Update and improve rustc_codegen_{llvm,ssa} docs

These docs were very out of date and misleading. They even said that
they codegen'd the *AST*!

For some reason, the `rustc_codegen_ssa::base` docs were exactly
identical to the `rustc_codegen_llvm::base` docs. They didn't really
make sense, because they had LLVM-specific information even though
`rustc_codegen_ssa` is supposed to be somewhat generic. So I removed
them as they were misleading.

Author: camelid

compiler/rustc_codegen_llvm/src/base.rs

@@ -1,17 +1,15 @@
-//! Codegen the completed AST to the LLVM IR.
-//!
-//! Some functions here, such as codegen_block and codegen_expr, return a value --
-//! the result of the codegen to LLVM -- while others, such as codegen_fn
-//! and mono_item, are called only for the side effect of adding a
-//! particular definition to the LLVM IR output we're producing.
+//! Codegen the MIR to the LLVM IR.
 //!
 //! Hopefully useful general knowledge about codegen:
 //!
-//! * There's no way to find out the `Ty` type of a Value. Doing so
+//! * There's no way to find out the [`Ty`] type of a [`Value`]. Doing so
 //!   would be "trying to get the eggs out of an omelette" (credit:
-//!   pcwalton). You can, instead, find out its `llvm::Type` by calling `val_ty`,
-//!   but one `llvm::Type` corresponds to many `Ty`s; for instance, `tup(int, int,
-//!   int)` and `rec(x=int, y=int, z=int)` will have the same `llvm::Type`.
+//!   pcwalton). You can, instead, find out its [`llvm::Type`] by calling [`val_ty`],
+//!   but one [`llvm::Type`] corresponds to many [`Ty`]s; for instance, `tup(int, int,
+//!   int)` and `rec(x=int, y=int, z=int)` will have the same [`llvm::Type`].
+//!
+//! [`Ty`]: rustc_middle::ty::Ty
+//! [`val_ty`]: common::val_ty
 
 use super::ModuleLlvm;
 

compiler/rustc_codegen_llvm/src/common.rs

@@ -314,6 +314,7 @@ impl ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
     }
 }
 
+/// Get the [LLVM type][Type] of a [`Value`].
 pub fn val_ty(v: &Value) -> &Type {
     unsafe { llvm::LLVMTypeOf(v) }
 }

compiler/rustc_codegen_ssa/src/base.rs

@@ -1,18 +1,3 @@
-//! Codegen the completed AST to the LLVM IR.
-//!
-//! Some functions here, such as `codegen_block` and `codegen_expr`, return a value --
-//! the result of the codegen to LLVM -- while others, such as `codegen_fn`
-//! and `mono_item`, are called only for the side effect of adding a
-//! particular definition to the LLVM IR output we're producing.
-//!
-//! Hopefully useful general knowledge about codegen:
-//!
-//! * There's no way to find out the `Ty` type of a `Value`. Doing so
-//!   would be "trying to get the eggs out of an omelette" (credit:
-//!   pcwalton). You can, instead, find out its `llvm::Type` by calling `val_ty`,
-//!   but one `llvm::Type` corresponds to many `Ty`s; for instance, `tup(int, int,
-//!   int)` and `rec(x=int, y=int, z=int)` will have the same `llvm::Type`.
-
 use crate::back::write::{
     compute_per_cgu_lto_type, start_async_codegen, submit_codegened_module_to_llvm,
     submit_post_lto_module_to_llvm, submit_pre_lto_module_to_llvm, ComputedLtoType, OngoingCodegen,