Pr/83 (#111)

Summary

This PR enhances the MIR graph visualization with richer allocation
information and adds D2 as an alternative output format.

Key changes:

- Index-first architecture: Build `AllocIndex`, `TypeIndex`, and
`GraphContext` upfront before graph traversal, enabling context-aware
rendering throughout
- **ALLOCS legend**: New yellow info node showing all `GlobalAlloc`
entries with decoded values (strings as ASCII, small integers as numeric
values)
- Enhanced constant labels: Constants now show provenance references
like `const [alloc0: Int(I32) = 42]` instead of opaque `const ?_i32`
- D2 output format (--d2 flag): Alternative to DOT with modern
diagramming, viewable in browser at https://play.d2lang.com

## Addresses
#83 

## Example

Before:
```rust
const ?_i32
```

After:
```rust
const [alloc0: Int(I32) = 42]
```

## New CLI usage

```console
cargo run -- --dot file.rs   # Graphviz DOT (existing)
cargo run -- --d2 file.rs    # D2 format (new)
```

## New Make targets
```console
make dot    # Generate all dot files
make svg    # Generate all svg files
make png    # Generate all png files
make d2      # Generate all D2 files
```

## Test plan

- [ ] Run --dot on test programs, verify ALLOCS legend appears
- [ ] Run --d2 on test programs, verify output renders in D2 playground
- [ ] Verify make integration-test passes

---------

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

Author: cds-amal

Makefile

@@ -6,7 +6,7 @@ default: build
 build:
 	cargo build ${RELEASE_FLAG}
 
-clean: rustup-clear-toolchain
+clean: rustup-clear-toolchain clean-graphs
 	cargo clean
 
 .PHONY: rustup-clear-toolchain
@@ -69,3 +69,47 @@ test-ui:
 	  exit 1; \
 	fi
 	bash tests/ui/run_ui_tests.sh "$$RUST_DIR_ROOT" "${VERBOSE}"
+
+.PHONY: dot svg png d2 clean-graphs
+
+OUTDIR_DOT=output-dot
+OUTDIR_SVG=output-svg
+OUTDIR_PNG=output-png
+OUTDIR_D2=output-d2
+
+clean-graphs:
+	@rm -rf $(OUTDIR_DOT) $(OUTDIR_SVG) $(OUTDIR_PNG) $(OUTDIR_D2)
+
+dot:
+	@mkdir -p $(OUTDIR_DOT)
+	@for rs in $(TESTDIR)/*.rs; do \
+		name=$$(basename $$rs .rs); \
+		echo "Generating $$name.smir.dot"; \
+		cargo run --release -- --dot -Zno-codegen $$rs 2>/dev/null; \
+		mv $$name.smir.dot $(OUTDIR_DOT)/ 2>/dev/null || true; \
+	done
+
+svg: dot
+	@mkdir -p $(OUTDIR_SVG)
+	@for dotfile in $(OUTDIR_DOT)/*.dot; do \
+		name=$$(basename $$dotfile .dot); \
+		echo "Converting $$name.dot -> $$name.svg"; \
+		dot -Tsvg $$dotfile -o $(OUTDIR_SVG)/$$name.svg; \
+	done
+
+png: dot
+	@mkdir -p $(OUTDIR_PNG)
+	@for dotfile in $(OUTDIR_DOT)/*.dot; do \
+		name=$$(basename $$dotfile .dot); \
+		echo "Converting $$name.dot -> $$name.png"; \
+		dot -Tpng $$dotfile -o $(OUTDIR_PNG)/$$name.png; \
+	done
+
+d2:
+	@mkdir -p $(OUTDIR_D2)
+	@for rs in $(TESTDIR)/*.rs; do \
+		name=$$(basename $$rs .rs); \
+		echo "Generating $$name.smir.d2"; \
+		cargo run --release -- --d2 -Zno-codegen $$rs 2>/dev/null; \
+		mv $$name.smir.d2 $(OUTDIR_D2)/ 2>/dev/null || true; \
+	done

README.md

@@ -30,6 +30,15 @@ _first_ argument `--dot`. When using `--json` as the first argument, the `*.smir
 will be written. Any other strings given as first argument will be passed to the compiler 
 (like all subsequent arguments).
 
+To generate visualizations for all test programs:
+
+```shell
+make dot   # Generate .dot files in output-dot/
+make svg   # Generate .svg files in output-svg/ (requires graphviz)
+make png   # Generate .png files in output-png/ (requires graphviz)
+make d2    # Generate .d2 files in output-d2/
+```
+
 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;

src/main.rs

@@ -4,7 +4,7 @@ pub mod driver;
 pub mod printer;
 use driver::stable_mir_driver;
 use printer::emit_smir;
-use stable_mir_json::mk_graph::emit_dotfile;
+use stable_mir_json::mk_graph::{emit_d2file, emit_dotfile};
 
 fn main() {
     let mut args: Vec<String> = env::args().collect();
@@ -19,6 +19,10 @@ fn main() {
             args.remove(1);
             stable_mir_driver(&args, emit_dotfile)
         }
+        Some(arg) if arg == "--d2" => {
+            args.remove(1);
+            stable_mir_driver(&args, emit_d2file)
+        }
         Some(_other) => stable_mir_driver(&args, emit_smir), // backward compatibility
     }
 }