Add support for building programs completely from source. (#24)

And add an example showing how to do this.

Cargo.toml

@@ -13,7 +13,7 @@ exclude = ["/.github", "ci"]
 keywords = ["linux"]
 
 [dependencies]
-c-gull = { version = "0.15.19", default-features = false, features = ["eyra"] }
+c-gull = { version = "0.15.20", default-features = false, features = ["eyra"] }
 
 [dev-dependencies]
 assert_cmd = "2.0.12"
@@ -33,7 +33,7 @@ which = "5.0.0"
 core_simd = { git = "https://github.com/rust-lang/portable-simd" }
 
 [features]
-default = ["be-std", "threadsafe-setenv"]
+default = ["be-std", "threadsafe-setenv", "use-compiler-builtins"]
 
 # This makes `setenv` and friends thread-safe by leaking memory.
 threadsafe-setenv = ["c-gull/threadsafe-setenv"]
@@ -57,5 +57,8 @@ experimental-relocate = ["c-gull/experimental-relocate"]
 # Have eyra do `use std::*;` so that it can be used as `std`.
 be-std = []
 
+# Should c-scape's `memcpy` etc. use compiler-builtins?
+use-compiler-builtins = ["c-gull/use-compiler-builtins"]
+
 # Enable `todo!()` stubs for unimplemented functions.
 todo = ["c-gull/todo"]

README.md

@@ -54,11 +54,13 @@ Other examples include
  - [enabling LTO],
  - [using min-sized-rust technique to produce small binaries], and
  - [adding Eyra as an optional dependency].
+ - [building a program completely from source].
 
 [this hello world example]: https://github.com/sunfishcode/eyra/tree/main/example-crates/hello-world#readme
 [enabling LTO]: https://github.com/sunfishcode/eyra/tree/main/example-crates/hello-world-lto#readme
 [using min-sized-rust technique to produce small binaries]: https://github.com/sunfishcode/eyra/tree/main/example-crates/hello-world-small#readme
 [adding Eyra as an optional dependency]: https://github.com/sunfishcode/eyra/tree/main/example-crates/eyra-optional-example#readme
+[building a program completely from source]: https://github.com/sunfishcode/eyra/tree/main/example-crates/all-from-source#readme
 
 ## Why?
 
@@ -76,8 +78,8 @@ Why use Eyra?
    For even more code-size reductions, see the techniques in
    [the hello-world-small example].
 
- - Support for compiling programs with alternate calling conventions, with
-   [Eyra and `-Zbuild-std`].
+ - Support for compiling programs with alternate calling conventions, using
+   [Eyra and `-Zbuild-std`] to [build a program completely from source].
 
  - [Fully static linking] that supports the platform NSS/DNS config. "Is such
    a thing even possible?", "Yes it is."
@@ -249,3 +251,4 @@ compatibility layer can use the underlying crates directly.
 [c-ward]: https://github.com/sunfishcode/c-ward#readme
 [rustix]: https://github.com/bytecodealliance/rustix#readme
 [supported by Rust]: https://doc.rust-lang.org/nightly/rustc/platform-support.html
+[build a program completely from source]: https://github.com/sunfishcode/eyra/tree/main/example-crates/all-from-source#readme

example-crates/all-from-source/.gitignore

@@ -0,0 +1,2 @@
+/target
+Cargo.lock

example-crates/all-from-source/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "all-from-source"
+version = "0.0.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+# Disable the "use-compiler-builtins" feature, to ensure that we build
+# `memcpy` etc. from source instead of using the prebuilt library.
+eyra = { path = "../..", default-features = false, features = ["be-std", "threadsafe-setenv"] }
+
+[profile.release]
+lto = true
+codegen-units = 1
+
+# This is just an example crate, and not part of the c-ward workspace.
+[workspace]

example-crates/all-from-source/README.md

@@ -0,0 +1,12 @@
+This example demonstrates building a program completely from source, without
+using any prebuilt libraries, if built with `-Zbuild-std`.
+
+For example, use `cargo rustc` to build the LLVM IR file and test that it has
+no external function declarations (other than LLVM intrinsics).
+
+```
+$ cargo +nightly rustc -Zbuild-std --release --target=x86_64-unknown-linux-gnu  -- --emit=llvm-ir
+[...]
+$ grep ^declare ./target/x86_64-unknown-linux-gnu/release/deps/all_from_source-*.ll | grep -v '@llvm'
+$
+``` 

example-crates/all-from-source/build.rs

@@ -0,0 +1,4 @@
+fn main() {
+    // Pass -nostartfiles to the linker.
+    println!("cargo:rustc-link-arg=-nostartfiles");
+}

example-crates/all-from-source/src/main.rs

@@ -0,0 +1,5 @@
+extern crate eyra;
+
+fn main() {
+    println!("Hello, world!");
+}