Rust AST Diff

About this tool

Rust AST Diff compares two Rust source snippets after parsing them into an abstract syntax tree and printing that tree in one canonical style. That means line wrapping, indentation, blank lines, and comments do not create noisy diff hunks. If the two snippets are the same program with different formatting, the tool says so; if a compiler-visible item changed, the unified diff is shown against the canonical Rust source.

Use it when reviewing generated Rust, checking whether a formatter changed behavior, or comparing examples where whitespace churn hides the actual code edit. Everything runs locally in your browser through WebAssembly.

Worked example — a real structural change:

Source A:

fn add(a: i32, b: i32) -> i32 {
    a + b
}

Source B:

fn add(left: i32, right: i32) -> i32 {
    left + right
}

With Output mode set to unified, the output is a git-style diff of the canonical forms:

--- a
+++ b
@@ -1,3 +1,3 @@
-fn add(a: i32, b: i32) -> i32 {
-    a + b
+fn add(left: i32, right: i32) -> i32 {
+    left + right
 }

If Source B were only reformatted or had a line comment added, summary mode would report formatting-only: same program, differs only in formatting/whitespace/comments.

Limits and edge cases

• This tool is intentionally Rust-only. Both inputs must parse as valid Rust source files or snippets accepted by syn::parse_file; parse errors include line and column when available.
• Formatting, whitespace, and comments are ignored because the parser discards them before canonical printing. Doc comments may become attributes in Rust's AST and can therefore appear as structural changes.
• The unified diff is produced from canonicalized lines, not original line numbers. Hunk locations refer to the canonical form.
• Large inputs are capped before building the line-diff table to avoid exhausting the browser sandbox. Diff smaller files or smaller modules if you hit the cell limit.
• context controls only unified diff hunks and is clamped to 0-100. canonical-a and canonical-b return one canonicalized source without comparing.

FAQ