In cases like:

func _() {
  if fo<>
}

func foo() {}

Completing at "<>" does not include "foo" because the missing "if"
opening curly brace renders the rest of the file unparseable. "foo"
doesn't exist in the AST, so as far as gopls is concerned it doesn't
exist at all.

To fix this, we detect when a block is missing opening "{" and we
insert stub "{}" to make things parse better. This is a different kind
of fix than our previous *ast.BadExpr handling because we must reparse
the file after tweaking the source. After reparsing we maintain the
original syntax error so the user sees consistent error messages. This
also avoids having the "{}" spring into existence when the user
formats the file.

Fixes golang/go#31907.
Updates golang/go#31687.

Change-Id: I95ba60a11f7dd23dc484c063b4fd7ad77daa4e08
Reviewed-on: https://go-review.googlesource.com/c/tools/+/216482
Run-TryBot: Muir Manders <muir@mnd.rs>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

In cases like:

  if i := foo<>

we get an *ast.BadExpr because the parser is expecting the condition
expression, but "i := foo" is not a valid expression. Now we move the
statement into the "init" field and add a dummy "cond" expression.

We also needed a slight tweak to our missing curly brace fix. Now we
insert an extra semicolon in cases like:

for i := 0; i < foo

yielding

for i := 0; i < foo;{}

The parser doesn't like having only two clauses in the three clause
"for" statement.

Updates golang/go#31687.

Change-Id: I12d51e0d8af03436741227753f8e71452a463b05
Reviewed-on: https://go-review.googlesource.com/c/tools/+/216483
Run-TryBot: Muir Manders <muir@mnd.rs>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

Dangling selectors such as:

    func _() {
      x.
    }
    var x struct { i int }

tend to wreak havoc on the AST. In the above example you didn't used
to get completions because the declaration of "x" was missing from the
AST.

We now work around this issue by inserting a "_" into the source code
before parsing to make the selector valid:

    func _() {
      x._ // <-- insert "_" here
    }
    var x struct { i int }

This makes completion work as expected because the declaration of "x"
is present in the AST.

I had to change fixAST() to be called before fixSrc() because
otherwise this new workaround in fixSrc() breaks the "accidental
keyword" countermeasures in fixAST().

Fixes golang/go#31973.
Updates golang/go#31687.

Change-Id: Ia7ef6c045a9c71502d1b8b36f187ac9b8a85fe21
Reviewed-on: https://go-review.googlesource.com/c/tools/+/216484
Run-TryBot: Muir Manders <muir@mnd.rs>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>