import old posts

content/posts/custom-targets-in-rust.md

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+date = "2020-12-29"
+draft = false
+path = "/blog/custom-targets-in-rust"
+tags = ["rust", "embedded"]
+title = "Custom targets in Rust"
++++
+
+I'm going to try a new style of blog post for this one, more of a lab notebook
+style. Expect normal posts for more interesting topics than fixing build
+systems.
+
+## ISSUE: workspaces interact very poorly with targets
+
+### BUGS:
+
+* https://github.com/rust-lang/cargo/issues/7004
+
+You can't make Cargo use a separate target per workspace crate. Thus, workspace
+wide builds like `cargo b --workspace` from the workspace root basically don't
+work. Thus, put `default-members = []` in the virtual manifest to stop those
+doing anything at all.
+
+Workspace wide documentation also doesn't work, so use `-p PACKAGENAME` with
+`cargo doc` which will document `PACKAGENAME` and all its dependencies,
+including transitive dependencies. This is very likely to actually build if the
+normal build works.
+
+## ISSUE: `RUST_TARGET_PATH`
+
+### BUGS
+
+* https://stackoverflow.com/questions/48040146/error-loading-target-specification-could-not-find-specification-for-target
+
+* https://github.com/japaric/xargo/issues/44
+
+* https://github.com/rust-lang/cargo/issues/4905
+
+This one was actually,,, attempted to be fixed, but the `xargo` PR got lost for
+a year in 2018 and got abandoned.
+
+Thus you should use `cargo -Z build-std` instead of any of `xargo` or
+`cargo-xbuild`, per the instructions in the [cargo-xbuild
+README](https://docs.rs/crate/cargo-xbuild/0.6.4).
+
+In particular, to not use `xargo` and not have to use `RUST_TARGET_PATH`:
+
+```text
+cargo build -Z build-std=core,compiler_builtins -Z build-std-features=compiler-builtins-mem --release --target ../riscv64imac-mu-kern-elf.json
+```
+
+## ISSUE: documentation for target spec files
+
+[This page wrongly suggests `xargo` which is outdated.](https://doc.rust-lang.org/stable/rustc/targets/custom.html)
+
+Some of the options are documented on
+[codegen-options](https://doc.rust-lang.org/stable/rustc/codegen-options/index.html)
+but not really,
+
+I just stole most of mine out of
+[SunriseOS](https://github.com/sunriseos/SunriseOS/blob/master/i386-unknown-none.json).
+
+There's some WEIRD caching going on with this, and you probably want to wipe
+`target` for each build while messing with this file.
+
+## ISSUE: how do you even get a target spec file?
+
+```text
+# Get a starting point for a target spec
+rustc +nightly -Zunstable-options --print target-spec-json --target riscv64imac-unknown-none-elf > ../riscv64imac-unknown-mukern-elf.json
+
+# Check if the target spec is round tripping properly
+RUST_TARGET_PATH=$(realpath ..) rustc --target riscv64imac-unknown-mukern-elf -Z unstable-options --print target-spec-json
+```
+
+## ISSUE: debugging `cargo check` issues in rust-analyzer
+
+Put this incantation in a shell startup file such that it ends up in your RA
+process's environment (🙃). I wish it was configurable in VSCode somewhere.
+
+```
+export RA_LOG='rust_analyzer=info,salsa=error,flycheck=debug'
+```
+
+The RA logs have useful config information, the `salsa` logs are extremely
+spammy, and `flycheck` is the `cargo check` module. [Its docs are
+here](https://rust-analyzer.github.io/rust-analyzer/flycheck/index.html).