[Kinflou]

Interpolation had two spellings. A constant needed f"page {N}", but an error's message and an assert's message interpolated in a plain "…", and f"…" there was a parse error. Now the rule is the same everywhere: only an f-string interpolates.

Grammar. A message (message = …, and an assert's second argument) is "…", printed as written, or f"…", with {path} placeholders and {{/}} escapes as for a constant. Both carry their span.

IR

• The template now keeps {{/}} escapes, and every brace in a plain message comes out escaped. A reader can always tell a literal brace from a placeholder. Before, reconstruct collapsed escapes into bare braces, the ambiguity comline-rust's message renderer worked around. Templates without literal braces are unchanged.
• New fields, all serde(default):
  • Error.message_span and Error.uninterpolated;
  • Assert.message_span and Assert.uninterpolated;
  • Validator.validate_span.

Diagnostics

• New warning validation::message-not-interpolated, the migration aid. A path written like a placeholder in a plain message, e.g. "{value.name} is empty", is printed as written. The warning sits on the message, and the help says to write f"…". A re-exported error is warned about only where it's declared.
• validator-checks-nothing with an empty validate block now points at the block and names the validator as a secondary location. Without a block it says so, and its example is now an f-string.
• The unused-property check reads placeholders with {{ skipped, so an escaped {{params.x}} doesn't count as a use.

Migration

• std's StringBounds now uses f-strings.
• Core's tests and fixtures that mean interpolation now use f"…"; expected IR templates are unchanged.
• The error and message vocabulary entries are reworded.

Tests: escape round-trips for both kinds of string, plain braces parsing, the new warning and its spans, the validate-block span, and literal braces not counting as placeholders. The full suite passes.

Follow-ups:

• comline-rust reads {{/}} in templates;
• the language server adds a quick fix that adds the f, f-string highlighting, and a VS Code </> fix;
• fixtures, docs and the cascade.