Rollup merge of #125510 - lcnr:change-proof-trees-to-be-shallow, r=compiler-errors

remove proof tree formatting, make em shallow

Debugging via tracing `RUSTC_LOG=rustc_trait_selection::solve=debug` is now imo slightly more readable then the actual proof tree formatter. Removing everything that's not needed for the `analyse` visitor allows us to remove a bunch of code.

I personally believe that we should continue to use tracing over proof trees for debugging:
- it eagerly prints, allowing us to debug ICEs
- the proof tree builder ends up going out of sync with the actual runtime behavior, which is confusing
- using shallow proof trees is a lot more performant as we frequently do not recurse into all nested goals when using an analyse visitor
- this allows us to clean up the implementation and remove some code

r? ```@compiler-errors```

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -7,15 +7,11 @@ pub mod suggestions;
 mod type_err_ctxt_ext;
 
 use super::{Obligation, ObligationCause, ObligationCauseCode, PredicateObligation};
-use crate::infer::InferCtxt;
-use crate::solve::{GenerateProofTree, InferCtxtEvalExt};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::Visitor;
-use rustc_middle::traits::solve::Goal;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::Span;
-use std::io::Write;
 use std::ops::ControlFlow;
 
 pub use self::infer_ctxt_ext::*;
@@ -184,16 +180,3 @@ pub enum DefIdOrName {
     DefId(DefId),
     Name(&'static str),
 }
-
-pub fn dump_proof_tree<'tcx>(o: &Obligation<'tcx, ty::Predicate<'tcx>>, infcx: &InferCtxt<'tcx>) {
-    infcx.probe(|_| {
-        let goal = Goal { predicate: o.predicate, param_env: o.param_env };
-        let tree = infcx
-            .evaluate_root_goal(goal, GenerateProofTree::Yes)
-            .1
-            .expect("proof tree should have been generated");
-        let mut lock = std::io::stdout().lock();
-        let _ = lock.write_fmt(format_args!("{tree:?}\n"));
-        let _ = lock.flush();
-    });
-}

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -46,7 +46,6 @@ use rustc_middle::ty::{
     TypeVisitableExt, Upcast,
 };
 use rustc_middle::{bug, span_bug};
-use rustc_session::config::DumpSolverProofTree;
 use rustc_session::Limit;
 use rustc_span::def_id::LOCAL_CRATE;
 use rustc_span::symbol::sym;
@@ -56,8 +55,8 @@ use std::fmt;
 use std::iter;
 
 use super::{
-    dump_proof_tree, ArgKind, CandidateSimilarity, FindExprBySpan, FindTypeParam,
-    GetSafeTransmuteErrorAndReason, HasNumericInferVisitor, ImplCandidate, UnsatisfiedConst,
+    ArgKind, CandidateSimilarity, FindExprBySpan, FindTypeParam, GetSafeTransmuteErrorAndReason,
+    HasNumericInferVisitor, ImplCandidate, UnsatisfiedConst,
 };
 
 pub use rustc_infer::traits::error_reporting::*;
@@ -369,13 +368,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         error: &SelectionError<'tcx>,
     ) -> ErrorGuaranteed {
         let tcx = self.tcx;
-
-        if tcx.sess.opts.unstable_opts.next_solver.map(|c| c.dump_tree).unwrap_or_default()
-            == DumpSolverProofTree::OnError
-        {
-            dump_proof_tree(root_obligation, self.infcx);
-        }
-
         let mut span = obligation.cause.span;
 
         let mut err = match *error {
@@ -1529,12 +1521,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
     #[instrument(skip(self), level = "debug")]
     fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>) -> ErrorGuaranteed {
-        if self.tcx.sess.opts.unstable_opts.next_solver.map(|c| c.dump_tree).unwrap_or_default()
-            == DumpSolverProofTree::OnError
-        {
-            dump_proof_tree(&error.root_obligation, self.infcx);
-        }
-
         match error.code {
             FulfillmentErrorCode::Select(ref selection_error) => self.report_selection_error(
                 error.obligation.clone(),