Auto merge of #93368 - eddyb:diagbld-guarantee, r=estebank

rustc_errors: let `DiagnosticBuilder::emit` return a "guarantee of emission".

That is, `DiagnosticBuilder` is now generic over the return type of `.emit()`, so we'll now have:
* `DiagnosticBuilder<ErrorReported>` for error (incl. fatal/bug) diagnostics
  * can only be created via a `const L: Level`-generic constructor, that limits allowed variants via a `where` clause, so not even `rustc_errors` can accidentally bypass this limitation
  * asserts `diagnostic.is_error()` on emission, just in case the construction restriction was bypassed (e.g. by replacing the whole `Diagnostic` inside `DiagnosticBuilder`)
  * `.emit()` returns `ErrorReported`, as a "proof" token that `.emit()` was called
    (though note that this isn't a real guarantee until after completing the work on
     #69426)
* `DiagnosticBuilder<()>` for everything else (warnings, notes, etc.)
  * can also be obtained from other `DiagnosticBuilder`s by calling `.forget_guarantee()`

This PR is a companion to other ongoing work, namely:
* #69426
  and it's ongoing implementation:
  #93222
  the API changes in this PR are needed to get statically-checked "only errors produce `ErrorReported` from `.emit()`", but doesn't itself provide any really strong guarantees without those other `ErrorReported` changes
* #93244
  would make the choices of API changes (esp. naming) in this PR fit better overall

In order to be able to let `.emit()` return anything trustable, several changes had to be made:
* `Diagnostic`'s `level` field is now private to `rustc_errors`, to disallow arbitrary "downgrade"s from "some kind of error" to "warning" (or anything else that doesn't cause compilation to fail)
  * it's still possible to replace the whole `Diagnostic` inside the `DiagnosticBuilder`, sadly, that's harder to fix, but it's unlikely enough that we can paper over it with asserts on `.emit()`
* `.cancel()` now consumes `DiagnosticBuilder`, preventing `.emit()` calls on a cancelled diagnostic
  * it's also now done internally, through `DiagnosticBuilder`-private state, instead of having a `Level::Cancelled` variant that can be read (or worse, written) by the user
  * this removes a hazard of calling `.cancel()` on an error then continuing to attach details to it, and even expect to be able to `.emit()` it
  * warnings were switched to *only* `can_emit_warnings` on emission (instead of pre-cancelling early)
  * `struct_dummy` was removed (as it relied on a pre-`Cancelled` `Diagnostic`)
* since `.emit()` doesn't consume the `DiagnosticBuilder` <sub>(I tried and gave up, it's much more work than this PR)</sub>,
  we have to make `.emit()` idempotent wrt the guarantees it returns
  * thankfully, `err.emit(); err.emit();` can return `ErrorReported` both times, as the second `.emit()` call has no side-effects *only* because the first one did do the appropriate emission
* `&mut Diagnostic` is now used in a lot of function signatures, which used to take `&mut DiagnosticBuilder` (in the interest of not having to make those functions generic)
  * the APIs were already mostly identical, allowing for low-effort porting to this new setup
  * only some of the suggestion methods needed some rework, to have the extra `DiagnosticBuilder` functionality on the `Diagnostic` methods themselves (that change is also present in #93259)
  * `.emit()`/`.cancel()` aren't available, but IMO calling them from an "error decorator/annotator" function isn't a good practice, and can lead to strange behavior (from the caller's perspective)
  * `.downgrade_to_delayed_bug()` was added, letting you convert any `.is_error()` diagnostic into a `delay_span_bug` one (which works because in both cases the guarantees available are the same)

This PR should ideally be reviewed commit-by-commit, since there is a lot of fallout in each.

r? `@estebank` cc `@Manishearth` `@nikomatsakis` `@mark-i-m`

compiler/rustc_lint/src/context.rs

@@ -319,7 +319,7 @@ impl LintStore {
     ) {
         let (tool_name, lint_name_only) = parse_lint_and_tool_name(lint_name);
         if lint_name_only == crate::WARNINGS.name_lower() && level == Level::ForceWarn {
-            return struct_span_err!(
+            struct_span_err!(
                 sess,
                 DUMMY_SP,
                 E0602,
@@ -327,6 +327,7 @@ impl LintStore {
                 crate::WARNINGS.name_lower()
             )
             .emit();
+            return;
         }
         let db = match self.check_lint_name(lint_name_only, tool_name, registered_tools) {
             CheckLintNameResult::Ok(_) => None,
@@ -339,7 +340,7 @@ impl LintStore {
                     err.help(&format!("did you mean: `{}`", suggestion));
                 }
 
-                Some(err)
+                Some(err.forget_guarantee())
             }
             CheckLintNameResult::Tool(result) => match result {
                 Err((Some(_), new_name)) => Some(sess.struct_warn(&format!(
@@ -350,13 +351,16 @@ impl LintStore {
                 ))),
                 _ => None,
             },
-            CheckLintNameResult::NoTool => Some(struct_span_err!(
-                sess,
-                DUMMY_SP,
-                E0602,
-                "unknown lint tool: `{}`",
-                tool_name.unwrap()
-            )),
+            CheckLintNameResult::NoTool => Some(
+                struct_span_err!(
+                    sess,
+                    DUMMY_SP,
+                    E0602,
+                    "unknown lint tool: `{}`",
+                    tool_name.unwrap()
+                )
+                .forget_guarantee(),
+            ),
         };
 
         if let Some(mut db) = db {