Auto merge of #110811 - compiler-errors:vars-are-question-mark, r=WaffleLapkin

Use `?0` notation for ty/ct/int/float/region vars

Aligns the notation for infer vars that T-types and friends most often uses for inference variables with the notation in the compiler (which is kinda a sigil nightmare IMO: `_#`) by adopting `?0` style infer vars.

This mostly affects debug output since verbose infer vars shouldn't show up in user-facing places.

Does this need an MCP? It's debug output, so I'm thinking no, but happy to open one. 🤔

r? types

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -95,7 +95,7 @@ impl<'tcx> fmt::Debug for ty::FnSig<'tcx> {
 
 impl<'tcx> fmt::Debug for ty::ConstVid<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "_#{}c", self.index)
+        write!(f, "?{}c", self.index)
     }
 }
 

compiler/rustc_middle/src/ty/sty.rs

@@ -1436,7 +1436,7 @@ pub struct ConstVid<'tcx> {
 rustc_index::newtype_index! {
     /// A **region** (lifetime) **v**ariable **ID**.
     #[derive(HashStable)]
-    #[debug_format = "'_#{}r"]
+    #[debug_format = "'?{}"]
     pub struct RegionVid {}
 }
 

compiler/rustc_type_ir/src/lib.rs

@@ -537,7 +537,7 @@ pub struct FloatVarValue(pub FloatTy);
 
 rustc_index::newtype_index! {
     /// A **ty**pe **v**ariable **ID**.
-    #[debug_format = "_#{}t"]
+    #[debug_format = "?{}t"]
     pub struct TyVid {}
 }
 
@@ -739,13 +739,13 @@ impl fmt::Debug for FloatVarValue {
 
 impl fmt::Debug for IntVid {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "_#{}i", self.index)
+        write!(f, "?{}i", self.index)
     }
 }
 
 impl fmt::Debug for FloatVid {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "_#{}f", self.index)
+        write!(f, "?{}f", self.index)
     }
 }