Auto merge of #122802 - estebank:unconstrained-generic-const, r=Nadrieril

Provide structured suggestion for unconstrained generic constant

```
error: unconstrained generic constant
  --> $DIR/const-argument-if-length.rs:18:10
   |
LL |     pad: [u8; is_zst::<T>()],
   |          ^^^^^^^^^^^^^^^^^^^
   |
help: try adding a `where` bound
   |
LL | pub struct AtLeastByte<T: ?Sized> where [(); is_zst::<T>()]: {
   |                                   ++++++++++++++++++++++++++
```

Detect when the constant expression isn't `usize` and suggest casting:

```
error: unconstrained generic constant
 --> f300.rs:6:10
  |
6 |     bb::<{!N}>();
  |          ^^^^
-Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs:3539:36
  |
help: try adding a `where` bound
  |
5 | fn b<const N: bool>() where [(); {!N} as usize]: {
  |                       ++++++++++++++++++++++++++
```

Fix #122395.

Author: bors

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

@@ -3536,12 +3536,39 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     let mut err =
                         self.dcx().struct_span_err(span, "unconstrained generic constant");
                     let const_span = self.tcx.def_span(uv.def);
+
+                    let const_ty = self.tcx.type_of(uv.def).instantiate(self.tcx, uv.args);
+                    let cast = if const_ty != self.tcx.types.usize { " as usize" } else { "" };
+                    let msg = "try adding a `where` bound";
                     match self.tcx.sess.source_map().span_to_snippet(const_span) {
-                            Ok(snippet) => err.help(format!(
-                                "try adding a `where` bound using this expression: `where [(); {snippet}]:`"
-                            )),
-                            _ => err.help("consider adding a `where` bound using this expression"),
-                        };
+                        Ok(snippet) => {
+                            let code = format!("[(); {snippet}{cast}]:");
+                            let def_id = if let ObligationCauseCode::CompareImplItemObligation {
+                                trait_item_def_id,
+                                ..
+                            } = obligation.cause.code()
+                            {
+                                trait_item_def_id.as_local()
+                            } else {
+                                Some(obligation.cause.body_id)
+                            };
+                            if let Some(def_id) = def_id
+                                && let Some(generics) = self.tcx.hir().get_generics(def_id)
+                            {
+                                err.span_suggestion_verbose(
+                                    generics.tail_span_for_predicate_suggestion(),
+                                    msg,
+                                    format!("{} {code}", generics.add_where_or_trailing_comma()),
+                                    Applicability::MaybeIncorrect,
+                                );
+                            } else {
+                                err.help(format!("{msg}: where {code}"));
+                            };
+                        }
+                        _ => {
+                            err.help(msg);
+                        }
+                    };
                     Ok(err)
                 }
                 ty::ConstKind::Expr(_) => {

tests/ui/const-generics/const-argument-if-length.full.stderr

@@ -4,7 +4,10 @@ error: unconstrained generic constant
 LL |     pad: [u8; is_zst::<T>()],
    |          ^^^^^^^^^^^^^^^^^^^
    |
-   = help: try adding a `where` bound using this expression: `where [(); is_zst::<T>()]:`
+help: try adding a `where` bound
+   |
+LL | pub struct AtLeastByte<T: ?Sized> where [(); is_zst::<T>()]: {
+   |                                   ++++++++++++++++++++++++++
 
 error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> $DIR/const-argument-if-length.rs:16:12