feat: Allow sub-diagnostic labels to refer to fields of their parent

guppylang/diagnostic.py

@@ -1,7 +1,17 @@
+import string
 import textwrap
-from dataclasses import dataclass, field, fields
+from collections.abc import Mapping, Sequence
+from dataclasses import dataclass, field
 from enum import Enum, auto
-from typing import ClassVar, Final, Literal, Protocol, overload, runtime_checkable
+from typing import (
+    Any,
+    ClassVar,
+    Final,
+    Literal,
+    Protocol,
+    overload,
+    runtime_checkable,
+)
 
 from typing_extensions import Self
 
@@ -54,6 +64,9 @@ class SubDiagnostic(Protocol):
     #: Message that is printed if no span is provided.
     message: ClassVar[str | None] = None
 
+    #: The parent main diagnostic this sub-diagnostic is attached to.
+    _parent: "Diagnostic | None" = field(default=None, init=False)
+
     def __post_init__(self) -> None:
         if self.span_label and self.span is None:
             raise InternalGuppyError("SubDiagnostic: Span label provided without span")
@@ -78,8 +91,17 @@ def _render(self, s: str | None) -> str | None:
         """Helper method to fill in placeholder values in strings with fields of this
         diagnostic.
         """
-        values = {f.name: getattr(self, f.name) for f in fields(self)}
-        return s.format(**values) if s is not None else None
+
+        class CustomFormatter(string.Formatter):
+            def get_value(
+                _self, key: int | str, args: Sequence[Any], kwargs: Mapping[str, Any]
+            ) -> Any:
+                assert isinstance(key, str)
+                if hasattr(self, key):
+                    return getattr(self, key)
+                return getattr(self._parent, key)
+
+        return CustomFormatter().format(s) if s is not None else None
 
 
 @runtime_checkable
@@ -114,6 +136,7 @@ def add_sub_diagnostic(self, sub: "SubDiagnostic") -> Self:
             raise InternalGuppyError(
                 "Diagnostic: Cross-file sub-diagnostics are not supported"
             )
+        object.__setattr__(sub, "_parent", self)
         self.children.append(sub)
         return self
 

tests/diagnostics/snapshots/test_advanced_formatting.txt

@@ -0,0 +1,7 @@
+Error: Can't compare apples with oranges (at <unknown>:1:0)
+  | 
+1 | apple == orange
+  | ^^^^^ This is an apple
+  | 
+1 | apple == orange
+  |          ------ This is not an apple

tests/diagnostics/test_diagnostics_rendering.py

@@ -96,6 +96,38 @@ class MySubDiagnostic(Note):
     run_test(source, diagnostic, snapshot, request)
 
 
+def test_advanced_formatting(snapshot, request):
+    @dataclass(frozen=True)
+    class MyDiagnostic(Error):
+        title: ClassVar[str] = "Can't compare apples with oranges"
+        span_label: ClassVar[str] = "This is an {a}{pp}{le}"
+        a: str
+
+        @property
+        def pp(self) -> str:
+            return "pp"
+
+        @property
+        def le(self) -> str:
+            return "le"
+
+    @dataclass(frozen=True)
+    class MySubDiagnostic(Note):
+        span_label: ClassVar[str] = "This is not an {a}{pp}{p}{le}"
+        p: str
+
+        @property
+        def pp(self) -> str:
+            return "p"
+
+    source = "apple == orange"
+    span_apple = Span(Loc(file, 1, 0), Loc(file, 1, 5))
+    span_orange = Span(Loc(file, 1, 9), Loc(file, 1, 15))
+    diagnostic = MyDiagnostic(span_apple, "a")
+    diagnostic.add_sub_diagnostic(MySubDiagnostic(span_orange, "p"))
+    run_test(source, diagnostic, snapshot, request)
+
+
 def test_long_label(snapshot, request):
     @dataclass(frozen=True)
     class MyDiagnostic(Error):