Add some documentation

src/config.rs

@@ -5,10 +5,14 @@ fn bool_env_var(key: &str) -> bool {
     env::var(key).as_ref().map(|val| &**val) == Ok("1")
 }
 
+/// The mode to use for compilation.
 #[derive(Copy, Clone, Debug)]
 pub enum CodegenMode {
+    /// AOT compile the crate. This is the default.
     Aot,
+    /// JIT compile and execute the crate.
     Jit,
+    /// JIT compile and execute the crate, but only compile functions the first time they are used.
     JitLazy,
 }
 
@@ -25,6 +29,7 @@ impl FromStr for CodegenMode {
     }
 }
 
+/// Configuration of cg_clif as passed in through `-Cllvm-args` and various env vars.
 #[derive(Clone, Debug)]
 pub struct BackendConfig {
     /// Should the crate be AOT compiled or JIT executed.
@@ -76,6 +81,7 @@ impl Default for BackendConfig {
 }
 
 impl BackendConfig {
+    /// Parse the configuration passed in using `-Cllvm-args`.
     pub fn from_opts(opts: &[String]) -> Result<Self, String> {
         fn parse_bool(name: &str, value: &str) -> Result<bool, String> {
             value.parse().map_err(|_| format!("failed to parse value `{}` for {}", value, name))

src/lib.rs

@@ -119,6 +119,8 @@ impl<F: Fn() -> String> Drop for PrintOnPanic<F> {
     }
 }
 
+/// The codegen context holds any information shared between the codegen of individual functions
+/// inside a single codegen unit with the exception of the Cranelift [`Module`](cranelift_module::Module).
 struct CodegenCx<'tcx> {
     tcx: TyCtxt<'tcx>,
     global_asm: String,

src/metadata.rs

@@ -14,6 +14,18 @@ use rustc_target::spec::Target;
 
 use crate::backend::WriteMetadata;
 
+/// The metadata loader used by cg_clif.
+///
+/// The metadata is stored in the same format as cg_llvm.
+///
+/// # Metadata location
+///
+/// <dl>
+/// <dt>rlib</dt>
+/// <dd>The metadata can be found in the `lib.rmeta` file inside of the ar archive.</dd>
+/// <dt>dylib</dt>
+/// <dd>The metadata can be found in the `.rustc` section of the shared library.</dd>
+/// </dl>
 pub(crate) struct CraneliftMetadataLoader;
 
 fn load_metadata_with(