Rollup merge of #147840 - jdonszelmann:unsizing-coercions, r=lcnr

Rework unsizing coercions in the new solver

Replaces https://github.com/rust-lang/rust/pull/141926, contains:

- a commit adding tests that fail before this work
- the two commits from the previous PR
- a commit in which these tests are fixed
- finally, a fixup for an in my opinion rather large regression in diagnostics. It's still not perfect, but better?

I hope this is roughly what you had in mind

Fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/241 and https://github.com/rust-lang/trait-system-refactor-initiative/issues/238, adding tests for both

r? ````@lcnr````

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -24,7 +24,7 @@ use tracing::instrument;
 use self::derive_errors::*;
 use super::Certainty;
 use super::delegate::SolverDelegate;
-use super::inspect::{self, ProofTreeInferCtxtExt};
+use super::inspect::{self, InferCtxtProofTreeExt};
 use crate::traits::{FulfillmentError, ScrubbedTraitError};
 
 mod derive_errors;

compiler/rustc_trait_selection/src/solve/fulfill/derive_errors.rs

@@ -15,7 +15,7 @@ use rustc_next_trait_solver::solve::{GoalEvaluation, SolverDelegateEvalExt as _}
 use tracing::{instrument, trace};
 
 use crate::solve::delegate::SolverDelegate;
-use crate::solve::inspect::{self, ProofTreeInferCtxtExt, ProofTreeVisitor};
+use crate::solve::inspect::{self, InferCtxtProofTreeExt, ProofTreeVisitor};
 use crate::solve::{Certainty, deeply_normalize_for_diagnostics};
 use crate::traits::{FulfillmentError, FulfillmentErrorCode, wf};
 

compiler/rustc_trait_selection/src/solve/inspect/analyse.rs

@@ -463,7 +463,7 @@ pub trait ProofTreeVisitor<'tcx> {
     fn visit_goal(&mut self, goal: &InspectGoal<'_, 'tcx>) -> Self::Result;
 }
 
-#[extension(pub trait ProofTreeInferCtxtExt<'tcx>)]
+#[extension(pub trait InferCtxtProofTreeExt<'tcx>)]
 impl<'tcx> InferCtxt<'tcx> {
     fn visit_proof_tree<V: ProofTreeVisitor<'tcx>>(
         &self,

compiler/rustc_trait_selection/src/solve/select.rs

@@ -12,7 +12,7 @@ use rustc_middle::{bug, span_bug};
 use rustc_span::Span;
 use thin_vec::thin_vec;
 
-use crate::solve::inspect::{self, ProofTreeInferCtxtExt};
+use crate::solve::inspect::{self, InferCtxtProofTreeExt};
 
 #[extension(pub trait InferCtxtSelectExt<'tcx>)]
 impl<'tcx> InferCtxt<'tcx> {

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -29,7 +29,7 @@ use tracing::{debug, instrument, warn};
 use super::ObligationCtxt;
 use crate::error_reporting::traits::suggest_new_overflow_limit;
 use crate::infer::InferOk;
-use crate::solve::inspect::{InspectGoal, ProofTreeInferCtxtExt, ProofTreeVisitor};
+use crate::solve::inspect::{InferCtxtProofTreeExt, InspectGoal, ProofTreeVisitor};
 use crate::solve::{SolverDelegate, deeply_normalize_for_diagnostics, inspect};
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::select::IntercrateAmbiguityCause;