Basic Description

This PR implements "postfix" adjustment hints:

They are identical to normal adjustment hints, but are rendered after the expression. E.g. expr.* instead of *expr. This mirrors "postfix deref" feature that I'm planning to eventually propose to the compiler.

Motivation

The advantage of being postfix is that you need to add parentheses less often:

This is because a lot of "reborrow" hints are caused by field access or method calls, both of which are postfix and have higher "precedence" than prefix & and *.

Also IMHO it just looks nicer and it's more clear what is happening (order of operations).

Modes

However, there are some cases where postfix hints need parentheses but prefix don't (for example &x being turned into (&x).*.*.& or &**&x).

This PR allows users to choose which look they like more. There are 4 options (rust-analyzer.inlayHints.expressionAdjustmentHints.mode setting):

• prefix — always use prefix hints (default, what was used before that PR)
• postfix — always use postfix hints
• prefer_prefix — try to minimize number of parentheses, breaking ties in favor of prefix
• prefer_postfix — try to minimize number of parentheses, breaking ties in favor of postfix

Comparison of all modes:

Edge cases

Where are some rare cases where chain hints weirdly interact with adjustment hints, for example (note SourceAnalyzer.&):

This is pre-existing, you can get the same effect with prefix hints (SourceAnalyzer)).

Another weird thing is this:

Here .& is a hint and ? is written in the source code. It looks like ? is part of the hint because ?. is ligature in my font. IMO this is a bug in vscode, but still worth mentioning (I'm also too lazy to report it there...).

Fixed bugs

I've used the "needs parens" API and this accidentally fixed a bug with parens around as, see the test diff:

     let _: *const u32  = &mut 0u32 as *mut u32;
                        //^^^^^^^^^^^^^^^^^^^^^<mut-ptr-to-const-ptr>
+                       //^^^^^^^^^^^^^^^^^^^^^(
+                       //^^^^^^^^^^^^^^^^^^^^^)
...
     let _: *const u32  = &mut 0u32 as *mut u32;
                        //^^^^^^^^^^^^^^^^^^^^^<mut-ptr-to-const-ptr>
+                       //^^^^^^^^^^^^^^^^^^^^^(
+                       //^^^^^^^^^^^^^^^^^^^^^)

Changelog

changelog feature Add an option to make adjustment hints (aka reborrow hints) postfix
changelog fix Fix placement of parentheses around as casts for adjustment hints