Make DiagnosticBuilder::emit consuming.

This works for most of its call sites. This is nice, because `emit` very
much makes sense as a consuming operation -- indeed,
`DiagnosticBuilderState` exists to ensure no diagnostic is emitted
twice, but it uses runtime checks.

For the small number of call sites where a consuming emit doesn't work,
the commit adds `DiagnosticBuilder::emit_without_consuming`. (This will
be removed in subsequent commits.)

Likewise, `emit_unless` becomes consuming. And `delay_as_bug` becomes
consuming, while `delay_as_bug_without_consuming` is added (which will
also be removed in subsequent commits.)

All this requires significant changes to `DiagnosticBuilder`'s chaining
methods. Currently `DiagnosticBuilder` method chaining uses a
non-consuming `&mut self -> &mut Self` style, which allows chaining to
be used when the chain ends in `emit()`, like so:
```
    struct_err(msg).span(span).emit();
```
But it doesn't work when producing a `DiagnosticBuilder` value,
requiring this:
```
    let mut err = self.struct_err(msg);
    err.span(span);
    err
```
This style of chaining won't work with consuming `emit` though. For
that, we need to use to a `self -> Self` style. That also would allow
`DiagnosticBuilder` production to be chained, e.g.:
```
    self.struct_err(msg).span(span)
```
However, removing the `&mut self -> &mut Self` style would require that
individual modifications of a `DiagnosticBuilder` go from this:
```
    err.span(span);
```
to this:
```
    err = err.span(span);
```
There are *many* such places. I have a high tolerance for tedious
refactorings, but even I gave up after a long time trying to convert
them all.

Instead, this commit has it both ways: the existing `&mut self -> Self`
chaining methods are kept, and new `self -> Self` chaining methods are
added, all of which have a `_mv` suffix (short for "move"). Changes to
the existing `forward!` macro lets this happen with very little
additional boilerplate code. I chose to add the suffix to the new
chaining methods rather than the existing ones, because the number of
changes required is much smaller that way.

This doubled chainging is a bit clumsy, but I think it is worthwhile
because it allows a *lot* of good things to subsequently happen. In this
commit, there are many `mut` qualifiers removed in places where
diagnostics are emitted without being modified. In subsequent commits:
- chaining can be used more, making the code more concise;
- more use of chaining also permits the removal of redundant diagnostic
  APIs like `struct_err_with_code`, which can be replaced easily with
  `struct_err` + `code_mv`;
- `emit_without_diagnostic` can be removed, which simplifies a lot of
  machinery, removing the need for `DiagnosticBuilderState`.

compiler/rustc_parse/src/parser/expr.rs

@@ -127,7 +127,7 @@ impl<'a> Parser<'a> {
     fn parse_expr_catch_underscore(&mut self, restrictions: Restrictions) -> PResult<'a, P<Expr>> {
         match self.parse_expr_res(restrictions, None) {
             Ok(expr) => Ok(expr),
-            Err(mut err) => match self.token.ident() {
+            Err(err) => match self.token.ident() {
                 Some((Ident { name: kw::Underscore, .. }, false))
                     if self.may_recover() && self.look_ahead(1, |t| t == &token::Comma) =>
                 {
@@ -1333,12 +1333,12 @@ impl<'a> Parser<'a> {
                                             .collect(),
                                     },
                                 });
-                            replacement_err.emit();
+                            replacement_err.emit_without_consuming();
 
                             let old_err = mem::replace(err, replacement_err);
                             old_err.cancel();
                         } else {
-                            err.emit();
+                            err.emit_without_consuming();
                         }
                         return Some(self.mk_expr_err(span));
                     }
@@ -1756,9 +1756,8 @@ impl<'a> Parser<'a> {
         mk_lit_char: impl FnOnce(Symbol, Span) -> L,
         err: impl FnOnce(&Self) -> DiagnosticBuilder<'a>,
     ) -> L {
-        if let Some(mut diag) = self.dcx().steal_diagnostic(lifetime.span, StashKey::LifetimeIsChar)
-        {
-            diag.span_suggestion_verbose(
+        if let Some(diag) = self.dcx().steal_diagnostic(lifetime.span, StashKey::LifetimeIsChar) {
+            diag.span_suggestion_verbose_mv(
                 lifetime.span.shrink_to_hi(),
                 "add `'` to close the char literal",
                 "'",
@@ -1767,7 +1766,7 @@ impl<'a> Parser<'a> {
             .emit();
         } else {
             err(self)
-                .span_suggestion_verbose(
+                .span_suggestion_verbose_mv(
                     lifetime.span.shrink_to_hi(),
                     "add `'` to close the char literal",
                     "'",
@@ -2903,7 +2902,7 @@ impl<'a> Parser<'a> {
         while self.token != token::CloseDelim(Delimiter::Brace) {
             match self.parse_arm() {
                 Ok(arm) => arms.push(arm),
-                Err(mut e) => {
+                Err(e) => {
                     // Recover by skipping to the end of the block.
                     e.emit();
                     self.recover_stmt();
@@ -3437,7 +3436,7 @@ impl<'a> Parser<'a> {
                 }
                 match self.parse_expr() {
                     Ok(e) => base = ast::StructRest::Base(e),
-                    Err(mut e) if recover => {
+                    Err(e) if recover => {
                         e.emit();
                         self.recover_stmt();
                     }