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Compiled functions #1555

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This PR introduces a proposed change of syntax from the confusing keyword phrases function method and predicate method to compiled function and compiled predicate, respectively. The PR allows both pairs of keyword phrases as synonyms, but gears up toward just accepting the compiled ... phrases in the future. In particular, documentation and all tests are changed to the new syntax, and the old syntax gives a deprecation warning under (the non-default) /deprecation:2.

In this PR, compiled is not a keyword by itself. It is only looked upon specially in the two new keyword phrases.

If accepted, this PR should be followed by an update of dafny-lang/ide-vscode to accept the new keyword, and corresponding updates in dafny-lang/libraries.

These replace the keyword phrases `function method` and `compiled predicate`, which are now marked as deprecated.
Note, in this commit, the tests still use the old syntax. However, this still causes a few changes in the expect test output, because of updated error messages and updated pretty printing.
except in `dafny4/RollYourOwnArrowType.dfy`, which will be completed in a separate commit
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Beyond reviewing the language change itself, I think the easiest way to review this PR is to look at each individual commit. Those commits are either boring (and huge) or interesting (and small).

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Just flagging that there is still some open discussion around whether we should also tweak the syntax for the function by method feature, since it is still arguably using the method keyword to indicate "when compiled", and we may want to align the concepts better.

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Also potentially blocked on #1564 - this PR is definitely an improvement, but we may want to consider deeper changes before making this change and then potentially undoing it shortly after.

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Given the discussion on #1564, I'm retracting this PR and hope to instead follow up with one based on that discussion.

RustanLeino added a commit that referenced this pull request Feb 22, 2022
As a result of discussion #1564, this PR implements a command-line option `/functionSyntax` that gives early access to a (syntactic) breaking change that we expect in Dafny version 4, namely changing function to be compiled (not ghost) by default.

This PR does not impose any changes on existing code. When (months from now) we are ready to release Dafny version 4, we can simply change the default from `/functionSyntax:3` to `/functionSyntax:4`, as well as update the test suite and documentation. When updating the tests, `/functionSyntax:migration3to4` or other options will be useful. When updating the documentation, it may be useful to look at what had been changed in the preliminary, and since abandoned, PR #1555, which had updated the documentation in a different direction.

The following table shows, for each mode of the new flag `/functionSyntax:<mode>`, what kind of declaration (ghost or compiled, or an error) you get from using the various keywords:

mode | `ghost function` | `function` | `function method`
-----|-----|-----|-----
`3` (default) | error | ghost | compiled
`4` | ghost | compiled | error
`migration3to4` | ghost | error | compiled
`experimentalDefaultGhost` | ghost | ghost | compiled
`experimentalDefaultCompiled` | ghost | compiled | compiled
`experimentalPredicateAlwaysGhost` | ghost | compiled | error

The same table applies if you replace `function` with `predicate`, except in the last line, which treats `function` and `predicate` in different ways:

mode | `ghost predicate` | `predicate` | `predicate method`
-----|-----|-----|-----
`experimentalPredicateAlwaysGhost` | error | ghost | error

## Other implementation level changes:

* Remove dangling `.expect` file

* Ignore generated `.py` files

* chore: Code improvements

* chore: Clean up some comments and formatting
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