ast: Require --help to parse more keywords as decorated statement (#10000)

This makes it so

```fish
if -e foo
    # do something
end
```

complains about `-e` not being a command instead of `end` being used
outside of an if-block.

That means both that `-e` could now be used as a command name (it
already can outside of `if`!) *and* that we get a better error!

The only way to get `if` to be a decorated statement now is to use `if
-h` or `if --help` specifically (with a literal option).

The same goes for switch, while and begin.

It would be possible, alternatively, to disallow `if -e` and point
towards using `test` instead, but the "unknown command" message should
already point towards using `test` more than pointing at the
"end" (that might be quite far away).

fish-rust/src/ast.rs

@@ -3503,20 +3503,27 @@ fn new_decorated_statement(slf: &mut Populator<'_>) -> Box<StatementVariant> {
             return got_error(self);
         }
 
-        // The only block-like builtin that takes any parameters is 'function'. So go to decorated
-        // statements if the subsequent token looks like '--'. The logic here is subtle:
+        // In some cases a block starter is a decorated statement instead, mostly if invoked with "--help".
+        // The logic here is subtle:
         //
-        // If we are 'begin', then we expect to be invoked with no arguments.
-        // If we are 'function', then we are a non-block if we are invoked with -h or --help
+        // If we are 'begin', it's only really a block if it has no arguments.
+        // If we are 'function' or another block starter, then we are a non-block if we are invoked with -h or --help
         // If we are anything else, we require an argument, so do the same thing if the subsequent
         // token is a statement terminator.
         if self.peek_token(0).typ == ParseTokenType::string {
-            // If we are a function, then look for help arguments. Otherwise, if the next token
+            // If we are one of these, then look for specifically help arguments. Otherwise, if the next token
             // looks like an option (starts with a dash), then parse it as a decorated statement.
-            if (self.peek_token(0).keyword == ParseKeyword::kw_function
+            let help_only_kws = [
+                ParseKeyword::kw_begin,
+                ParseKeyword::kw_function,
+                ParseKeyword::kw_if,
+                ParseKeyword::kw_switch,
+                ParseKeyword::kw_while,
+            ];
+            if (help_only_kws.contains(&self.peek_token(0).keyword)
                 && self.peek_token(1).is_help_argument)
-                || (self.peek_token(0).keyword != ParseKeyword::kw_function
-                    && self.peek_token(1).has_dash_prefix)
+                || (!help_only_kws.contains(&self.peek_token(0).keyword)
+                    && self.peek_token(1).is_dash_prefix_string())
             {
                 return new_decorated_statement(self);
             }

tests/checks/syntax-error-location.fish

@@ -73,3 +73,19 @@ echo "bind -M" | $fish
 # CHECKERR: ^
 # CHECKERR: (Type 'help bind' for related documentation)
 
+$fish -c 'if -e; end'
+# CHECKERR: fish: Unknown command: -e
+# CHECKERR: fish:
+# CHECKERR: if -e; end
+# CHECKERR:    ^^
+
+$fish -c 'begin --notanoption; end'
+# CHECKERR: fish: Unknown command: --notanoption
+# CHECKERR: fish:
+# CHECKERR: begin --notanoption; end
+# CHECKERR:       ^~~~~~~~~~~~^
+
+$fish -c 'begin --help'
+# CHECKERR: fish: begin: missing man page
+# CHECKERR: Documentation may not be installed.
+# CHECKERR: `help begin` will show an online version