Add helpful comments to tt_prepend_space.

In particular, I found some places where it treats `!` inappropriately,
which are worth documenting.

Author: nnethercote

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -150,6 +150,8 @@ pub fn print_crate<'a>(
 /// and also addresses some specific regressions described in #63896 and #73345.
 fn tt_prepend_space(tt: &TokenTree, prev: &TokenTree) -> bool {
     if let TokenTree::Token(token, _) = prev {
+        // No space after these tokens, e.g. `x.y`, `$e`
+        // (The carets point to `prev`.)       ^     ^
         if matches!(token.kind, token::Dot | token::Dollar) {
             return false;
         }
@@ -158,10 +160,19 @@ fn tt_prepend_space(tt: &TokenTree, prev: &TokenTree) -> bool {
         }
     }
     match tt {
+        // No space before these tokens, e.g. `foo,`, `println!`, `x.y`
+        // (The carets point to `token`.)         ^           ^     ^
+        //
+        // FIXME: having `Not` here works well for macro invocations like
+        // `println!`, but is bad when `!` means "logical not" or "the never
+        // type", where the lack of space causes ugliness like this:
+        // `Fn() ->!`, `x =! y`, `if! x { f(); }`.
         TokenTree::Token(token, _) => !matches!(token.kind, token::Comma | token::Not | token::Dot),
+        // No space before parentheses if preceded by these tokens, e.g. `foo(...)`
         TokenTree::Delimited(_, Delimiter::Parenthesis, _) => {
             !matches!(prev, TokenTree::Token(Token { kind: token::Ident(..), .. }, _))
         }
+        // No space before brackets if preceded by these tokens, e.g. `#[...]`
         TokenTree::Delimited(_, Delimiter::Bracket, _) => {
             !matches!(prev, TokenTree::Token(Token { kind: token::Pound, .. }, _))
         }