analyze: rewrite::expr: add examples to unlower and distribute doc co…

…mments

c2rust-analyze/src/rewrite/expr/distribute.rs

@@ -1,6 +1,22 @@
 //! Distributes MIR rewrites to HIR nodes.  This takes a list of MIR rewrites (from `mir_op`) and a
 //! map from MIR location to `HirId` (from `unlower`) and produces a map from `HirId` to a list of
 //! MIR rewrites.
+//!
+//! Using the example from `unlower`:
+//!
+//! ```
+//! bb0[5]: Terminator { source_info: ..., kind: _4 = f(move _5) -> [return: bb1, unwind: bb2] }
+//!   []: StoreIntoLocal, `f(y)`
+//!   [Rvalue]: Expr, `f(y)`
+//!   [Rvalue, CallArg(0)]: Expr, `y`
+//! ```
+//!
+//! A MIR rewrite on `bb0[5]` `[Rvalue, CallArg(0)]` would be attached to the MIR
+//! `Expr` `y`, and a rewrite on `bb0[5]` `[Rvalue`] would be attached to `f(y)`.
+//! A MIR rewrite on `bb0[5]` `[]` (i.e. on the call terminator itself) would
+//! result in an error, since there is no good place in the HIR to attach such a
+//! rewrite.
+
 use crate::rewrite::expr::mir_op::{self, MirRewrite, SubLoc};
 use crate::rewrite::expr::unlower::{MirOrigin, MirOriginDesc};
 use log::*;

c2rust-analyze/src/rewrite/expr/unlower.rs

@@ -1,5 +1,56 @@
 //! Builds the *unlowering map*, which maps each piece of the MIR to the HIR `Expr` that was
 //! lowered to produce it.
+//!
+//! For example:
+//!
+//! ```Rust
+//! fn f(a: i32) -> i32 {
+//!     a + 1
+//! }
+//!
+//! fn g(x: i32, y: i32) -> i32 {
+//!     x + f(y)
+//! }
+//! ```
+//!
+//! For `f`, the unlowering map annotates the MIR as follows:
+//!
+//! ```
+//! block bb0:
+//!   bb0[0]: StorageLive(_2)
+//!   bb0[1]: _2 = _1
+//!     []: StoreIntoLocal, `a`
+//!     [Rvalue]: Expr, `a`
+//!   bb0[2]: _0 = Add(move _2, const 1_i32)
+//!     []: StoreIntoLocal, `a + 1`
+//!     [Rvalue]: Expr, `a + 1`
+//!   bb0[3]: StorageDead(_2)
+//!   bb0[4]: Terminator { source_info: ..., kind: return }
+//! ```
+//!
+//! The statement `_2 = _1` is associated with the expression `a`; the statement
+//! as a whole is storing the result of evaluating `a` into a MIR local, and the
+//! statement's rvalue `_1` represents the expression `a` itself.  Similarly, `_0 =
+//! Add(move _2, const 1)` stores the result of `a + 1` into a local.  If needed,
+//! we could extend the `unlower` pass to also record that `move _2` (a.k.a. `bb0[2]`
+//! `[Rvalue, RvalueOperand(0)]`) is lowered from the `Expr` `a`.
+//!
+//! On `g`, the unlowering map includes the following (among other entries):
+//!
+//! ```
+//! bb0[5]: Terminator { source_info: ..., kind: _4 = f(move _5) -> [return: bb1, unwind: bb2] }
+//!   []: StoreIntoLocal, `f(y)`
+//!   [Rvalue]: Expr, `f(y)`
+//!   [Rvalue, CallArg(0)]: Expr, `y`
+//! bb1[1]: _0 = Add(move _3, move _4)
+//!   []: StoreIntoLocal, `x + f(y)`
+//!   [Rvalue]: Expr, `x + f(y)`
+//! ```
+//!
+//! The call terminator `_4 = f(move _5)` computes `f(y)` and stores the result
+//! into a local; its rvalue is `f(y)` itself, and the first argument of the rvalue
+//! is `y`.
+
 use crate::rewrite::build_span_index;
 use crate::rewrite::expr::mir_op::SubLoc;
 use crate::rewrite::span_index::SpanIndex;