Merge pull request #21732 from Wilfred/add_claude_rules

internal: Define rules for LLMs

Author: Veykril

AGENTS.md

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+CLAUDE.md

CLAUDE.md

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+**Reminder: All AI usage must be disclosed in commit messages, see
+CONTRIBUTING.md for more details.**
+
+## Build Commands
+
+```bash
+cargo build                    # Build all crates
+cargo test                     # Run all tests
+cargo test -p <crate>          # Run tests for a specific crate (e.g., cargo test -p hir-ty)
+cargo lint                     # Run clippy on all targets
+cargo xtask codegen            # Run code generation
+cargo xtask tidy               # Run tidy checks
+UPDATE_EXPECT=1 cargo test     # Update test expectations (snapshot tests)
+RUN_SLOW_TESTS=1 cargo test    # Run heavy/slow tests
+```
+
+## Key Architectural Invariants
+
+- Typing in a function body never invalidates global derived data
+- Parser/syntax tree is built per-file to enable parallel parsing
+- The server is stateless (HTTP-like); context must be re-created from request parameters
+- Cancellation uses salsa's cancellation mechanism; computations panic with a `Cancelled` payload
+
+### Code Generation
+
+Generated code is committed to the repo. Grammar and AST are generated from `ungrammar`. Run `cargo test -p xtask` after adding inline parser tests (`// test test_name` comments).
+
+## Testing
+
+Tests are snapshot-based using `expect-test`. Test fixtures use a mini-language:
+- `$0` marks cursor position
+- `// ^^^^` labels attach to the line above
+- `//- minicore: sized, fn` includes parts of minicore (minimal core library)
+- `//- /path/to/file.rs crate:name deps:dep1,dep2` declares files/crates
+
+## Style Notes
+
+- Use `stdx::never!` and `stdx::always!` instead of `assert!` for recoverable invariants
+- Use `T![fn]` macro instead of `SyntaxKind::FN_KW`
+- Use keyword name mangling over underscore prefixing for identifiers: `crate` → `krate`, `fn` → `func`, `struct` → `strukt`, `type` → `ty`