(Mostly) revert "Account for type param from other item in `note_and_explain`"

This mostly reverts commit 7449478c2f6fd2d72c12a51d8562f1e6108facab.
It also removes an `opt_param_at` that really is unnecessary given our
ICE policy for malformed intrinsics.

3 files changed, 45 insertions, 93 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/intrinsic.rs b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
index 00ff470a0a7..7b9a5b84b43 100644
--- a/compiler/rustc_hir_analysis/src/check/intrinsic.rs
+++ b/compiler/rustc_hir_analysis/src/check/intrinsic.rs
@@ -164,9 +164,8 @@ pub fn check_intrinsic_type(
 ) {
     let generics = tcx.generics_of(intrinsic_id);
     let param = |n| {
-        if let Some(&ty::GenericParamDef {
-            name, kind: ty::GenericParamDefKind::Type { .. }, ..
-        }) = generics.opt_param_at(n as usize, tcx)
+        if let &ty::GenericParamDef { name, kind: ty::GenericParamDefKind::Type { .. }, .. } =
+            generics.param_at(n as usize, tcx)
         {
             Ty::new_param(tcx, n, name)
         } else {
diff --git a/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs b/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs
index c24ad1fa1e7..7ccb78a7d0b 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs
@@ -54,17 +54,13 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     }
                     (ty::Param(expected), ty::Param(found)) => {
                         let generics = tcx.generics_of(body_owner_def_id);
-                        if let Some(param) = generics.opt_type_param(expected, tcx) {
-                            let e_span = tcx.def_span(param.def_id);
-                            if !sp.contains(e_span) {
-                                diag.span_label(e_span, "expected type parameter");
-                            }
+                        let e_span = tcx.def_span(generics.type_param(expected, tcx).def_id);
+                        if !sp.contains(e_span) {
+                            diag.span_label(e_span, "expected type parameter");
                         }
-                        if let Some(param) = generics.opt_type_param(found, tcx) {
-                            let f_span = tcx.def_span(param.def_id);
-                            if !sp.contains(f_span) {
-                                diag.span_label(f_span, "found type parameter");
-                            }
+                        let f_span = tcx.def_span(generics.type_param(found, tcx).def_id);
+                        if !sp.contains(f_span) {
+                            diag.span_label(f_span, "found type parameter");
                         }
                         diag.note(
                             "a type parameter was expected, but a different one was found; \
@@ -87,29 +83,22 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     | (ty::Alias(ty::Projection, proj), ty::Param(p))
                         if !tcx.is_impl_trait_in_trait(proj.def_id) =>
                     {
-                        let parent = tcx
-                            .generics_of(body_owner_def_id)
-                            .opt_type_param(p, tcx)
-                            .and_then(|param| {
-                                let p_def_id = param.def_id;
-                                let p_span = tcx.def_span(p_def_id);
-                                let expected = match (values.expected.kind(), values.found.kind()) {
-                                    (ty::Param(_), _) => "expected ",
-                                    (_, ty::Param(_)) => "found ",
-                                    _ => "",
-                                };
-                                if !sp.contains(p_span) {
-                                    diag.span_label(
-                                        p_span,
-                                        format!("{expected}this type parameter"),
-                                    );
-                                }
-                                p_def_id.as_local().and_then(|id| {
-                                    let local_id = tcx.local_def_id_to_hir_id(id);
-                                    let generics = tcx.parent_hir_node(local_id).generics()?;
-                                    Some((id, generics))
-                                })
-                            });
+                        let param = tcx.generics_of(body_owner_def_id).type_param(p, tcx);
+                        let p_def_id = param.def_id;
+                        let p_span = tcx.def_span(p_def_id);
+                        let expected = match (values.expected.kind(), values.found.kind()) {
+                            (ty::Param(_), _) => "expected ",
+                            (_, ty::Param(_)) => "found ",
+                            _ => "",
+                        };
+                        if !sp.contains(p_span) {
+                            diag.span_label(p_span, format!("{expected}this type parameter"));
+                        }
+                        let parent = p_def_id.as_local().and_then(|id| {
+                            let local_id = tcx.local_def_id_to_hir_id(id);
+                            let generics = tcx.parent_hir_node(local_id).generics()?;
+                            Some((id, generics))
+                        });
                         let mut note = true;
                         if let Some((local_id, generics)) = parent {
                             // Synthesize the associated type restriction `Add<Output = Expected>`.
@@ -183,16 +172,14 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     (ty::Param(p), ty::Dynamic(..) | ty::Alias(ty::Opaque, ..))
                     | (ty::Dynamic(..) | ty::Alias(ty::Opaque, ..), ty::Param(p)) => {
                         let generics = tcx.generics_of(body_owner_def_id);
-                        if let Some(param) = generics.opt_type_param(p, tcx) {
-                            let p_span = tcx.def_span(param.def_id);
-                            let expected = match (values.expected.kind(), values.found.kind()) {
-                                (ty::Param(_), _) => "expected ",
-                                (_, ty::Param(_)) => "found ",
-                                _ => "",
-                            };
-                            if !sp.contains(p_span) {
-                                diag.span_label(p_span, format!("{expected}this type parameter"));
-                            }
+                        let p_span = tcx.def_span(generics.type_param(p, tcx).def_id);
+                        let expected = match (values.expected.kind(), values.found.kind()) {
+                            (ty::Param(_), _) => "expected ",
+                            (_, ty::Param(_)) => "found ",
+                            _ => "",
+                        };
+                        if !sp.contains(p_span) {
+                            diag.span_label(p_span, format!("{expected}this type parameter"));
                         }
                         diag.help("type parameters must be constrained to match other types");
                         if tcx.sess.teach(diag.code.unwrap()) {
@@ -233,11 +220,9 @@ impl<T> Trait<T> for X {
                         ty::Closure(..) | ty::CoroutineClosure(..) | ty::Coroutine(..),
                     ) => {
                         let generics = tcx.generics_of(body_owner_def_id);
-                        if let Some(param) = generics.opt_type_param(p, tcx) {
-                            let p_span = tcx.def_span(param.def_id);
-                            if !sp.contains(p_span) {
-                                diag.span_label(p_span, "expected this type parameter");
-                            }
+                        let p_span = tcx.def_span(generics.type_param(p, tcx).def_id);
+                        if !sp.contains(p_span) {
+                            diag.span_label(p_span, "expected this type parameter");
                         }
                         diag.help(format!(
                             "every closure has a distinct type and so could not always match the \
@@ -246,16 +231,14 @@ impl<T> Trait<T> for X {
                     }
                     (ty::Param(p), _) | (_, ty::Param(p)) => {
                         let generics = tcx.generics_of(body_owner_def_id);
-                        if let Some(param) = generics.opt_type_param(p, tcx) {
-                            let p_span = tcx.def_span(param.def_id);
-                            let expected = match (values.expected.kind(), values.found.kind()) {
-                                (ty::Param(_), _) => "expected ",
-                                (_, ty::Param(_)) => "found ",
-                                _ => "",
-                            };
-                            if !sp.contains(p_span) {
-                                diag.span_label(p_span, format!("{expected}this type parameter"));
-                            }
+                        let p_span = tcx.def_span(generics.type_param(p, tcx).def_id);
+                        let expected = match (values.expected.kind(), values.found.kind()) {
+                            (ty::Param(_), _) => "expected ",
+                            (_, ty::Param(_)) => "found ",
+                            _ => "",
+                        };
+                        if !sp.contains(p_span) {
+                            diag.span_label(p_span, format!("{expected}this type parameter"));
                         }
                     }
                     (ty::Alias(ty::Projection | ty::Inherent, proj_ty), _)
@@ -545,10 +528,8 @@ impl<T> Trait<T> for X {
             return false;
         };
         let generics = tcx.generics_of(body_owner_def_id);
-        let Some(param) = generics.opt_type_param(param_ty, tcx) else {
-            return false;
-        };
-        let Some(def_id) = param.def_id.as_local() else {
+        let def_id = generics.type_param(param_ty, tcx).def_id;
+        let Some(def_id) = def_id.as_local() else {
             return false;
         };
 
diff --git a/compiler/rustc_middle/src/ty/generics.rs b/compiler/rustc_middle/src/ty/generics.rs
index 04655c5d20b..0b447b28cd4 100644
--- a/compiler/rustc_middle/src/ty/generics.rs
+++ b/compiler/rustc_middle/src/ty/generics.rs
@@ -244,20 +244,6 @@ impl<'tcx> Generics {
         }
     }
 
-    /// Returns the `GenericParamDef` with the given index if available.
-    pub fn opt_param_at(
-        &'tcx self,
-        param_index: usize,
-        tcx: TyCtxt<'tcx>,
-    ) -> Option<&'tcx GenericParamDef> {
-        if let Some(index) = param_index.checked_sub(self.parent_count) {
-            self.own_params.get(index)
-        } else {
-            tcx.generics_of(self.parent.expect("parent_count > 0 but no parent?"))
-                .opt_param_at(param_index, tcx)
-        }
-    }
-
     pub fn params_to(&'tcx self, param_index: usize, tcx: TyCtxt<'tcx>) -> &'tcx [GenericParamDef] {
         if let Some(index) = param_index.checked_sub(self.parent_count) {
             &self.own_params[..index]
@@ -289,20 +275,6 @@ impl<'tcx> Generics {
         }
     }
 
-    /// Returns the `GenericParamDef` associated with this `ParamTy` if it belongs to this
-    /// `Generics`.
-    pub fn opt_type_param(
-        &'tcx self,
-        param: ParamTy,
-        tcx: TyCtxt<'tcx>,
-    ) -> Option<&'tcx GenericParamDef> {
-        let param = self.opt_param_at(param.index as usize, tcx)?;
-        match param.kind {
-            GenericParamDefKind::Type { .. } => Some(param),
-            _ => None,
-        }
-    }
-
     /// Returns the `GenericParamDef` associated with this `ParamConst`.
     pub fn const_param(&'tcx self, param: ParamConst, tcx: TyCtxt<'tcx>) -> &GenericParamDef {
         let param = self.param_at(param.index as usize, tcx);