Rollup merge of rust-lang#122197 - lcnr:proof-tree-braces, r=BoxyUwU

inspect formatter: add braces

This makes it easier to hide subtrees when looking through proof trees. Looking at the proof tree for `usize: Clone`:

nightly:
```
ROOT GOAL: Goal { predicate: Binder { value: TraitPredicate(<usize as std::clone::Clone>, polarity:Positive), bound_vars: [] }, param_env: ParamEnv { caller_bounds: [], reveal: UserFacing } }
    GOAL: Canonical { value: QueryInput { goal: Goal { predicate: Binder { value: TraitPredicate(<usize as std::clone::Clone>, polarity:Positive), bound_vars: [] }, param_env: ParamEnv { caller_bounds: [], reveal: UserFacing } }, anchor: Bind(DefId(0:5 ~ main[8e4b]::main)), predefined_opaques_in_body: PredefinedOpaques(PredefinedOpaquesData { opaque_types: [] }) }, max_universe: U0, variables: [] }
    REVISION 0
        INSTANTIATED: QueryInput { goal: Goal { predicate: Binder { value: TraitPredicate(<usize as std::clone::Clone>, polarity:Positive), bound_vars: [] }, param_env: ParamEnv { caller_bounds: [], reveal: UserFacing } }, anchor: Bind(DefId(0:5 ~ main[8e4b]::main)), predefined_opaques_in_body: PredefinedOpaques(PredefinedOpaquesData { opaque_types: [] }) }
        ROOT RESULT: Ok(Canonical { value: Response { certainty: Yes, var_values: CanonicalVarValues { var_values: [] }, external_constraints: ExternalConstraints(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }) }, max_universe: U0, variables: [] })
            CANDIDATE Impl(DefId(2:30665 ~ core[a9f5]::clone::impls::{impl#5})): Ok(Canonical { value: Response { certainty: Yes, var_values: CanonicalVarValues { var_values: [] }, external_constraints: ExternalConstraints(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }) }, max_universe: U0, variables: [] })
                TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                ITERATION 0
            CANDIDATE constituent tys: Err(NoSolution)
            NORMALIZING SELF TY FOR ASSEMBLY:
            NORMALIZING SELF TY FOR ASSEMBLY:
    RESULT: Ok(Canonical { value: Response { certainty: Yes, var_values: CanonicalVarValues { var_values: [] }, external_constraints: ExternalConstraints(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }) }, max_universe: U0, variables: [] })
```

with this PR:
```
ROOT GOAL: Goal { predicate: Binder { value: TraitPredicate(<usize as std::clone::Clone>, polarity:Positive), bound_vars: [] }, param_env: ParamEnv { caller_bounds: [], reveal: UserFacing } } {
    GOAL: Canonical { value: QueryInput { goal: Goal { predicate: Binder { value: TraitPredicate(<usize as std::clone::Clone>, polarity:Positive), bound_vars: [] }, param_env: ParamEnv { caller_bounds: [], reveal: UserFacing } }, anchor: Bind(DefId(0:5 ~ main[4f74]::main)), predefined_opaques_in_body: PredefinedOpaques(PredefinedOpaquesData { opaque_types: [] }) }, max_universe: U0, variables: [] }
    REVISION 0: {
        INSTANTIATED: QueryInput { goal: Goal { predicate: Binder { value: TraitPredicate(<usize as std::clone::Clone>, polarity:Positive), bound_vars: [] }, param_env: ParamEnv { caller_bounds: [], reveal: UserFacing } }, anchor: Bind(DefId(0:5 ~ main[4f74]::main)), predefined_opaques_in_body: PredefinedOpaques(PredefinedOpaquesData { opaque_types: [] }) }
        ROOT RESULT: Ok(Canonical { value: Response { certainty: Yes, var_values: CanonicalVarValues { var_values: [] }, external_constraints: ExternalConstraints(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }) }, max_universe: U0, variables: [] }) {
            CANDIDATE Impl(DefId(2:30468 ~ core[d1d7]::clone::impls::{impl#5})): Ok(Canonical { value: Response { certainty: Yes, var_values: CanonicalVarValues { var_values: [] }, external_constraints: ExternalConstraints(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }) }, max_universe: U0, variables: [] }) {
                TRY_EVALUATE_ADDED_GOALS: Ok(Yes)
                ITERATION 0 {}
            }
            CANDIDATE constituent tys: Err(NoSolution) {}
            NORMALIZING SELF TY FOR ASSEMBLY: {}
            NORMALIZING SELF TY FOR ASSEMBLY: {}
        }
    }
    RESULT: Ok(Canonical { value: Response { certainty: Yes, var_values: CanonicalVarValues { var_values: [] }, external_constraints: ExternalConstraints(ExternalConstraintsData { region_constraints: QueryRegionConstraints { outlives: [], member_constraints: [] }, opaque_types: [] }) }, max_universe: U0, variables: [] })
}
```

r? `@BoxyUwU`

compiler/rustc_middle/src/traits/solve/inspect/format.rs

@@ -4,22 +4,31 @@ pub(super) struct ProofTreeFormatter<'a, 'b> {
     f: &'a mut (dyn Write + 'b),
 }
 
+enum IndentorState {
+    StartWithNewline,
+    OnNewline,
+    Inline,
+}
+
 /// A formatter which adds 4 spaces of indentation to its input before
 /// passing it on to its nested formatter.
 ///
 /// We can use this for arbitrary levels of indentation by nesting it.
 struct Indentor<'a, 'b> {
     f: &'a mut (dyn Write + 'b),
-    on_newline: bool,
+    state: IndentorState,
 }
 
 impl Write for Indentor<'_, '_> {
     fn write_str(&mut self, s: &str) -> std::fmt::Result {
         for line in s.split_inclusive('\n') {
-            if self.on_newline {
-                self.f.write_str("    ")?;
+            match self.state {
+                IndentorState::StartWithNewline => self.f.write_str("\n    ")?,
+                IndentorState::OnNewline => self.f.write_str("    ")?,
+                IndentorState::Inline => {}
             }
-            self.on_newline = line.ends_with('\n');
+            self.state =
+                if line.ends_with('\n') { IndentorState::OnNewline } else { IndentorState::Inline };
             self.f.write_str(line)?;
         }
 
@@ -32,11 +41,15 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
         ProofTreeFormatter { f }
     }
 
-    fn nested<F, R>(&mut self, func: F) -> R
+    fn nested<F>(&mut self, func: F) -> std::fmt::Result
     where
-        F: FnOnce(&mut ProofTreeFormatter<'_, '_>) -> R,
+        F: FnOnce(&mut ProofTreeFormatter<'_, '_>) -> std::fmt::Result,
     {
-        func(&mut ProofTreeFormatter { f: &mut Indentor { f: self.f, on_newline: true } })
+        write!(self.f, " {{")?;
+        func(&mut ProofTreeFormatter {
+            f: &mut Indentor { f: self.f, state: IndentorState::StartWithNewline },
+        })?;
+        writeln!(self.f, "}}")
     }
 
     pub(super) fn format_goal_evaluation(&mut self, eval: &GoalEvaluation<'_>) -> std::fmt::Result {
@@ -47,7 +60,7 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
                 IsNormalizesToHack::Yes => "NORMALIZES-TO HACK GOAL",
             },
         };
-        writeln!(self.f, "{}: {:?}", goal_text, eval.uncanonicalized_goal)?;
+        write!(self.f, "{}: {:?}", goal_text, eval.uncanonicalized_goal)?;
         self.nested(|this| this.format_canonical_goal_evaluation(&eval.evaluation))
     }
 
@@ -69,7 +82,7 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
             }
             CanonicalGoalEvaluationKind::Evaluation { revisions } => {
                 for (n, step) in revisions.iter().enumerate() {
-                    writeln!(self.f, "REVISION {n}")?;
+                    write!(self.f, "REVISION {n}")?;
                     self.nested(|this| this.format_evaluation_step(step))?;
                 }
                 writeln!(self.f, "RESULT: {:?}", eval.result)
@@ -88,25 +101,25 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
     pub(super) fn format_probe(&mut self, probe: &Probe<'_>) -> std::fmt::Result {
         match &probe.kind {
             ProbeKind::Root { result } => {
-                writeln!(self.f, "ROOT RESULT: {result:?}")
+                write!(self.f, "ROOT RESULT: {result:?}")
             }
             ProbeKind::NormalizedSelfTyAssembly => {
-                writeln!(self.f, "NORMALIZING SELF TY FOR ASSEMBLY:")
+                write!(self.f, "NORMALIZING SELF TY FOR ASSEMBLY:")
             }
             ProbeKind::UnsizeAssembly => {
-                writeln!(self.f, "ASSEMBLING CANDIDATES FOR UNSIZING:")
+                write!(self.f, "ASSEMBLING CANDIDATES FOR UNSIZING:")
             }
             ProbeKind::UpcastProjectionCompatibility => {
-                writeln!(self.f, "PROBING FOR PROJECTION COMPATIBILITY FOR UPCASTING:")
+                write!(self.f, "PROBING FOR PROJECTION COMPATIBILITY FOR UPCASTING:")
             }
             ProbeKind::CommitIfOk => {
-                writeln!(self.f, "COMMIT_IF_OK:")
+                write!(self.f, "COMMIT_IF_OK:")
             }
             ProbeKind::MiscCandidate { name, result } => {
-                writeln!(self.f, "CANDIDATE {name}: {result:?}")
+                write!(self.f, "CANDIDATE {name}: {result:?}")
             }
             ProbeKind::TraitCandidate { source, result } => {
-                writeln!(self.f, "CANDIDATE {source:?}: {result:?}")
+                write!(self.f, "CANDIDATE {source:?}: {result:?}")
             }
         }?;
 
@@ -137,7 +150,7 @@ impl<'a, 'b> ProofTreeFormatter<'a, 'b> {
         writeln!(self.f, "TRY_EVALUATE_ADDED_GOALS: {:?}", added_goals_evaluation.result)?;
 
         for (n, iterations) in added_goals_evaluation.evaluations.iter().enumerate() {
-            writeln!(self.f, "ITERATION {n}")?;
+            write!(self.f, "ITERATION {n}")?;
             self.nested(|this| {
                 for goal_evaluation in iterations {
                     this.format_goal_evaluation(goal_evaluation)?;