• VSCode Version: Code 1.15.1 (41abd21, 2017-08-16T18:05:38.342Z)
• OS Version: Windows_NT ia32 10.0.15063
• Extensions:

initially found with VSCode Version: Code - Insiders 1.16.0-insider (aa42e6e, 2017-09-01T15:40:17.463Z)

workspace.asRelativePath returns unexpected values in two ways:

workspace.asRelativePath(workspace.rootPath) does not return '' or './'

ok, so the documentation says

...or when the path * is not a child of that folder, the input is returned.

I'll give that part of it a pass, as the root is technically not a child.

It does, however, return a modified string, instead of the input: the input's drive backslash is returned as a forward slash.

const wksRoot = workspace.rootPath;  // e:\foo
const rel = workspace.asRelativePath(wksRoot) // e:/foo

This is not specific to the root, however. Any \ gets converted to /

const rel = workspace.asRelativePath('D:\\BAR\\FOO') // d:/bar/foo

expected behavior is to actually return the input as specified, so the following code would be valid:

const rel = workspace.asRelativePath(testPath);
if (testPath === rel) { // do non-relative path things }

having to do a separate test for (testPath === workspace.rootPath) is not ideal, either. How is this intended to be handled for the multi-root feature? Will it still return the testPath?
Perhaps now would be a great time to make this feature more usable, and create a better function that clarifies and makes more usable the different use cases, rather than the current solution of having asRelativePath report different things depending on the project setup (yuk!) and environment

suggested api:

export interface IRelativePathResult {
  relativePath: string,
  foundRoot: string
}
export function getRelativePath(pathOrUri: string | Uri): IRelativePathResult;

result is null for path not related
result.relativePath is '' if the input was a root path, or the full relative path
result.foundRoot is the closest root (or the input if exact match)

ideally:

if(typeof inputPath === 'string') {
  assert (result.foundRoot + result.relativePath === inputPath);
}

but that could be a stretch goal