Auto merge of rust-lang#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
     rust-lang#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:
* rust-lang#69426
  and it's ongoing implementation:
  rust-lang#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
* rust-lang#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 rust-lang#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`

clippy_lints/src/copies.rs

@@ -6,7 +6,7 @@ use clippy_utils::{
 };
 use if_chain::if_chain;
 use rustc_data_structures::fx::FxHashSet;
-use rustc_errors::{Applicability, DiagnosticBuilder};
+use rustc_errors::{Applicability, Diagnostic};
 use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{Block, Expr, ExprKind, HirId};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
@@ -489,7 +489,7 @@ fn emit_branches_sharing_code_lint(
         add_expr_note = !cx.typeck_results().expr_ty(if_expr).is_unit();
     }
 
-    let add_optional_msgs = |diag: &mut DiagnosticBuilder<'_>| {
+    let add_optional_msgs = |diag: &mut Diagnostic| {
         if add_expr_note {
             diag.note("The end suggestion probably needs some adjustments to use the expression result correctly");
         }

clippy_lints/src/doc.rs

@@ -628,9 +628,7 @@ fn check_code(cx: &LateContext<'_>, text: &str, edition: Edition, span: Span) {
                 let mut parser = match maybe_new_parser_from_source_str(&sess, filename, code) {
                     Ok(p) => p,
                     Err(errs) => {
-                        for mut err in errs {
-                            err.cancel();
-                        }
+                        drop(errs);
                         return false;
                     },
                 };
@@ -668,7 +666,7 @@ fn check_code(cx: &LateContext<'_>, text: &str, edition: Edition, span: Span) {
                             _ => {},
                         },
                         Ok(None) => break,
-                        Err(mut e) => {
+                        Err(e) => {
                             e.cancel();
                             return false;
                         },

clippy_lints/src/implicit_hasher.rs

@@ -1,7 +1,7 @@
 use std::borrow::Cow;
 use std::collections::BTreeMap;
 
-use rustc_errors::DiagnosticBuilder;
+use rustc_errors::Diagnostic;
 use rustc_hir as hir;
 use rustc_hir::intravisit::{walk_body, walk_expr, walk_inf, walk_ty, Visitor};
 use rustc_hir::{Body, Expr, ExprKind, GenericArg, Item, ItemKind, QPath, TyKind};
@@ -68,7 +68,7 @@ impl<'tcx> LateLintPass<'tcx> for ImplicitHasher {
 
         fn suggestion<'tcx>(
             cx: &LateContext<'tcx>,
-            diag: &mut DiagnosticBuilder<'_>,
+            diag: &mut Diagnostic,
             generics_span: Span,
             generics_suggestion_span: Span,
             target: &ImplicitHasherType<'_>,

clippy_lints/src/inline_fn_without_body.rs

@@ -1,7 +1,7 @@
 //! checks for `#[inline]` on trait methods without bodies
 
 use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::sugg::DiagnosticBuilderExt;
+use clippy_utils::sugg::DiagnosticExt;
 use rustc_ast::ast::Attribute;
 use rustc_errors::Applicability;
 use rustc_hir::{TraitFn, TraitItem, TraitItemKind};

clippy_lints/src/needless_pass_by_value.rs

@@ -6,7 +6,7 @@ use clippy_utils::{get_trait_def_id, is_self, paths};
 use if_chain::if_chain;
 use rustc_ast::ast::Attribute;
 use rustc_data_structures::fx::FxHashSet;
-use rustc_errors::{Applicability, DiagnosticBuilder};
+use rustc_errors::{Applicability, Diagnostic};
 use rustc_hir::intravisit::FnKind;
 use rustc_hir::{BindingAnnotation, Body, FnDecl, GenericArg, HirId, Impl, ItemKind, Node, PatKind, QPath, TyKind};
 use rustc_hir::{HirIdMap, HirIdSet};
@@ -196,7 +196,7 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
                     }
 
                     // Dereference suggestion
-                    let sugg = |diag: &mut DiagnosticBuilder<'_>| {
+                    let sugg = |diag: &mut Diagnostic| {
                         if let ty::Adt(def, ..) = ty.kind() {
                             if let Some(span) = cx.tcx.hir().span_if_local(def.did) {
                                 if can_type_implement_copy(cx.tcx, cx.param_env, ty, traits::ObligationCause::dummy_with_span(span)).is_ok() {

clippy_lints/src/new_without_default.rs

@@ -1,7 +1,7 @@
 use clippy_utils::diagnostics::span_lint_hir_and_then;
 use clippy_utils::return_ty;
 use clippy_utils::source::snippet;
-use clippy_utils::sugg::DiagnosticBuilderExt;
+use clippy_utils::sugg::DiagnosticExt;
 use if_chain::if_chain;
 use rustc_errors::Applicability;
 use rustc_hir as hir;

clippy_lints/src/write.rs

@@ -534,7 +534,7 @@ impl Write {
             match parser
                 .parse_expr()
                 .map(rustc_ast::ptr::P::into_inner)
-                .map_err(|mut e| e.cancel())
+                .map_err(|e| e.cancel())
             {
                 // write!(e, ...)
                 Ok(p) if parser.eat(&token::Comma) => Some(p),
@@ -563,7 +563,7 @@ impl Write {
             }
 
             let comma_span = parser.prev_token.span;
-            let token_expr = if let Ok(expr) = parser.parse_expr().map_err(|mut err| err.cancel()) {
+            let token_expr = if let Ok(expr) = parser.parse_expr().map_err(|err| err.cancel()) {
                 expr
             } else {
                 return (Some(fmtstr), None);

clippy_utils/src/diagnostics.rs

@@ -8,13 +8,13 @@
 //! Thank you!
 //! ~The `INTERNAL_METADATA_COLLECTOR` lint
 
-use rustc_errors::{Applicability, DiagnosticBuilder};
+use rustc_errors::{Applicability, Diagnostic};
 use rustc_hir::HirId;
 use rustc_lint::{LateContext, Lint, LintContext};
 use rustc_span::source_map::{MultiSpan, Span};
 use std::env;
 
-fn docs_link(diag: &mut DiagnosticBuilder<'_>, lint: &'static Lint) {
+fn docs_link(diag: &mut Diagnostic, lint: &'static Lint) {
     if env::var("CLIPPY_DISABLE_DOCS_LINKS").is_err() {
         if let Some(lint) = lint.name_lower().strip_prefix("clippy::") {
             diag.help(&format!(
@@ -145,7 +145,7 @@ pub fn span_lint_and_then<C, S, F>(cx: &C, lint: &'static Lint, sp: S, msg: &str
 where
     C: LintContext,
     S: Into<MultiSpan>,
-    F: FnOnce(&mut DiagnosticBuilder<'_>),
+    F: FnOnce(&mut Diagnostic),
 {
     cx.struct_span_lint(lint, sp, |diag| {
         let mut diag = diag.build(msg);
@@ -169,7 +169,7 @@ pub fn span_lint_hir_and_then(
     hir_id: HirId,
     sp: impl Into<MultiSpan>,
     msg: &str,
-    f: impl FnOnce(&mut DiagnosticBuilder<'_>),
+    f: impl FnOnce(&mut Diagnostic),
 ) {
     cx.tcx.struct_span_lint_hir(lint, hir_id, sp, |diag| {
         let mut diag = diag.build(msg);
@@ -219,7 +219,7 @@ pub fn span_lint_and_sugg<'a, T: LintContext>(
 /// appear once per
 /// replacement. In human-readable format though, it only appears once before
 /// the whole suggestion.
-pub fn multispan_sugg<I>(diag: &mut DiagnosticBuilder<'_>, help_msg: &str, sugg: I)
+pub fn multispan_sugg<I>(diag: &mut Diagnostic, help_msg: &str, sugg: I)
 where
     I: IntoIterator<Item = (Span, String)>,
 {
@@ -232,7 +232,7 @@ where
 /// multiple spans. This is tracked in issue [rustfix#141](https://github.com/rust-lang/rustfix/issues/141).
 /// Suggestions with multiple spans will be silently ignored.
 pub fn multispan_sugg_with_applicability<I>(
-    diag: &mut DiagnosticBuilder<'_>,
+    diag: &mut Diagnostic,
     help_msg: &str,
     applicability: Applicability,
     sugg: I,

clippy_utils/src/sugg.rs

@@ -673,8 +673,8 @@ fn indentation<T: LintContext>(cx: &T, span: Span) -> Option<String> {
         })
 }
 
-/// Convenience extension trait for `DiagnosticBuilder`.
-pub trait DiagnosticBuilderExt<T: LintContext> {
+/// Convenience extension trait for `Diagnostic`.
+pub trait DiagnosticExt<T: LintContext> {
     /// Suggests to add an attribute to an item.
     ///
     /// Correctly handles indentation of the attribute and item.
@@ -721,7 +721,7 @@ pub trait DiagnosticBuilderExt<T: LintContext> {
     fn suggest_remove_item(&mut self, cx: &T, item: Span, msg: &str, applicability: Applicability);
 }
 
-impl<T: LintContext> DiagnosticBuilderExt<T> for rustc_errors::DiagnosticBuilder<'_> {
+impl<T: LintContext> DiagnosticExt<T> for rustc_errors::Diagnostic {
     fn suggest_item_with_attr<D: Display + ?Sized>(
         &mut self,
         cx: &T,