annotation'], None] = None,
+                 **kwargs: Any):
+        super().__init__(**kwargs)
+        self.content = content
+        self.annotations = annotations or []
+
+    def add_annotation(self, annot: 'Annotation') -> None:
+        self.annotations.append(annot)
+
+
+class Span(BaseNode):
+    def __init__(self, start: int, end: int, **kwargs: Any):
+        super().__init__(**kwargs)
+        self.start = start
+        self.end = end
+
+
+class Annotation(SyntaxNode):
+    def __init__(self,
+                 code: str,
+                 arguments: Union[List[Any], None] = None,
+                 message: Union[str, None] = None,
+                 **kwargs: Any):
+        super().__init__(**kwargs)
+        self.code = code
+        self.arguments = arguments or []
+        self.message = message
+
+
+EntryType = Union[Message, Term, Comment, GroupComment, ResourceComment, Junk]
+InlineExpression = Union[NumberLiteral, StringLiteral, MessageReference,
+                         TermReference, VariableReference, FunctionReference]
diff --git a/third_party/python/fluent.syntax/fluent/syntax/errors.py b/third_party/python/fluent.syntax/fluent/syntax/errors.py
new file mode 100644
index 0000000000..010374828f
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent/syntax/errors.py
@@ -0,0 +1,70 @@
+from typing import Tuple, Union
+
+
+class ParseError(Exception):
+    def __init__(self, code: str, *args: Union[str, None]):
+        self.code = code
+        self.args = args
+        self.message = get_error_message(code, args)
+
+
+def get_error_message(code: str, args: Tuple[Union[str, None], ...]) -> str:
+    if code == 'E00001':
+        return 'Generic error'
+    if code == 'E0002':
+        return 'Expected an entry start'
+    if code == 'E0003':
+        return 'Expected token: "{}"'.format(args[0])
+    if code == 'E0004':
+        return 'Expected a character from range: "{}"'.format(args[0])
+    if code == 'E0005':
+        msg = 'Expected message "{}" to have a value or attributes'
+        return msg.format(args[0])
+    if code == 'E0006':
+        msg = 'Expected term "-{}" to have a value'
+        return msg.format(args[0])
+    if code == 'E0007':
+        return 'Keyword cannot end with a whitespace'
+    if code == 'E0008':
+        return 'The callee has to be an upper-case identifier or a term'
+    if code == 'E0009':
+        return 'The argument name has to be a simple identifier'
+    if code == 'E0010':
+        return 'Expected one of the variants to be marked as default (*)'
+    if code == 'E0011':
+        return 'Expected at least one variant after "->"'
+    if code == 'E0012':
+        return 'Expected value'
+    if code == 'E0013':
+        return 'Expected variant key'
+    if code == 'E0014':
+        return 'Expected literal'
+    if code == 'E0015':
+        return 'Only one variant can be marked as default (*)'
+    if code == 'E0016':
+        return 'Message references cannot be used as selectors'
+    if code == 'E0017':
+        return 'Terms cannot be used as selectors'
+    if code == 'E0018':
+        return 'Attributes of messages cannot be used as selectors'
+    if code == 'E0019':
+        return 'Attributes of terms cannot be used as placeables'
+    if code == 'E0020':
+        return 'Unterminated string expression'
+    if code == 'E0021':
+        return 'Positional arguments must not follow named arguments'
+    if code == 'E0022':
+        return 'Named arguments must be unique'
+    if code == 'E0024':
+        return 'Cannot access variants of a message.'
+    if code == 'E0025':
+        return 'Unknown escape sequence: \\{}.'.format(args[0])
+    if code == 'E0026':
+        return 'Invalid Unicode escape sequence: {}.'.format(args[0])
+    if code == 'E0027':
+        return 'Unbalanced closing brace in TextElement.'
+    if code == 'E0028':
+        return 'Expected an inline expression'
+    if code == 'E0029':
+        return 'Expected simple expression as selector'
+    return code
diff --git a/third_party/python/fluent.syntax/fluent/syntax/parser.py b/third_party/python/fluent.syntax/fluent/syntax/parser.py
new file mode 100644
index 0000000000..87075409f1
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent/syntax/parser.py
@@ -0,0 +1,701 @@
+import re
+from typing import Any, Callable, List, Set, TypeVar, Union, cast
+from . import ast
+from .stream import EOL, FluentParserStream
+from .errors import ParseError
+
+R = TypeVar("R", bound=ast.SyntaxNode)
+
+
+def with_span(fn: Callable[..., R]) -> Callable[..., R]:
+    def decorated(self: 'FluentParser', ps: FluentParserStream, *args: Any, **kwargs: Any) -> Any:
+        if not self.with_spans:
+            return fn(self, ps, *args, **kwargs)
+
+        start = ps.index
+        node = fn(self, ps, *args, **kwargs)
+
+        # Don't re-add the span if the node already has it. This may happen
+        # when one decorated function calls another decorated function.
+        if node.span is not None:
+            return node
+
+        end = ps.index
+        node.add_span(start, end)
+        return node
+
+    return decorated
+
+
+class FluentParser:
+    """This class is used to parse Fluent source content.
+
+    ``with_spans`` enables source information in the form of
+    :class:`.ast.Span` objects for each :class:`.ast.SyntaxNode`.
+    """
+
+    def __init__(self, with_spans: bool = True):
+        self.with_spans = with_spans
+
+    def parse(self, source: str) -> ast.Resource:
+        """Create a :class:`.ast.Resource` from a Fluent source.
+        """
+        ps = FluentParserStream(source)
+        ps.skip_blank_block()
+
+        entries: List[ast.EntryType] = []
+        last_comment = None
+
+        while ps.current_char:
+            entry = self.get_entry_or_junk(ps)
+            blank_lines = ps.skip_blank_block()
+
+            # Regular Comments require special logic. Comments may be attached
+            # to Messages or Terms if they are followed immediately by them.
+            # However they should parse as standalone when they're followed by
+            # Junk. Consequently, we only attach Comments once we know that the
+            # Message or the Term parsed successfully.
+            if isinstance(entry, ast.Comment) and len(blank_lines) == 0 \
+                    and ps.current_char:
+                # Stash the comment and decide what to do with it
+                # in the next pass.
+                last_comment = entry
+                continue
+
+            if last_comment is not None:
+                if isinstance(entry, (ast.Message, ast.Term)):
+                    entry.comment = last_comment
+                    if self.with_spans:
+                        cast(ast.Span, entry.span).start = cast(ast.Span, entry.comment.span).start
+                else:
+                    entries.append(last_comment)
+                # In either case, the stashed comment has been dealt with;
+                # clear it.
+                last_comment = None
+
+            entries.append(entry)
+
+        res = ast.Resource(entries)
+
+        if self.with_spans:
+            res.add_span(0, ps.index)
+
+        return res
+
+    def parse_entry(self, source: str) -> ast.EntryType:
+        """Parse the first :class:`.ast.Entry` in source.
+
+        Skip all encountered comments and start parsing at the first :class:`.ast.Message`
+        or :class:`.ast.Term` start. Return :class:`.ast.Junk` if the parsing is not successful.
+
+        Preceding comments are ignored unless they contain syntax errors
+        themselves, in which case :class:`.ast.Junk` for the invalid comment is returned.
+        """
+        ps = FluentParserStream(source)
+        ps.skip_blank_block()
+
+        while ps.current_char == '#':
+            skipped = self.get_entry_or_junk(ps)
+            if isinstance(skipped, ast.Junk):
+                # Don't skip Junk comments.
+                return skipped
+            ps.skip_blank_block()
+
+        return self.get_entry_or_junk(ps)
+
+    def get_entry_or_junk(self, ps: FluentParserStream) -> ast.EntryType:
+        entry_start_pos = ps.index
+
+        try:
+            entry = self.get_entry(ps)
+            ps.expect_line_end()
+            return entry
+        except ParseError as err:
+            error_index = ps.index
+            ps.skip_to_next_entry_start(entry_start_pos)
+            next_entry_start = ps.index
+            if next_entry_start < error_index:
+                # The position of the error must be inside of the Junk's span.
+                error_index = next_entry_start
+
+            # Create a Junk instance
+            slice = ps.string[entry_start_pos:next_entry_start]
+            junk = ast.Junk(slice)
+            if self.with_spans:
+                junk.add_span(entry_start_pos, next_entry_start)
+            annot = ast.Annotation(err.code, list(err.args) if err.args else None, err.message)
+            annot.add_span(error_index, error_index)
+            junk.add_annotation(annot)
+            return junk
+
+    def get_entry(self, ps: FluentParserStream) -> ast.EntryType:
+        if ps.current_char == '#':
+            return self.get_comment(ps)
+
+        if ps.current_char == '-':
+            return self.get_term(ps)
+
+        if ps.is_identifier_start():
+            return self.get_message(ps)
+
+        raise ParseError('E0002')
+
+    @with_span
+    def get_comment(self, ps: FluentParserStream) -> Union[ast.Comment, ast.GroupComment, ast.ResourceComment]:
+        # 0 - comment
+        # 1 - group comment
+        # 2 - resource comment
+        level = -1
+        content = ''
+
+        while True:
+            i = -1
+            while ps.current_char == '#' \
+                    and (i < (2 if level == -1 else level)):
+                ps.next()
+                i += 1
+
+            if level == -1:
+                level = i
+
+            if ps.current_char != EOL:
+                ps.expect_char(' ')
+                ch = ps.take_char(lambda x: x != EOL)
+                while ch:
+                    content += ch
+                    ch = ps.take_char(lambda x: x != EOL)
+
+            if ps.is_next_line_comment(level=level):
+                content += cast(str, ps.current_char)
+                ps.next()
+            else:
+                break
+
+        if level == 0:
+            return ast.Comment(content)
+        elif level == 1:
+            return ast.GroupComment(content)
+        elif level == 2:
+            return ast.ResourceComment(content)
+
+        # never happens if ps.current_char == '#' when called
+        return cast(ast.Comment, None)
+
+    @with_span
+    def get_message(self, ps: FluentParserStream) -> ast.Message:
+        id = self.get_identifier(ps)
+        ps.skip_blank_inline()
+        ps.expect_char('=')
+
+        value = self.maybe_get_pattern(ps)
+        attrs = self.get_attributes(ps)
+
+        if value is None and len(attrs) == 0:
+            raise ParseError('E0005', id.name)
+
+        return ast.Message(id, value, attrs)
+
+    @with_span
+    def get_term(self, ps: FluentParserStream) -> ast.Term:
+        ps.expect_char('-')
+        id = self.get_identifier(ps)
+
+        ps.skip_blank_inline()
+        ps.expect_char('=')
+
+        value = self.maybe_get_pattern(ps)
+        if value is None:
+            raise ParseError('E0006', id.name)
+
+        attrs = self.get_attributes(ps)
+        return ast.Term(id, value, attrs)
+
+    @with_span
+    def get_attribute(self, ps: FluentParserStream) -> ast.Attribute:
+        ps.expect_char('.')
+
+        key = self.get_identifier(ps)
+
+        ps.skip_blank_inline()
+        ps.expect_char('=')
+
+        value = self.maybe_get_pattern(ps)
+        if value is None:
+            raise ParseError('E0012')
+
+        return ast.Attribute(key, value)
+
+    def get_attributes(self, ps: FluentParserStream) -> List[ast.Attribute]:
+        attrs: List[ast.Attribute] = []
+        ps.peek_blank()
+
+        while ps.is_attribute_start():
+            ps.skip_to_peek()
+            attr = self.get_attribute(ps)
+            attrs.append(attr)
+            ps.peek_blank()
+
+        return attrs
+
+    @with_span
+    def get_identifier(self, ps: FluentParserStream) -> ast.Identifier:
+        name = ps.take_id_start()
+        if name is None:
+            raise ParseError('E0004', 'a-zA-Z')
+
+        ch = ps.take_id_char()
+        while ch:
+            name += ch
+            ch = ps.take_id_char()
+
+        return ast.Identifier(name)
+
+    def get_variant_key(self, ps: FluentParserStream) -> Union[ast.Identifier, ast.NumberLiteral]:
+        ch = ps.current_char
+
+        if ch is None:
+            raise ParseError('E0013')
+
+        cc = ord(ch)
+        if ((cc >= 48 and cc <= 57) or cc == 45):  # 0-9, -
+            return self.get_number(ps)
+
+        return self.get_identifier(ps)
+
+    @with_span
+    def get_variant(self, ps: FluentParserStream, has_default: bool) -> ast.Variant:
+        default_index = False
+
+        if ps.current_char == '*':
+            if has_default:
+                raise ParseError('E0015')
+            ps.next()
+            default_index = True
+
+        ps.expect_char('[')
+        ps.skip_blank()
+
+        key = self.get_variant_key(ps)
+
+        ps.skip_blank()
+        ps.expect_char(']')
+
+        value = self.maybe_get_pattern(ps)
+        if value is None:
+            raise ParseError('E0012')
+
+        return ast.Variant(key, value, default_index)
+
+    def get_variants(self, ps: FluentParserStream) -> List[ast.Variant]:
+        variants: List[ast.Variant] = []
+        has_default = False
+
+        ps.skip_blank()
+        while ps.is_variant_start():
+            variant = self.get_variant(ps, has_default)
+
+            if variant.default:
+                has_default = True
+
+            variants.append(variant)
+            ps.expect_line_end()
+            ps.skip_blank()
+
+        if len(variants) == 0:
+            raise ParseError('E0011')
+
+        if not has_default:
+            raise ParseError('E0010')
+
+        return variants
+
+    def get_digits(self, ps: FluentParserStream) -> str:
+        num = ''
+
+        ch = ps.take_digit()
+        while ch:
+            num += ch
+            ch = ps.take_digit()
+
+        if len(num) == 0:
+            raise ParseError('E0004', '0-9')
+
+        return num
+
+    @with_span
+    def get_number(self, ps: FluentParserStream) -> ast.NumberLiteral:
+        num = ''
+
+        if ps.current_char == '-':
+            num += '-'
+            ps.next()
+
+        num += self.get_digits(ps)
+
+        if ps.current_char == '.':
+            num += '.'
+            ps.next()
+            num += self.get_digits(ps)
+
+        return ast.NumberLiteral(num)
+
+    def maybe_get_pattern(self, ps: FluentParserStream) -> Union[ast.Pattern, None]:
+        '''Parse an inline or a block Pattern, or None
+
+        maybe_get_pattern distinguishes between patterns which start on the
+        same line as the indentifier (aka inline singleline patterns and inline
+        multiline patterns), and patterns which start on a new line (aka block
+        patterns). The distinction is important for the dedentation logic: the
+        indent of the first line of a block pattern must be taken into account
+        when calculating the maximum common indent.
+        '''
+        ps.peek_blank_inline()
+        if ps.is_value_start():
+            ps.skip_to_peek()
+            return self.get_pattern(ps, is_block=False)
+
+        ps.peek_blank_block()
+        if ps.is_value_continuation():
+            ps.skip_to_peek()
+            return self.get_pattern(ps, is_block=True)
+
+        return None
+
+    @with_span
+    def get_pattern(self, ps: FluentParserStream, is_block: bool) -> ast.Pattern:
+        elements: List[Any] = []
+        if is_block:
+            # A block pattern is a pattern which starts on a new line. Measure
+            # the indent of this first line for the dedentation logic.
+            blank_start = ps.index
+            first_indent = ps.skip_blank_inline()
+            elements.append(self.Indent(first_indent, blank_start, ps.index))
+            common_indent_length = len(first_indent)
+        else:
+            # Should get fixed by the subsequent min() operation
+            common_indent_length = cast(int, float('infinity'))
+
+        while ps.current_char:
+            if ps.current_char == EOL:
+                blank_start = ps.index
+                blank_lines = ps.peek_blank_block()
+                if ps.is_value_continuation():
+                    ps.skip_to_peek()
+                    indent = ps.skip_blank_inline()
+                    common_indent_length = min(common_indent_length, len(indent))
+                    elements.append(self.Indent(blank_lines + indent, blank_start, ps.index))
+                    continue
+
+                # The end condition for get_pattern's while loop is a newline
+                # which is not followed by a valid pattern continuation.
+                ps.reset_peek()
+                break
+
+            if ps.current_char == '}':
+                raise ParseError('E0027')
+
+            element: Union[ast.TextElement, ast.Placeable]
+            if ps.current_char == '{':
+                element = self.get_placeable(ps)
+            else:
+                element = self.get_text_element(ps)
+
+            elements.append(element)
+
+        dedented = self.dedent(elements, common_indent_length)
+        return ast.Pattern(dedented)
+
+    class Indent(ast.SyntaxNode):
+        def __init__(self, value: str, start: int, end: int):
+            super(FluentParser.Indent, self).__init__()
+            self.value = value
+            self.add_span(start, end)
+
+    def dedent(self,
+               elements: List[Union[ast.TextElement, ast.Placeable, Indent]],
+               common_indent: int
+               ) -> List[Union[ast.TextElement, ast.Placeable]]:
+        '''Dedent a list of elements by removing the maximum common indent from
+        the beginning of text lines. The common indent is calculated in
+        get_pattern.
+        '''
+        trimmed: List[Union[ast.TextElement, ast.Placeable]] = []
+
+        for element in elements:
+            if isinstance(element, ast.Placeable):
+                trimmed.append(element)
+                continue
+
+            if isinstance(element, self.Indent):
+                # Strip the common indent.
+                element.value = element.value[:len(element.value) - common_indent]
+                if len(element.value) == 0:
+                    continue
+
+            prev = trimmed[-1] if len(trimmed) > 0 else None
+            if isinstance(prev, ast.TextElement):
+                # Join adjacent TextElements by replacing them with their sum.
+                sum = ast.TextElement(prev.value + element.value)
+                if self.with_spans:
+                    sum.add_span(cast(ast.Span, prev.span).start, cast(ast.Span, element.span).end)
+                trimmed[-1] = sum
+                continue
+
+            if isinstance(element, self.Indent):
+                # If the indent hasn't been merged into a preceding
+                # TextElements, convert it into a new TextElement.
+                text_element = ast.TextElement(element.value)
+                if self.with_spans:
+                    text_element.add_span(cast(ast.Span, element.span).start, cast(ast.Span, element.span).end)
+                element = text_element
+
+            trimmed.append(element)
+
+        # Trim trailing whitespace from the Pattern.
+        last_element = trimmed[-1] if len(trimmed) > 0 else None
+        if isinstance(last_element, ast.TextElement):
+            last_element.value = last_element.value.rstrip(' \n\r')
+            if last_element.value == "":
+                trimmed.pop()
+
+        return trimmed
+
+    @with_span
+    def get_text_element(self, ps: FluentParserStream) -> ast.TextElement:
+        buf = ''
+
+        while ps.current_char:
+            ch = ps.current_char
+
+            if ch == '{' or ch == '}':
+                return ast.TextElement(buf)
+
+            if ch == EOL:
+                return ast.TextElement(buf)
+
+            buf += ch
+            ps.next()
+
+        return ast.TextElement(buf)
+
+    def get_escape_sequence(self, ps: FluentParserStream) -> str:
+        next = ps.current_char
+
+        if next == '\\' or next == '"':
+            ps.next()
+            return f'\\{next}'
+
+        if next == 'u':
+            return self.get_unicode_escape_sequence(ps, next, 4)
+
+        if next == 'U':
+            return self.get_unicode_escape_sequence(ps, next, 6)
+
+        raise ParseError('E0025', next)
+
+    def get_unicode_escape_sequence(self, ps: FluentParserStream, u: str, digits: int) -> str:
+        ps.expect_char(u)
+        sequence = ''
+        for _ in range(digits):
+            ch = ps.take_hex_digit()
+            if not ch:
+                raise ParseError('E0026', f'\\{u}{sequence}{ps.current_char}')
+            sequence += ch
+
+        return f'\\{u}{sequence}'
+
+    @with_span
+    def get_placeable(self, ps: FluentParserStream) -> ast.Placeable:
+        ps.expect_char('{')
+        ps.skip_blank()
+        expression = self.get_expression(ps)
+        ps.expect_char('}')
+        return ast.Placeable(expression)
+
+    @with_span
+    def get_expression(self, ps: FluentParserStream) -> Union[ast.InlineExpression,
+                                                              ast.Placeable,
+                                                              ast.SelectExpression]:
+        selector = self.get_inline_expression(ps)
+
+        ps.skip_blank()
+
+        if ps.current_char == '-':
+            if ps.peek() != '>':
+                ps.reset_peek()
+                return selector
+
+            if isinstance(selector, ast.MessageReference):
+                if selector.attribute is None:
+                    raise ParseError('E0016')
+                else:
+                    raise ParseError('E0018')
+
+            elif (
+                isinstance(selector, ast.TermReference)
+            ):
+                if selector.attribute is None:
+                    raise ParseError('E0017')
+            elif not (
+                isinstance(selector, (
+                    ast.StringLiteral,
+                    ast.NumberLiteral,
+                    ast.VariableReference,
+                    ast.FunctionReference,
+                ))
+            ):
+                raise ParseError('E0029')
+
+            ps.next()
+            ps.next()
+
+            ps.skip_blank_inline()
+            ps.expect_line_end()
+
+            variants = self.get_variants(ps)
+            return ast.SelectExpression(selector, variants)
+
+        if (
+            isinstance(selector, ast.TermReference)
+            and selector.attribute is not None
+        ):
+            raise ParseError('E0019')
+
+        return selector
+
+    @with_span
+    def get_inline_expression(self, ps: FluentParserStream) -> Union[ast.InlineExpression, ast.Placeable]:
+        if ps.current_char == '{':
+            return self.get_placeable(ps)
+
+        if ps.is_number_start():
+            return self.get_number(ps)
+
+        if ps.current_char == '"':
+            return self.get_string(ps)
+
+        if ps.current_char == '$':
+            ps.next()
+            id = self.get_identifier(ps)
+            return ast.VariableReference(id)
+
+        if ps.current_char == '-':
+            ps.next()
+            id = self.get_identifier(ps)
+            attribute = None
+            if ps.current_char == '.':
+                ps.next()
+                attribute = self.get_identifier(ps)
+            arguments = None
+            ps.peek_blank()
+            if ps.current_peek == '(':
+                ps.skip_to_peek()
+                arguments = self.get_call_arguments(ps)
+            return ast.TermReference(id, attribute, arguments)
+
+        if ps.is_identifier_start():
+            id = self.get_identifier(ps)
+            ps.peek_blank()
+
+            if ps.current_peek == '(':
+                # It's a Function. Ensure it's all upper-case.
+                if not re.match('^[A-Z][A-Z0-9_-]*$', id.name):
+                    raise ParseError('E0008')
+                ps.skip_to_peek()
+                args = self.get_call_arguments(ps)
+                return ast.FunctionReference(id, args)
+
+            attribute = None
+            if ps.current_char == '.':
+                ps.next()
+                attribute = self.get_identifier(ps)
+
+            return ast.MessageReference(id, attribute)
+
+        raise ParseError('E0028')
+
+    @with_span
+    def get_call_argument(self,
+                          ps: FluentParserStream
+                          ) -> Union[ast.InlineExpression, ast.NamedArgument, ast.Placeable]:
+        exp = self.get_inline_expression(ps)
+
+        ps.skip_blank()
+
+        if ps.current_char != ':':
+            return exp
+
+        if isinstance(exp, ast.MessageReference) and exp.attribute is None:
+            ps.next()
+            ps.skip_blank()
+
+            value = self.get_literal(ps)
+            return ast.NamedArgument(exp.id, value)
+
+        raise ParseError('E0009')
+
+    @with_span
+    def get_call_arguments(self, ps: FluentParserStream) -> ast.CallArguments:
+        positional: List[Union[ast.InlineExpression, ast.Placeable]] = []
+        named: List[ast.NamedArgument] = []
+        argument_names: Set[str] = set()
+
+        ps.expect_char('(')
+        ps.skip_blank()
+
+        while True:
+            if ps.current_char == ')':
+                break
+
+            arg = self.get_call_argument(ps)
+            if isinstance(arg, ast.NamedArgument):
+                if arg.name.name in argument_names:
+                    raise ParseError('E0022')
+                named.append(arg)
+                argument_names.add(arg.name.name)
+            elif len(argument_names) > 0:
+                raise ParseError('E0021')
+            else:
+                positional.append(arg)
+
+            ps.skip_blank()
+
+            if ps.current_char == ',':
+                ps.next()
+                ps.skip_blank()
+                continue
+
+            break
+
+        ps.expect_char(')')
+        return ast.CallArguments(positional, named)
+
+    @with_span
+    def get_string(self, ps: FluentParserStream) -> ast.StringLiteral:
+        value = ''
+
+        ps.expect_char('"')
+
+        while True:
+            ch = ps.take_char(lambda x: x != '"' and x != EOL)
+            if not ch:
+                break
+            if ch == '\\':
+                value += self.get_escape_sequence(ps)
+            else:
+                value += ch
+
+        if ps.current_char == EOL:
+            raise ParseError('E0020')
+
+        ps.expect_char('"')
+
+        return ast.StringLiteral(value)
+
+    @with_span
+    def get_literal(self, ps: FluentParserStream) -> Union[ast.NumberLiteral, ast.StringLiteral]:
+        if ps.is_number_start():
+            return self.get_number(ps)
+        if ps.current_char == '"':
+            return self.get_string(ps)
+        raise ParseError('E0014')
diff --git a/third_party/python/fluent.syntax/fluent/syntax/py.typed b/third_party/python/fluent.syntax/fluent/syntax/py.typed
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent/syntax/py.typed
diff --git a/third_party/python/fluent.syntax/fluent/syntax/serializer.py b/third_party/python/fluent.syntax/fluent/syntax/serializer.py
new file mode 100644
index 0000000000..68ea89b3d3
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent/syntax/serializer.py
@@ -0,0 +1,237 @@
+from typing import List, Union
+from . import ast
+
+
+def indent_except_first_line(content: str) -> str:
+    return "    ".join(
+        content.splitlines(True)
+    )
+
+
+def includes_new_line(elem: Union[ast.TextElement, ast.Placeable]) -> bool:
+    return isinstance(elem, ast.TextElement) and "\n" in elem.value
+
+
+def is_select_expr(elem: Union[ast.TextElement, ast.Placeable]) -> bool:
+    return (
+        isinstance(elem, ast.Placeable) and
+        isinstance(elem.expression, ast.SelectExpression))
+
+
+def should_start_on_new_line(pattern: ast.Pattern) -> bool:
+    is_multiline = any(is_select_expr(elem) for elem in pattern.elements) \
+        or any(includes_new_line(elem) for elem in pattern.elements)
+
+    if is_multiline:
+        first_element = pattern.elements[0]
+        if isinstance(first_element, ast.TextElement):
+            first_char = first_element.value[0]
+            if first_char in ("[", ".", "*"):
+                return False
+        return True
+    return False
+
+
+class FluentSerializer:
+    """FluentSerializer converts :class:`.ast.SyntaxNode` objects to unicode strings.
+
+    `with_junk` controls if parse errors are written back or not.
+    """
+    HAS_ENTRIES = 1
+
+    def __init__(self, with_junk: bool = False):
+        self.with_junk = with_junk
+
+    def serialize(self, resource: ast.Resource) -> str:
+        "Serialize a :class:`.ast.Resource` to a string."
+        if not isinstance(resource, ast.Resource):
+            raise Exception('Unknown resource type: {}'.format(type(resource)))
+
+        state = 0
+
+        parts: List[str] = []
+        for entry in resource.body:
+            if not isinstance(entry, ast.Junk) or self.with_junk:
+                parts.append(self.serialize_entry(entry, state))
+                if not state & self.HAS_ENTRIES:
+                    state |= self.HAS_ENTRIES
+
+        return "".join(parts)
+
+    def serialize_entry(self, entry: ast.EntryType, state: int = 0) -> str:
+        "Serialize an :class:`.ast.Entry` to a string."
+        if isinstance(entry, ast.Message):
+            return serialize_message(entry)
+        if isinstance(entry, ast.Term):
+            return serialize_term(entry)
+        if isinstance(entry, ast.Comment):
+            if state & self.HAS_ENTRIES:
+                return "\n{}\n".format(serialize_comment(entry, "#"))
+            return "{}\n".format(serialize_comment(entry, "#"))
+        if isinstance(entry, ast.GroupComment):
+            if state & self.HAS_ENTRIES:
+                return "\n{}\n".format(serialize_comment(entry, "##"))
+            return "{}\n".format(serialize_comment(entry, "##"))
+        if isinstance(entry, ast.ResourceComment):
+            if state & self.HAS_ENTRIES:
+                return "\n{}\n".format(serialize_comment(entry, "###"))
+            return "{}\n".format(serialize_comment(entry, "###"))
+        if isinstance(entry, ast.Junk):
+            return serialize_junk(entry)
+        raise Exception('Unknown entry type: {}'.format(type(entry)))
+
+
+def serialize_comment(comment: Union[ast.Comment, ast.GroupComment, ast.ResourceComment], prefix: str = "#") -> str:
+    if not comment.content:
+        return f'{prefix}\n'
+
+    prefixed = "\n".join([
+        prefix if len(line) == 0 else f"{prefix} {line}"
+        for line in comment.content.split("\n")
+    ])
+    # Add the trailing line break.
+    return f'{prefixed}\n'
+
+
+def serialize_junk(junk: ast.Junk) -> str:
+    return junk.content or ''
+
+
+def serialize_message(message: ast.Message) -> str:
+    parts: List[str] = []
+
+    if message.comment:
+        parts.append(serialize_comment(message.comment))
+
+    parts.append(f"{message.id.name} =")
+
+    if message.value:
+        parts.append(serialize_pattern(message.value))
+
+    if message.attributes:
+        for attribute in message.attributes:
+            parts.append(serialize_attribute(attribute))
+
+    parts.append("\n")
+    return ''.join(parts)
+
+
+def serialize_term(term: ast.Term) -> str:
+    parts: List[str] = []
+
+    if term.comment:
+        parts.append(serialize_comment(term.comment))
+
+    parts.append(f"-{term.id.name} =")
+    parts.append(serialize_pattern(term.value))
+
+    if term.attributes:
+        for attribute in term.attributes:
+            parts.append(serialize_attribute(attribute))
+
+    parts.append("\n")
+    return ''.join(parts)
+
+
+def serialize_attribute(attribute: ast.Attribute) -> str:
+    return "\n    .{} ={}".format(
+        attribute.id.name,
+        indent_except_first_line(serialize_pattern(attribute.value))
+    )
+
+
+def serialize_pattern(pattern: ast.Pattern) -> str:
+    content = "".join(serialize_element(elem) for elem in pattern.elements)
+    content = indent_except_first_line(content)
+
+    if should_start_on_new_line(pattern):
+        return f'\n    {content}'
+
+    return f' {content}'
+
+
+def serialize_element(element: ast.PatternElement) -> str:
+    if isinstance(element, ast.TextElement):
+        return element.value
+    if isinstance(element, ast.Placeable):
+        return serialize_placeable(element)
+    raise Exception('Unknown element type: {}'.format(type(element)))
+
+
+def serialize_placeable(placeable: ast.Placeable) -> str:
+    expr = placeable.expression
+    if isinstance(expr, ast.Placeable):
+        return "{{{}}}".format(serialize_placeable(expr))
+    if isinstance(expr, ast.SelectExpression):
+        # Special-case select expressions to control the withespace around the
+        # opening and the closing brace.
+        return "{{ {}}}".format(serialize_expression(expr))
+    if isinstance(expr, ast.Expression):
+        return "{{ {} }}".format(serialize_expression(expr))
+    raise Exception('Unknown expression type: {}'.format(type(expr)))
+
+
+def serialize_expression(expression: Union[ast.Expression, ast.Placeable]) -> str:
+    if isinstance(expression, ast.StringLiteral):
+        return f'"{expression.value}"'
+    if isinstance(expression, ast.NumberLiteral):
+        return expression.value
+    if isinstance(expression, ast.VariableReference):
+        return f"${expression.id.name}"
+    if isinstance(expression, ast.TermReference):
+        out = f"-{expression.id.name}"
+        if expression.attribute is not None:
+            out += f".{expression.attribute.name}"
+        if expression.arguments is not None:
+            out += serialize_call_arguments(expression.arguments)
+        return out
+    if isinstance(expression, ast.MessageReference):
+        out = expression.id.name
+        if expression.attribute is not None:
+            out += f".{expression.attribute.name}"
+        return out
+    if isinstance(expression, ast.FunctionReference):
+        args = serialize_call_arguments(expression.arguments)
+        return f"{expression.id.name}{args}"
+    if isinstance(expression, ast.SelectExpression):
+        out = "{} ->".format(
+            serialize_expression(expression.selector))
+        for variant in expression.variants:
+            out += serialize_variant(variant)
+        return f"{out}\n"
+    if isinstance(expression, ast.Placeable):
+        return serialize_placeable(expression)
+    raise Exception('Unknown expression type: {}'.format(type(expression)))
+
+
+def serialize_variant(variant: ast.Variant) -> str:
+    return "\n{}[{}]{}".format(
+        "   *" if variant.default else "    ",
+        serialize_variant_key(variant.key),
+        indent_except_first_line(serialize_pattern(variant.value))
+    )
+
+
+def serialize_call_arguments(expr: ast.CallArguments) -> str:
+    positional = ", ".join(
+        serialize_expression(arg) for arg in expr.positional)
+    named = ", ".join(
+        serialize_named_argument(arg) for arg in expr.named)
+    if len(expr.positional) > 0 and len(expr.named) > 0:
+        return f'({positional}, {named})'
+    return '({})'.format(positional or named)
+
+
+def serialize_named_argument(arg: ast.NamedArgument) -> str:
+    return "{}: {}".format(
+        arg.name.name,
+        serialize_expression(arg.value)
+    )
+
+
+def serialize_variant_key(key: Union[ast.Identifier, ast.NumberLiteral]) -> str:
+    if isinstance(key, ast.Identifier):
+        return key.name
+    if isinstance(key, ast.NumberLiteral):
+        return key.value
+    raise Exception('Unknown variant key type: {}'.format(type(key)))
diff --git a/third_party/python/fluent.syntax/fluent/syntax/stream.py b/third_party/python/fluent.syntax/fluent/syntax/stream.py
new file mode 100644
index 0000000000..150ac933ca
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent/syntax/stream.py
@@ -0,0 +1,283 @@
+from typing import Callable, Union
+from typing_extensions import Literal
+from .errors import ParseError
+
+
+class ParserStream:
+    def __init__(self, string: str):
+        self.string = string
+        self.index = 0
+        self.peek_offset = 0
+
+    def get(self, offset: int) -> Union[str, None]:
+        try:
+            return self.string[offset]
+        except IndexError:
+            return None
+
+    def char_at(self, offset: int) -> Union[str, None]:
+        # When the cursor is at CRLF, return LF but don't move the cursor. The
+        # cursor still points to the EOL position, which in this case is the
+        # beginning of the compound CRLF sequence. This ensures slices of
+        # [inclusive, exclusive) continue to work properly.
+        if self.get(offset) == '\r' \
+                and self.get(offset + 1) == '\n':
+            return '\n'
+
+        return self.get(offset)
+
+    @property
+    def current_char(self) -> Union[str, None]:
+        return self.char_at(self.index)
+
+    @property
+    def current_peek(self) -> Union[str, None]:
+        return self.char_at(self.index + self.peek_offset)
+
+    def next(self) -> Union[str, None]:
+        self.peek_offset = 0
+        # Skip over CRLF as if it was a single character.
+        if self.get(self.index) == '\r' \
+                and self.get(self.index + 1) == '\n':
+            self.index += 1
+        self.index += 1
+        return self.get(self.index)
+
+    def peek(self) -> Union[str, None]:
+        # Skip over CRLF as if it was a single character.
+        if self.get(self.index + self.peek_offset) == '\r' \
+                and self.get(self.index + self.peek_offset + 1) == '\n':
+            self.peek_offset += 1
+        self.peek_offset += 1
+        return self.get(self.index + self.peek_offset)
+
+    def reset_peek(self, offset: int = 0) -> None:
+        self.peek_offset = offset
+
+    def skip_to_peek(self) -> None:
+        self.index += self.peek_offset
+        self.peek_offset = 0
+
+
+EOL = '\n'
+EOF = None
+SPECIAL_LINE_START_CHARS = ('}', '.', '[', '*')
+
+
+class FluentParserStream(ParserStream):
+
+    def peek_blank_inline(self) -> str:
+        start = self.index + self.peek_offset
+        while self.current_peek == ' ':
+            self.peek()
+        return self.string[start:self.index + self.peek_offset]
+
+    def skip_blank_inline(self) -> str:
+        blank = self.peek_blank_inline()
+        self.skip_to_peek()
+        return blank
+
+    def peek_blank_block(self) -> str:
+        blank = ""
+        while True:
+            line_start = self.peek_offset
+            self.peek_blank_inline()
+
+            if self.current_peek == EOL:
+                blank += EOL
+                self.peek()
+                continue
+
+            if self.current_peek is EOF:
+                # Treat the blank line at EOF as a blank block.
+                return blank
+
+            # Any other char; reset to column 1 on this line.
+            self.reset_peek(line_start)
+            return blank
+
+    def skip_blank_block(self) -> str:
+        blank = self.peek_blank_block()
+        self.skip_to_peek()
+        return blank
+
+    def peek_blank(self) -> None:
+        while self.current_peek in (" ", EOL):
+            self.peek()
+
+    def skip_blank(self) -> None:
+        self.peek_blank()
+        self.skip_to_peek()
+
+    def expect_char(self, ch: str) -> Literal[True]:
+        if self.current_char == ch:
+            self.next()
+            return True
+
+        raise ParseError('E0003', ch)
+
+    def expect_line_end(self) -> Literal[True]:
+        if self.current_char is EOF:
+            # EOF is a valid line end in Fluent.
+            return True
+
+        if self.current_char == EOL:
+            self.next()
+            return True
+
+        # Unicode Character 'SYMBOL FOR NEWLINE' (U+2424)
+        raise ParseError('E0003', '\u2424')
+
+    def take_char(self, f: Callable[[str], bool]) -> Union[str, Literal[False], None]:
+        ch = self.current_char
+        if ch is None:
+            return EOF
+        if f(ch):
+            self.next()
+            return ch
+        return False
+
+    def is_char_id_start(self, ch: Union[str, None]) -> bool:
+        if ch is None:
+            return False
+
+        cc = ord(ch)
+        return (cc >= 97 and cc <= 122) or \
+               (cc >= 65 and cc <= 90)
+
+    def is_identifier_start(self) -> bool:
+        return self.is_char_id_start(self.current_peek)
+
+    def is_number_start(self) -> bool:
+        ch = self.peek() if self.current_char == '-' else self.current_char
+        if ch is None:
+            self.reset_peek()
+            return False
+
+        cc = ord(ch)
+        is_digit = cc >= 48 and cc <= 57
+        self.reset_peek()
+        return is_digit
+
+    def is_char_pattern_continuation(self, ch: Union[str, None]) -> bool:
+        if ch is EOF:
+            return False
+
+        return ch not in SPECIAL_LINE_START_CHARS
+
+    def is_value_start(self) -> bool:
+        # Inline Patterns may start with any char.
+        return self.current_peek is not EOF and self.current_peek != EOL
+
+    def is_value_continuation(self) -> bool:
+        column1 = self.peek_offset
+        self.peek_blank_inline()
+
+        if self.current_peek == '{':
+            self.reset_peek(column1)
+            return True
+
+        if self.peek_offset - column1 == 0:
+            return False
+
+        if self.is_char_pattern_continuation(self.current_peek):
+            self.reset_peek(column1)
+            return True
+
+        return False
+
+    # -1 - any
+    #  0 - comment
+    #  1 - group comment
+    #  2 - resource comment
+    def is_next_line_comment(self, level: int = -1) -> bool:
+        if self.current_peek != EOL:
+            return False
+
+        i = 0
+
+        while (i <= level or (level == -1 and i < 3)):
+            if self.peek() != '#':
+                if i <= level and level != -1:
+                    self.reset_peek()
+                    return False
+                break
+            i += 1
+
+        # The first char after #, ## or ###.
+        if self.peek() in (' ', EOL):
+            self.reset_peek()
+            return True
+
+        self.reset_peek()
+        return False
+
+    def is_variant_start(self) -> bool:
+        current_peek_offset = self.peek_offset
+        if self.current_peek == '*':
+            self.peek()
+        if self.current_peek == '[' and self.peek() != '[':
+            self.reset_peek(current_peek_offset)
+            return True
+
+        self.reset_peek(current_peek_offset)
+        return False
+
+    def is_attribute_start(self) -> bool:
+        return self.current_peek == '.'
+
+    def skip_to_next_entry_start(self, junk_start: int) -> None:
+        last_newline = self.string.rfind(EOL, 0, self.index)
+        if junk_start < last_newline:
+            # Last seen newline is _after_ the junk start. It's safe to rewind
+            # without the risk of resuming at the same broken entry.
+            self.index = last_newline
+
+        while self.current_char:
+            # We're only interested in beginnings of line.
+            if self.current_char != EOL:
+                self.next()
+                continue
+
+            # Break if the first char in this line looks like an entry start.
+            first = self.next()
+            if self.is_char_id_start(first) or first == '-' or first == '#':
+                break
+
+            # Syntax 0.4 compatibility
+            peek = self.peek()
+            self.reset_peek()
+            if (first, peek) == ('/', '/') or (first, peek) == ('[', '['):
+                break
+
+    def take_id_start(self) -> Union[str, None]:
+        if self.is_char_id_start(self.current_char):
+            ret = self.current_char
+            self.next()
+            return ret
+
+        raise ParseError('E0004', 'a-zA-Z')
+
+    def take_id_char(self) -> Union[str, Literal[False], None]:
+        def closure(ch: str) -> bool:
+            cc = ord(ch)
+            return ((cc >= 97 and cc <= 122) or
+                    (cc >= 65 and cc <= 90) or
+                    (cc >= 48 and cc <= 57) or
+                    cc == 95 or cc == 45)
+        return self.take_char(closure)
+
+    def take_digit(self) -> Union[str, Literal[False], None]:
+        def closure(ch: str) -> bool:
+            cc = ord(ch)
+            return (cc >= 48 and cc <= 57)
+        return self.take_char(closure)
+
+    def take_hex_digit(self) -> Union[str, Literal[False], None]:
+        def closure(ch: str) -> bool:
+            cc = ord(ch)
+            return (
+                (cc >= 48 and cc <= 57)   # 0-9
+                or (cc >= 65 and cc <= 70)  # A-F
+                or (cc >= 97 and cc <= 102))  # a-f
+        return self.take_char(closure)
diff --git a/third_party/python/fluent.syntax/fluent/syntax/visitor.py b/third_party/python/fluent.syntax/fluent/syntax/visitor.py
new file mode 100644
index 0000000000..0df9f5963e
--- /dev/null
+++ b/third_party/python/fluent.syntax/fluent/syntax/visitor.py
@@ -0,0 +1,65 @@
+from typing import Any, List
+from .ast import BaseNode, Node
+
+
+class Visitor:
+    '''Read-only visitor pattern.
+
+    Subclass this to gather information from an AST.
+    To generally define which nodes not to descend in to, overload
+    `generic_visit`.
+    To handle specific node types, add methods like `visit_Pattern`.
+    If you want to still descend into the children of the node, call
+    `generic_visit` of the superclass.
+    '''
+
+    def visit(self, node: Any) -> None:
+        if isinstance(node, list):
+            for child in node:
+                self.visit(child)
+            return
+        if not isinstance(node, BaseNode):
+            return
+        nodename = type(node).__name__
+        visit = getattr(self, f'visit_{nodename}', self.generic_visit)
+        visit(node)
+
+    def generic_visit(self, node: BaseNode) -> None:
+        for propvalue in vars(node).values():
+            self.visit(propvalue)
+
+
+class Transformer(Visitor):
+    '''In-place AST Transformer pattern.
+
+    Subclass this to create an in-place modified variant
+    of the given AST.
+    If you need to keep the original AST around, pass
+    a `node.clone()` to the transformer.
+    '''
+
+    def visit(self, node: Any) -> Any:
+        if not isinstance(node, BaseNode):
+            return node
+
+        nodename = type(node).__name__
+        visit = getattr(self, f'visit_{nodename}', self.generic_visit)
+        return visit(node)
+
+    def generic_visit(self, node: Node) -> Node:  # type: ignore
+        for propname, propvalue in vars(node).items():
+            if isinstance(propvalue, list):
+                new_vals: List[Any] = []
+                for child in propvalue:
+                    new_val = self.visit(child)
+                    if new_val is not None:
+                        new_vals.append(new_val)
+                # in-place manipulation
+                propvalue[:] = new_vals
+            elif isinstance(propvalue, BaseNode):
+                new_val = self.visit(propvalue)
+                if new_val is None:
+                    delattr(node, propname)
+                else:
+                    setattr(node, propname, new_val)
+        return node