10440: Fix Clippy warnings and replace some `if let`s with `match` r=Veykril a=arzg
I decided to try fixing a bunch of Clippy warnings. I am aware of this project’s opinion of Clippy (I have read both [rust-lang/clippy#5537](https://github.com/rust-lang/rust-clippy/issues/5537) and [rust-analyzer/rowan#57 (comment)](https://github.com/rust-analyzer/rowan/pull/57#discussion_r415676159)), so I totally understand if part of or the entirety of this PR is rejected. In particular, I can see how the semicolons and `if let` vs `match` commits provide comparatively little benefit when compared to the ensuing churn.
I tried to separate each kind of change into its own commit to make it easier to discard certain changes. I also only applied Clippy suggestions where I thought they provided a definite improvement to the code (apart from semicolons, which is IMO more of a formatting/consistency question than a linting question). In the end I accumulated a list of 28 Clippy lints I ignored entirely.
Sidenote: I should really have asked about this on Zulip before going through all 1,555 `if let`s in the codebase to decide which ones definitely look better as `match` :P
Co-authored-by: Aramis Razzaghipour <aramisnoah@gmail.com>
Consider these expples
{ 92 }
async { 92 }
'a: { 92 }
#[a] { 92 }
Previously the tree for them were
BLOCK_EXPR
{ ... }
EFFECT_EXPR
async
BLOCK_EXPR
{ ... }
EFFECT_EXPR
'a:
BLOCK_EXPR
{ ... }
BLOCK_EXPR
#[a]
{ ... }
As you see, it gets progressively worse :) The last two items are
especially odd. The last one even violates the balanced curleys
invariant we have (#10357) The new approach is to say that the stuff in
`{}` is stmt_list, and the block is stmt_list + optional modifiers
BLOCK_EXPR
STMT_LIST
{ ... }
BLOCK_EXPR
async
STMT_LIST
{ ... }
BLOCK_EXPR
'a:
STMT_LIST
{ ... }
BLOCK_EXPR
#[a]
STMT_LIST
{ ... }
Previously we swapped to events in the buffer, but that might be wrong
if there aer `forward_parent` links pointing to the swapped-out node.
Let's do the same via parent links instead, keeping the nodes in place
The code here is intentionally dense and does exactly what is written.
Explaining semantic difference between Rust 2015 and 2018 doesn't help
with understanding syntax. Better to just add more targeted tests.
FragmentKind played two roles:
* entry point to the parser
* syntactic category of a macro call
These are different use-cases, and warrant different types. For example,
macro can't expand to visibility, but we have such fragment today.
This PR introduces `ExpandsTo` enum to separate this two use-cases.
I suspect we might further split `FragmentKind` into `$x:specifier` enum
specific to MBE, and a general parser entry point, but that's for
another PR!
