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`

6 files changed, 39 insertions, 41 deletions

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 6b70c983344..2c3ddae9cb4 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -1070,8 +1070,9 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
                             .emit();
                     }
                 }
-                let ill_formed =
-                    |span| struct_span_err!(self.r.session, span, E0466, "bad macro import").emit();
+                let ill_formed = |span| {
+                    struct_span_err!(self.r.session, span, E0466, "bad macro import").emit();
+                };
                 match attr.meta() {
                     Some(meta) => match meta.kind {
                         MetaItemKind::Word => {
diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index 9a2fb3b86e2..b366473cb30 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -3,7 +3,7 @@ use std::ptr;
 use rustc_ast::{self as ast, Path};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashSet;
-use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder};
+use rustc_errors::{struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorReported};
 use rustc_feature::BUILTIN_ATTRIBUTES;
 use rustc_hir::def::Namespace::{self, *};
 use rustc_hir::def::{self, CtorKind, CtorOf, DefKind, NonMacroAttrKind};
@@ -110,7 +110,7 @@ impl<'a> Resolver<'a> {
         &self,
         span: Span,
         resolution_error: ResolutionError<'_>,
-    ) -> DiagnosticBuilder<'_> {
+    ) -> DiagnosticBuilder<'_, ErrorReported> {
         match resolution_error {
             ResolutionError::GenericParamsFromOuterFunction(outer_res, has_generic_params) => {
                 let mut err = struct_span_err!(
@@ -624,7 +624,10 @@ impl<'a> Resolver<'a> {
         }
     }
 
-    crate fn report_vis_error(&self, vis_resolution_error: VisResolutionError<'_>) {
+    crate fn report_vis_error(
+        &self,
+        vis_resolution_error: VisResolutionError<'_>,
+    ) -> ErrorReported {
         match vis_resolution_error {
             VisResolutionError::Relative2018(span, path) => {
                 let mut err = self.session.struct_span_err(
@@ -1031,7 +1034,7 @@ impl<'a> Resolver<'a> {
 
     crate fn unresolved_macro_suggestions(
         &mut self,
-        err: &mut DiagnosticBuilder<'a>,
+        err: &mut Diagnostic,
         macro_kind: MacroKind,
         parent_scope: &ParentScope<'a>,
         ident: Ident,
@@ -1120,7 +1123,7 @@ impl<'a> Resolver<'a> {
 
     crate fn add_typo_suggestion(
         &self,
-        err: &mut DiagnosticBuilder<'_>,
+        err: &mut Diagnostic,
         suggestion: Option<TypoSuggestion>,
         span: Span,
     ) -> bool {
@@ -1817,7 +1820,7 @@ fn find_span_immediately_after_crate_name(
 crate fn show_candidates(
     definitions: &rustc_hir::definitions::Definitions,
     session: &Session,
-    err: &mut DiagnosticBuilder<'_>,
+    err: &mut Diagnostic,
     // This is `None` if all placement locations are inside expansions
     use_placement_span: Option<Span>,
     candidates: &[ImportSuggestion],
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 9ac3e6e22bd..91695257137 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -2001,13 +2001,11 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             // into a single one.
             let mut parent_err = this.r.into_struct_error(parent_err.span, parent_err.node);
 
-            parent_err.cancel();
-
             err.message = take(&mut parent_err.message);
             err.code = take(&mut parent_err.code);
             err.children = take(&mut parent_err.children);
 
-            drop(parent_err);
+            parent_err.cancel();
 
             let def_id = this.parent_scope.module.nearest_parent_mod();
 
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index f20cf29cc89..b71776c1615 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -12,7 +12,9 @@ use rustc_ast::{
 };
 use rustc_ast_pretty::pprust::path_segment_to_string;
 use rustc_data_structures::fx::FxHashSet;
-use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder};
+use rustc_errors::{
+    pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorReported,
+};
 use rustc_hir as hir;
 use rustc_hir::def::Namespace::{self, *};
 use rustc_hir::def::{self, CtorKind, CtorOf, DefKind};
@@ -133,7 +135,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         span: Span,
         source: PathSource<'_>,
         res: Option<Res>,
-    ) -> (DiagnosticBuilder<'a>, Vec<ImportSuggestion>) {
+    ) -> (DiagnosticBuilder<'a, ErrorReported>, Vec<ImportSuggestion>) {
         let ident_span = path.last().map_or(span, |ident| ident.ident.span);
         let ns = source.namespace();
         let is_expected = &|res| source.is_expected(res);
@@ -606,11 +608,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         (err, candidates)
     }
 
-    fn detect_assoct_type_constraint_meant_as_path(
-        &self,
-        base_span: Span,
-        err: &mut DiagnosticBuilder<'_>,
-    ) {
+    fn detect_assoct_type_constraint_meant_as_path(&self, base_span: Span, err: &mut Diagnostic) {
         let Some(ty) = self.diagnostic_metadata.current_type_path else { return; };
         let TyKind::Path(_, path) = &ty.kind else { return; };
         for segment in &path.segments {
@@ -675,11 +673,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
     }
 
     /// Given `where <T as Bar>::Baz: String`, suggest `where T: Bar<Baz = String>`.
-    fn restrict_assoc_type_in_where_clause(
-        &mut self,
-        span: Span,
-        err: &mut DiagnosticBuilder<'_>,
-    ) -> bool {
+    fn restrict_assoc_type_in_where_clause(&mut self, span: Span, err: &mut Diagnostic) -> bool {
         // Detect that we are actually in a `where` predicate.
         let (bounded_ty, bounds, where_span) =
             if let Some(ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
@@ -875,7 +869,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
     /// Returns `true` if able to provide context-dependent help.
     fn smart_resolve_context_dependent_help(
         &mut self,
-        err: &mut DiagnosticBuilder<'a>,
+        err: &mut Diagnostic,
         span: Span,
         source: PathSource<'_>,
         res: Res,
@@ -885,7 +879,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         let ns = source.namespace();
         let is_expected = &|res| source.is_expected(res);
 
-        let path_sep = |err: &mut DiagnosticBuilder<'_>, expr: &Expr| match expr.kind {
+        let path_sep = |err: &mut Diagnostic, expr: &Expr| match expr.kind {
             ExprKind::Field(_, ident) => {
                 err.span_suggestion(
                     expr.span,
@@ -908,7 +902,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
             _ => false,
         };
 
-        let find_span = |source: &PathSource<'_>, err: &mut DiagnosticBuilder<'_>| {
+        let find_span = |source: &PathSource<'_>, err: &mut Diagnostic| {
             match source {
                 PathSource::Expr(Some(Expr { span, kind: ExprKind::Call(_, _), .. }))
                 | PathSource::TupleStruct(span, _) => {
@@ -1066,7 +1060,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                     })
                     .unwrap_or(false)
                 {
-                    err.delay_as_bug();
+                    err.downgrade_to_delayed_bug();
                     // We already suggested changing `:` into `::` during parsing.
                     return false;
                 }
@@ -1435,7 +1429,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         start.to(sm.next_point(start))
     }
 
-    fn type_ascription_suggestion(&self, err: &mut DiagnosticBuilder<'_>, base_span: Span) -> bool {
+    fn type_ascription_suggestion(&self, err: &mut Diagnostic, base_span: Span) -> bool {
         let sm = self.r.session.source_map();
         let base_snippet = sm.span_to_snippet(base_span);
         if let Some(&sp) = self.diagnostic_metadata.current_type_ascription.last() {
@@ -1472,7 +1466,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                                 .borrow_mut()
                                 .insert(colon_sp)
                             {
-                                err.delay_as_bug();
+                                err.downgrade_to_delayed_bug();
                             }
                         }
                         if let Ok(base_snippet) = base_snippet {
@@ -1577,7 +1571,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
     /// Adds a suggestion for using an enum's variant when an enum is used instead.
     fn suggest_using_enum_variant(
         &mut self,
-        err: &mut DiagnosticBuilder<'a>,
+        err: &mut Diagnostic,
         source: PathSource<'_>,
         def_id: DefId,
         span: Span,
@@ -1825,7 +1819,7 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
         &self,
         spans: Vec<Span>,
         count: usize,
-    ) -> DiagnosticBuilder<'tcx> {
+    ) -> DiagnosticBuilder<'tcx, ErrorReported> {
         struct_span_err!(
             self.tcx.sess,
             spans,
@@ -1910,7 +1904,8 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
     /// Returns whether to add `'static` lifetime to the suggested lifetime list.
     crate fn report_elision_failure(
         &mut self,
-        db: &mut DiagnosticBuilder<'_>,
+        // FIXME(eddyb) rename this since it's no longer a `DiagnosticBuilder`.
+        db: &mut Diagnostic,
         params: &[ElisionFailureInfo],
     ) -> bool {
         let mut m = String::new();
@@ -2059,7 +2054,7 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
 
     crate fn add_missing_lifetime_specifiers_label(
         &self,
-        err: &mut DiagnosticBuilder<'_>,
+        err: &mut Diagnostic,
         mut spans_with_counts: Vec<(Span, usize)>,
         lifetime_names: &FxHashSet<Symbol>,
         lifetime_spans: Vec<Span>,
@@ -2090,7 +2085,7 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
         }
 
         let suggest_existing =
-            |err: &mut DiagnosticBuilder<'_>,
+            |err: &mut Diagnostic,
              name: &str,
              formatters: Vec<Option<Box<dyn Fn(&str) -> String>>>| {
                 if let Some(MissingLifetimeSpot::HigherRanked { span: for_span, span_type }) =
@@ -2174,7 +2169,7 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
                     Applicability::MaybeIncorrect,
                 );
             };
-        let suggest_new = |err: &mut DiagnosticBuilder<'_>, suggs: Vec<Option<String>>| {
+        let suggest_new = |err: &mut Diagnostic, suggs: Vec<Option<String>>| {
             for missing in self.missing_named_lifetime_spots.iter().rev() {
                 let mut introduce_suggestion = vec![];
                 let msg;
diff --git a/compiler/rustc_resolve/src/late/lifetimes.rs b/compiler/rustc_resolve/src/late/lifetimes.rs
index 2f0ad60709d..23a8189f62f 100644
--- a/compiler/rustc_resolve/src/late/lifetimes.rs
+++ b/compiler/rustc_resolve/src/late/lifetimes.rs
@@ -9,7 +9,7 @@
 use crate::late::diagnostics::{ForLifetimeSpanType, MissingLifetimeSpot};
 use rustc_ast::walk_list;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
-use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder};
+use rustc_errors::{struct_span_err, Applicability, Diagnostic};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefIdMap, LocalDefId};
@@ -1572,6 +1572,7 @@ fn signal_shadowing_problem(tcx: TyCtxt<'_>, name: Symbol, orig: Original, shado
             name,
             orig.kind.desc()
         )
+        .forget_guarantee()
     } else {
         // shadowing involving a label is only a warning, due to issues with
         // labels and lifetimes not being macro-hygienic.
@@ -1873,7 +1874,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
     // or from `fn rah<'a>(T<'a>)` to `fn rah(T<'_>)`
     fn suggest_eliding_single_use_lifetime(
         &self,
-        err: &mut DiagnosticBuilder<'_>,
+        err: &mut Diagnostic,
         def_id: DefId,
         lifetime: &hir::Lifetime,
     ) {
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index eed8aaed4ee..4823b889207 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -40,7 +40,7 @@ use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
 use rustc_data_structures::intern::Interned;
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder};
+use rustc_errors::{struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorReported};
 use rustc_expand::base::{DeriveResolutions, SyntaxExtension, SyntaxExtensionKind};
 use rustc_hir::def::Namespace::*;
 use rustc_hir::def::{self, CtorOf, DefKind, NonMacroAttrKind, PartialRes};
@@ -713,7 +713,7 @@ struct PrivacyError<'a> {
 }
 
 struct UseError<'a> {
-    err: DiagnosticBuilder<'a>,
+    err: DiagnosticBuilder<'a, ErrorReported>,
     /// Candidates which user could `use` to access the missing type.
     candidates: Vec<ImportSuggestion>,
     /// The `DefId` of the module to place the use-statements in.
@@ -3173,7 +3173,7 @@ impl<'a> Resolver<'a> {
     /// ```
     fn add_suggestion_for_rename_of_use(
         &self,
-        err: &mut DiagnosticBuilder<'_>,
+        err: &mut Diagnostic,
         name: Symbol,
         import: &Import<'_>,
         binding_span: Span,
@@ -3252,7 +3252,7 @@ impl<'a> Resolver<'a> {
     /// as characters expected by span manipulations won't be present.
     fn add_suggestion_for_duplicate_nested_use(
         &self,
-        err: &mut DiagnosticBuilder<'_>,
+        err: &mut Diagnostic,
         import: &Import<'_>,
         binding_span: Span,
     ) {