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Rollup merge of #77934 - XAMPPRocky:codegen-backend-docs, r=jonas-schievink

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Document -Z codegen-backend in the unstable book

### [Rendered](https://github.com/XAMPPRocky/rust/blob/codegen-backend-docs/src/doc/unstable-book/src/compiler-flags/codegen-backend.md)

Companion PR to #77933 tracking issue.

cc @bjorn3
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Yuki Okushi 2020-10-15 07:32:36 +09:00 committed by GitHub
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# `codegen-backend`
The tracking issue for this feature is: [#77933](https://github.com/rust-lang/rust/issues/77933).
------------------------
This feature allows you to specify a path to a dynamic library to use as rustc's
code generation backend at runtime.
Set the `-Zcodegen-backend=<path>` compiler flag to specify the location of the
backend. The library must be of crate type `dylib` and must contain a function
named `__rustc_codegen_backend` with a signature of `fn() -> Box<dyn rustc_codegen_ssa::traits::CodegenBackend>`.
## Example
See also the [`hotplug_codegen_backend`](https://github.com/rust-lang/rust/tree/master/src/test/run-make-fulldeps/hotplug_codegen_backend) test
for a full example.
```rust,ignore
use rustc_codegen_ssa::traits::CodegenBackend;
struct MyBackend;
impl CodegenBackend for MyBackend {
// Implement codegen methods
}
#[no_mangle]
pub fn __rustc_codegen_backend() -> Box<dyn CodegenBackend> {
Box::new(MyBackend)
}
```