Option to generate a dot file representing the smir-json (#34)

In this first version we don't draw the statements of blocks, just the
block structure and caller/callee relations between blocks/items.

In terms of interface, the `main` program now has options `--json` and
`--dot` which have to come first, and we default to `--json` if they are
absent.

Output example: (main calls `foo(0)` calls `bar(0)`)

![image](https://github.com/user-attachments/assets/114bb9cb-e21a-4fd2-af4a-e2f8f0c87c1c)

The HS version had more color but was otherwise the same:

![image](https://github.com/user-attachments/assets/c391e9a7-4129-4702-a3c2-feb8a896ed78)

---------

Co-authored-by: Daniel Cumming <124537596+dkcumming@users.noreply.github.com>

Author: jberthold

Cargo.lock

@@ -7,6 +7,7 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+dot-writer = "0.1.4"
 tracing = "0.1"
 # serde = { version = "=1.0.202", features = ["derive"] }
 # serde_cbor = "0.11"

Cargo.toml

@@ -39,6 +39,14 @@ To generate stable MIR output without building a binary, you can invoke the tool
 ./run.sh -Z no-codegen <crate_root>
 ```
 
+There is experimental support for rendering the Stable-MIR items and their basic blocks as a 
+call graph in graphviz' dot format. 
+
+To produce a dot file `*.smir.dot` (instead of `*.smir.json`), one can invoke the driver with
+_first_ argument `--dot`. When using `--json` as the first argument, the `*.smir.json` file
+will be written. Any other strings given as first argument will be passed to the compiler 
+(like all subsequent arguments).
+
 There are a few environment variables that can be set to control the tools output:
 
 1.  `LINK_ITEMS` - add entries to the link-time `functions` map for each monomorphic item in the crate;

README.md

@@ -1,5 +1,6 @@
 #![feature(rustc_private)]
 pub mod driver;
+pub mod mk_graph;
 pub mod printer;
 pub use driver::stable_mir_driver;
 pub use printer::*;

src/lib.rs

@@ -4,8 +4,23 @@ pub mod driver;
 pub mod printer;
 use driver::stable_mir_driver;
 use printer::emit_smir;
+use smir_pretty::mk_graph::emit_dotfile;
 
 fn main() {
-    let args: Vec<_> = env::args().collect();
-    stable_mir_driver(&args, emit_smir)
+    let mut args: Vec<String> = env::args().collect();
+
+    match args.get(1) {
+        None =>
+            stable_mir_driver(&args, emit_smir), // backward compatibility
+        Some(arg) if arg == "--json" => {
+            args.remove(1);
+            stable_mir_driver(&args, emit_smir)
+        }
+        Some(arg) if arg == "--dot" => {
+            args.remove(1);
+            stable_mir_driver(&args, emit_dotfile)
+        }
+        Some(_other) =>
+            stable_mir_driver(&args, emit_smir), // backward compatibility
+    }
 }