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Diagnostic rendering was part of the data model: `Loc::FormatLocation` and `Loc::FormatSnippet` wrote plain text to a stream, so no capability, color, or terminal width could reach them. A diagnostic is now one message with labels, contexts, and location information attached, and `Diagnostics::Renderer` draws it into a `Terminal::Buffer`. The design is in `toolchain/docs/diagnostics_rendering.md` (#7668), authoring in `toolchain/docs/diagnostics.md`. Before: ``` example.carbon:6:3: error: 1 argument passed to function expecting 0 arguments Run0(1); ^~~~~~~ example.carbon:3:1: note: calling function declared here fn Run0() {} ^~~~~~~~~~~ ``` After: ``` error: 1 argument passed to function expecting 0 arguments ╭─┤ example.carbon:6:3 ┆ 3 │ fn Run0() {} · ─────┬───── · ╰──┤ calling function declared here ┆ -> 6 │ Run0(1); · ─────── ╰──────────── ``` `--no-diagnostic-snippets` gives a compact form, `example.carbon:6:3-9`. File tests use it; the rendering tests under `toolchain/diagnostics/testdata` ask for snippets. C++ interop diagnostics are data now rather than text Clang drew: `SemIR::ClangSourceLocId` names a `clang::SourceRange`, and a Clang location is marked as the point it names. `StreamConsumer` moves to a `:renderer` target that `:emitter` does not depend on, keeping `common/terminal` out of `consumer.h`. Capabilities come from the file behind the error stream, so tests get plain ASCII. `renderer_fuzzer` draws diagnostics built from fuzzer bytes with every capability combination, since a shape the renderer doesn't expect must never be why the compiler dies while reporting a problem. TODOs mark the follow-ups: line weights and crossing lines, wrapping that keeps a quoted type whole, and ordering diagnostics that share a byte offset. Also: `IncorrectExtension`'s hint moves into its message, since its note had no location of its own, and `strip_noise` in `check_diagnostics.py` blanks multi-line `#define`s instead of deleting them. Assisted-by: Claude Code