/* ... */
}

struct Builder<'a, 'll, 'tcx> {
  cx: &'a CodegenCx<'ll, 'tcx>,
  /* ... */
}
```

`CodegenCx` is used to compile one codegen-unit that can contain multiple
functions, whereas `Builder` is created to compile one basic block.

The code in `rustc_codegen_llvm` has to deal with multiple explicit lifetime
parameters, that correspond to the following:
* `'tcx` is the longest lifetime, that corresponds to the original `TyCtxt`
  containing the program's information;
* `'a` is a short-lived reference of a `CodegenCx` or another object inside a
  struct;
* `'ll` is the lifetime of references to LLVM objects such as `Value` or
  `Type`.

Although there are already many lifetime parameters in the code, making it
generic uncovered situations where the borrow-checker was passing only due to
the special nature of the LLVM objects manipulated (they are extern pointers).
For instance, an additional lifetime parameter had to be added to
`LocalAnalyser` in `analyse.rs`, leading to the definition:

```rust,ignore
struct LocalAnalyzer<'mir, 'a, 'tcx> {
  /* ... */
}
```

However, the two most important structures `CodegenCx` and `Builder` are not
defined in the backend-agnostic code. Indeed, their content is highly specific
of the backend and it makes more sense to leave their definition to the backend
implementor than to allow just a narrow spot via a generic field for the
backend's context.

### Traits and interface

Because they have to be defined by the backend, `CodegenCx` and `Builder` will
be the structures implementing all the traits defining the backend's interface.
These traits are defined in the folder `rustc_codegen_ssa/traits` and all the
backend-agnostic code is parametrized by them. For instance, let us explain how
a function in `base.rs` is parametrized:

```rust,ignore
pub fn codegen_instance<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
    cx: &'a Bx::CodegenCx,
    instance: Instance<'tcx>
) {
    /* ... */
}
```

In this signature, we have the two lifetime parameters explained earlier and
the master type `Bx` which satisfies the trait `BuilderMethods` corresponding
to the interface satisfied by the `Builder` struct. The `BuilderMethods`
defines an associated type `Bx::CodegenCx` that itself satisfies the
`CodegenMethods` traits implemented by the struct `CodegenCx`.

On the trait side, here is an example with part of the definition of
`BuilderMethods` in `traits/builder.rs`:

```rust,ignore
pub trait BuilderMethods<'a, 'tcx>:
    HasCodegen<'tcx>
    + DebugInfoBuilderMethods<'tcx>
    + ArgTypeMethods<'tcx>
    + AbiBuilderMethods<'tcx>
    + IntrinsicCallMethods<'tcx>
    + AsmBuilderMethods<'tcx>
{
    fn new_block<'b>(
        cx: &'a Self::CodegenCx,
        llfn: Self::Function,