Is there any way to fold/collapse Rust documentation comments in Visual Studio Code?

I was concerned by this as well, so I looked around. The good surprise is that since March 2018, it is possible to implement this thanks to a new folding provider API.

I did a little POC (the code is horrible and not secure, it is not intended to be used as is):

class CommentProvider implements vscode.FoldingRangeProvider {
    // This method must return a list of the foldable ranges
    provideFoldingRanges(document: vscode.TextDocument, context: vscode.FoldingContext, token: vscode.CancellationToken): vscode.ProviderResult<vscode.FoldingRange[]> {
        let ret: vscode.FoldingRange[] = [];
        for (let i = 0; i < document.lineCount; ++i) {
            let line = document.lineAt(i).text.trim();
            if (line.startsWith("//!")) {
                let from = i;
                do {
                    ++i;
                } while (document.lineAt(i).text.trim().startsWith("//!"));
                ret.push(new vscode.FoldingRange(from, i - 1));
            }
        }
        return ret;
    }
}

export function activate(context: vscode.ExtensionContext) {
    let sel: vscode.DocumentSelector = { scheme: 'file', language: 'rust' };
    let pro = new CommentProvider();
    vscode.languages.registerFoldingRangeProvider(sel, pro);
}

and it must be registered correctly in the package.json:

"activationEvents": [
    "onLanguage:rust"
],
"contributes": {
    "languages": [
        {
            "id": "rust",
            "aliases": [
                "rs"
            ],
            "extensions": [
                "rs"
            ],
            "configuration": "./language-configuration.json"
        }
    ]
},

I am not sure about the values, though.

The best would be to ask to the team doing the official rust extension to integrate this.