Auto merge of #119146 - nnethercote:rm-DiagCtxt-api-duplication, r=compiler-errors

Remove `DiagCtxt` API duplication

`DiagCtxt` defines the internal API for creating and emitting diagnostics: methods like `struct_err`, `struct_span_warn`, `note`, `create_fatal`, `emit_bug`. There are over 50 methods.

Some of these methods are then duplicated across several other types: `Session`, `ParseSess`, `Parser`, `ExtCtxt`, and `MirBorrowckCtxt`. `Session` duplicates the most, though half the ones it does are unused. Each duplicated method just calls forward to the corresponding method in `DiagCtxt`. So this duplication exists to (in the best case) shorten chains like `ecx.tcx.sess.parse_sess.dcx.emit_err()` to `ecx.emit_err()`.

This API duplication is ugly and has been bugging me for a while. And it's inconsistent: there's no real logic about which methods are duplicated, and the use of `#[rustc_lint_diagnostic]` and `#[track_caller]` attributes vary across the duplicates.

This PR removes the duplicated API methods and makes all diagnostic creation and emission go through `DiagCtxt`. It also adds `dcx` getter methods to several types to shorten chains. This approach scales *much* better than API duplication; indeed, the PR adds `dcx()` to numerous types that didn't have API duplication: `TyCtxt`, `LoweringCtxt`, `ConstCx`, `FnCtxt`, `TypeErrCtxt`, `InferCtxt`, `CrateLoader`, `CheckAttrVisitor`, and `Resolver`. These result in a lot of changes from `foo.tcx.sess.emit_err()` to `foo.dcx().emit_err()`. (You could do this with more types, but it gets into diminishing returns territory for types that don't emit many diagnostics.)

After all these changes, some call sites are more verbose, some are less verbose, and many are the same. The total number of lines is reduced, mostly because of the removed API duplication. And consistency is increased, because calls to `emit_err` and friends are always preceded with `.dcx()` or `.dcx`.

r? `@compiler-errors`

compiler/rustc_const_eval/src/const_eval/error.rs

@@ -151,10 +151,10 @@ where
             let (our_span, frames) = get_span_and_frames();
             let span = span.unwrap_or(our_span);
             let err = mk(span, frames);
-            let mut err = tcx.sess.create_err(err);
+            let mut err = tcx.dcx().create_err(err);
 
             let msg = error.diagnostic_message();
-            error.add_args(tcx.sess.dcx(), &mut err);
+            error.add_args(tcx.dcx(), &mut err);
 
             // Use *our* span to label the interp error
             err.span_label(our_span, msg);

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -391,7 +391,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                 if ecx.tcx.is_ctfe_mir_available(def) {
                     Ok(ecx.tcx.mir_for_ctfe(def))
                 } else if ecx.tcx.def_kind(def) == DefKind::AssocConst {
-                    let guar = ecx.tcx.sess.span_delayed_bug(
+                    let guar = ecx.tcx.dcx().span_delayed_bug(
                         rustc_span::DUMMY_SP,
                         "This is likely a const item that is missing from its impl",
                     );
@@ -621,7 +621,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                 if is_error {
                     let guard = ecx
                         .tcx
-                        .sess
+                        .dcx()
                         .span_delayed_bug(span, "The deny lint should have already errored");
                     throw_inval!(AlreadyReported(guard.into()));
                 }
@@ -630,7 +630,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
                 // current number of evaluated terminators is a power of 2. The latter gives us a cheap
                 // way to implement exponential backoff.
                 let span = ecx.cur_span();
-                ecx.tcx.sess.emit_warning(LongRunningWarn { span, item_span: ecx.tcx.span });
+                ecx.tcx.dcx().emit_warning(LongRunningWarn { span, item_span: ecx.tcx.span });
             }
         }
 

compiler/rustc_const_eval/src/const_eval/mod.rs

@@ -61,7 +61,7 @@ pub(crate) fn eval_to_valtree<'tcx>(
             match err {
                 ValTreeCreationError::NodesOverflow => {
                     let span = tcx.hir().span_if_local(did);
-                    tcx.sess.emit_err(MaxNumNodesInConstErr { span, global_const_id });
+                    tcx.dcx().emit_err(MaxNumNodesInConstErr { span, global_const_id });
 
                     Ok(None)
                 }