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README.md · 105 lines · 4.3 KBmarkdown Blame HistoryRaw
Add a README b66da6b nandithebull 7h ago1# rustnim
2
3A Rust → Nim transpiler. It parses Rust with `syn` and emits Nim source that
4behaves the same way — same stdout, same exit status — when both are built and
5run.
6
7The rule the whole thing is organised around: **never approximate a semantic
8you cannot represent.** If a construct doesn't map, `rustnim` says so and exits
9non-zero. It never writes a file it can't stand behind.
10
11That rule comes from a real failure. We tried an existing transpiler that
12advertises Rust support on the `base16ct` crate; it emitted an empty file and
13exited 0. The write-up is in [`findings/`](findings/).
14
15## Build and run
16
17```bash
18cargo build --release
19./target/release/rustnim input.rs -o out.nim
20nim c out.nim
21```
22
23Several inputs are concatenated into one Nim module, in the order given — Nim
24has no per-file `mod`, so items are flattened and names must not collide:
25
26```bash
27rustnim src/lib.rs src/error.rs --cfg feature=alloc -o crate.nim
28```
29
30## What works
31
32Functions and `impl` methods, structs, enums (C-like and data-carrying),
33`Option`/`Result` with `?`, `let`/`let mut`, the integer and float operators at
34exact widths, `as` casts, `if`/`while`/`loop`/`for`, `match` including binding
35patterns, `Vec`/slices/arrays, type aliases, function-typed parameters
36(`impl Fn(A) -> B`), multi-file input, `#[cfg]`, and `println!`/`format!` with
37`{}`, `{:?}`, `{:x}`, `{:b}`, positional and inline-named arguments, and
38zero/space padding.
39
Correct the README where trait impls and iterators are listed as rejected 428f374 nandithebull 6h ago40Also: the formatting trait impls (`Display`, `Debug`, `LowerHex`, `UpperHex`,
41`Binary`, `Octal`) and `From`; and slice iterators — `iter`, `iter_mut`,
42`enumerate`, `zip`, `chunks_exact`, `chunks_exact_mut`, `windows` — resolved
43into a single index loop where each binding is an lvalue into the original
44container, so `*d = v` through `iter_mut()` reaches the caller's slice.
45Borrowed slices are views, not copies, including as return types.
46
Add closures and unsafe; base16ct's lower.rs and upper.rs go through 0a375d8 nandithebull 6h ago47Closures and `unsafe` blocks, and `&str` as a borrowed view rather than an
48owned copy. Modules: pass the crate root first and each further file after it,
49and items are scoped by module, so `lower::decode` and `mixed::decode` stay
50distinct.
51
Correct the README where trait impls and iterators are listed as rejected 428f374 nandithebull 6h ago52Rejected with a reason, rather than guessed at: `i128`/`u128`, generics,
Add closures and unsafe; base16ct's lower.rs and upper.rs go through 0a375d8 nandithebull 6h ago53`move` closures, trait impls other than the ones above, iterator adaptors with
54no index-loop equivalent (`map`, `filter`, `take_while`), float→int casts, and
55any standard-library method that isn't mapped.
Correct the README where trait impls and iterators are listed as rejected 428f374 nandithebull 6h ago56
57## base16ct
58
Add closures and unsafe; base16ct's lower.rs and upper.rs go through 0a375d8 nandithebull 6h ago59`tests/cases/026-base16ct-crate/` transpiles four of base16ct 1.0.0's six
60modules — `error.rs`, `lower.rs`, `upper.rs`, `mixed.rs` — byte-for-byte as
61published on crates.io, plus `lib.rs`'s `decoded_len`, `encoded_len` and
62`decode_inner` verbatim. Decoding and encoding are byte-identical to rustc's,
63including `encode_str`, which is a closure over an `unsafe` block returning a
64borrowed `&str` view of the bytes just written. That is the crate the
65transpiler in [`findings/`](findings/) emitted an empty file for.
Correct the README where trait impls and iterators are listed as rejected 428f374 nandithebull 6h ago66
Add closures and unsafe; base16ct's lower.rs and upper.rs go through 0a375d8 nandithebull 6h ago67`display.rs` and the `alloc` half are not through yet;
68[`DESIGN.md`](DESIGN.md) says exactly what each still needs.
Add a README b66da6b nandithebull 7h ago69
70## Tests
71
72```bash
73cargo test
74```
75
76The tests are differential, with rustc as the oracle. Each case in
77`tests/cases/` is compiled by rustc and run, transpiled and compiled by Nim and
78run, and the two stdouts and exit statuses must match. Building anything less
79than that — checking the Nim output "looks right", say — is how you end up
80shipping the bug in `findings/`. `rustnim` exiting 0 with a missing or empty
81output file is a hard failure, and so is an empty corpus.
82
83rustc is invoked without `-O` so debug-profile overflow checks are on, which is
84the matching pair for `nim c`'s defaults.
85
86Nim is found at `.nim-toolchain/bin/nim` in the repo root or any parent, or via
87`RUSTNIM_NIM`. Run a single case with:
88
89```bash
90RUSTNIM_CASE=005 cargo test --test differential -- --nocapture
91```
92
93## Layout
94
95| file | role |
96|---|---|
97| `src/ty.rs` | Rust type → Nim type, exact widths, explicit rejections |
98| `src/lower.rs` | items, statements, expressions → Nim |
99| `src/fmt.rs` | `println!`/`format!` format strings |
100| `src/prelude.nim` | `Option`/`Result`, panic, `Display`/`Debug` runtime |
101| `tests/differential.rs` | the runner above |
102
103[`DESIGN.md`](DESIGN.md) has the reasoning: how each construct is mapped, what
104was settled by running both compilers rather than by reading docs, and what's
105still open.