bug: `forge flatten` should ignore intermediary files for imports

[kopy-kat]

Component

Forge

Have you ensured that all of these are up to date?

• Foundry
• Foundryup

What version of Foundry are you on?

0.2.0

What command(s) is the bug in?

forge flatten

Operating System

macOS (Apple Silicon)

Describe the bug

In more complex codebases, you might want to import objects from files other than the file in which an object is originally defined, in order to make the codebase easier to read. A simplified example:

File A:

import {Test} from "./Intermediary";

File Intermediary:

import {Test} from "./Test";
import {Test2} from "./Test2";

File Test:

contract Test {}

However, when running forge flatten (and when using --verify), all files are treated equally, even when they are only used as "intermediary files". This means that the flattened version of file A will also include Test2, which is never imported into A.

Proposed solution

The flattener should determine whether a file is used only as an intermediary and ignore it if it is, jumping to the file from which the used import is exported from.

[mattsse]

isn't this solveable by not importing import {Test2} from "./Test2"; in Intermediary?

[kopy-kat]

No, the point is to have intermediary files from which you can selectively import in order to make the code more readable.

One analogy is index.js files in js which can remap different exported functions or objects from a folder. For example, instead of the import path being package/api/component/function.js you can create package/index.js which declares exports to be made and then import from package. This is especially useful for sdks.

I appreciate that this is not very common in solidity codebases today but makes code much more readable imo.

[mattsse]

ah I think I get it now,

the important part is this

import {Test} from "./Intermediary";

not sure if this can be done easily/safely because I think this would require resolving reexports like detecting that Test is not used from Intermediary, but is reexported in Intermediary

wdyt @klkvr

[kopy-kat]

But Test cant be used in Intermediary right? Otherwise the compiler should throw an Identifier already declared error.

As long as named imports are used I think this should be relatively straightforward, but if a file has multiple unnamed imports then it could be complicated to figure out where a specific object is coming from.

Another edge case would be aliased imports but this should also be relatively straightforward to handle

[klkvr]

Yeah, we can keep track of all file-level identifiers locations and exclude files which don't have any declarations used by flattening target or its dependencies.

We can think about this as a broader task on minimizing flattener output

The idea is to keep track of all file-level declarations (contracts, structs, functions, etc) and find which of them are actually used by flattening target or its dependencies by building a graph.

Then we can only write needed items to the resulted flattened file.

That way we can not only filter out "intermediary" files like in this example, but also any unused items from any of the files

[kopy-kat]

hey guys, any update on this? I also believe this is the same as foundry-rs/compilers#253