Resolve panic in `generate_delegate_methods`
Fixes#16276
This PR addresses two issues:
1. When using `PathTransform`, it searches for the node corresponding to the `path` in the `source_scope` during `make::fn_`. Therefore, we need to perform the transform before `make::fn_` (similar to the problem in issue #15804). Otherwise, even though the tokens are the same, their offsets (i.e., `span`) differ, resulting in the error "Can't find CONST_ARG@xxx."
2. As mentioned in the first point, `PathTransform` searches for the node corresponding to the `path` in the `source_scope`. Thus, when transforming paths, we should update nodes from right to left (i.e., use **reverse of preorder** (right -> left -> root) instead of **postorder** (left -> right -> root)). Reasons are as follows:
In the red-green tree (rowan), we do not store absolute ranges but instead store the length of each node and dynamically calculate offsets (spans). Therefore, when modifying the left-side node (such as nodes are inserted or deleted), it causes all right-side nodes' spans to change. This, in turn, leads to PathTransform being unable to find nodes with the same paths (due to different spans), resulting in errors.
fix: Fix `ast::Path::segments` implementation
calling `ast::Path::segments` on a qualifier currently returns all the segments of the top path instead of just the segments of the qualifier.
The issue can be summarized by the simple failing test below:
```rust
#[test]
fn path_segments() {
//use ra_ap_syntax::ast;
let path: ast::Path = ...; // e.g. `ast::Path` for "foo::bar::item".
let path_segments: Vec<_> = path.segments().collect();
let qualifier_segments: Vec<_> = path.qualifier().unwrap().segments().collect();
assert_eq!(path_segments.len(), qualifier_segments.len() + 1); // Fails because `LHS = RHS`.
}
```
This PR:
- Fixes the implementation of `ast::Path::segments`
- Fixes `ast::Path::segments` callers that either implicitly relied on behavior of previous implementation or exhibited other "wrong" behavior directly related to the result of `ast::Path::segments` (all callers have been reviewed, only one required modification)
- Removes unnecessary (and now unused) `ast::Path::segments` alternatives
fix: Differentiate between vfs config load and file changed events
Kind of fixes https://github.com/rust-lang/rust-analyzer/issues/14730 in a pretty bad way. We need to rethink the vfs-notify layer entirely. For a decent fix.
