Rollup merge of #122527 - fmease:clean-up-hir-ty-lowering, r=compiler…

…-errors

Clean up AstConv

Split off from #120926 to make it only contain the renaming & (doc) comment updates.
Any changes other than that which have accumulated over time are now part of this PR.
Let's be disciplined ;) Inspired by #120926 (comment).

---

* Remove `hir_trait_to_predicates`
  * Unused since #113671
* Inline `create_args_for_ast_trait_ref`
  * Only had a single call site
  * Having it as a separate method didn't gain us anything
* Use an if-let guard somewhere to avoid unwrapping
* Avoid explicit trait object lifetimes
  * More legible, stylistic-only (the updated code is 100% semantically identical)
  * Use explicitly elided lifetimes in impl headers, they get elaborated to distinct lifetimes
  * Make use of [object lifetime defaulting](https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes) for a trait object type inside of a reference type somewhere
* Use preexisting dedicated method `ItemCtxt::to_ty` over `<dyn AstConv<'_>>::ast_ty_to_ty`
* Use preexisting dedicated method `AstConv::astconv` over explicit coercions
* Simplify the function signature of `create_args_for_ast_path` and of `check_generic_arg_count`
  * In both cases redundant information was passed rendering the call sites verbose and confusing
  * No perf impact (tested in [#120926](#120926))
* Move diagnostic method `report_ambiguous_associated_type` from `astconv` to `astconv::errors`
  * The submodule `errors` exists specifically for that purpose
  * Use it to keep the main module clean & short

compiler/rustc_hir_analysis/src/astconv/errors.rs

@@ -22,7 +22,7 @@ use rustc_span::symbol::{sym, Ident};
 use rustc_span::{Span, Symbol, DUMMY_SP};
 use rustc_trait_selection::traits::object_safety_violations_for_assoc_item;
 
-impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
+impl<'tcx> dyn AstConv<'tcx> + '_ {
     /// On missing type parameters, emit an E0393 error and provide a structured suggestion using
     /// the type parameter's name as a placeholder.
     pub(crate) fn complain_about_missing_type_params(
@@ -349,6 +349,118 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         })
     }
 
+    pub(super) fn report_ambiguous_associated_type(
+        &self,
+        span: Span,
+        types: &[String],
+        traits: &[String],
+        name: Symbol,
+    ) -> ErrorGuaranteed {
+        let mut err =
+            struct_span_code_err!(self.tcx().dcx(), span, E0223, "ambiguous associated type");
+        if self
+            .tcx()
+            .resolutions(())
+            .confused_type_with_std_module
+            .keys()
+            .any(|full_span| full_span.contains(span))
+        {
+            err.span_suggestion_verbose(
+                span.shrink_to_lo(),
+                "you are looking for the module in `std`, not the primitive type",
+                "std::",
+                Applicability::MachineApplicable,
+            );
+        } else {
+            let mut types = types.to_vec();
+            types.sort();
+            let mut traits = traits.to_vec();
+            traits.sort();
+            match (&types[..], &traits[..]) {
+                ([], []) => {
+                    err.span_suggestion_verbose(
+                        span,
+                        format!(
+                            "if there were a type named `Type` that implements a trait named \
+                             `Trait` with associated type `{name}`, you could use the \
+                             fully-qualified path",
+                        ),
+                        format!("<Type as Trait>::{name}"),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                ([], [trait_str]) => {
+                    err.span_suggestion_verbose(
+                        span,
+                        format!(
+                            "if there were a type named `Example` that implemented `{trait_str}`, \
+                             you could use the fully-qualified path",
+                        ),
+                        format!("<Example as {trait_str}>::{name}"),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                ([], traits) => {
+                    err.span_suggestions(
+                        span,
+                        format!(
+                            "if there were a type named `Example` that implemented one of the \
+                             traits with associated type `{name}`, you could use the \
+                             fully-qualified path",
+                        ),
+                        traits
+                            .iter()
+                            .map(|trait_str| format!("<Example as {trait_str}>::{name}"))
+                            .collect::<Vec<_>>(),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                ([type_str], []) => {
+                    err.span_suggestion_verbose(
+                        span,
+                        format!(
+                            "if there were a trait named `Example` with associated type `{name}` \
+                             implemented for `{type_str}`, you could use the fully-qualified path",
+                        ),
+                        format!("<{type_str} as Example>::{name}"),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                (types, []) => {
+                    err.span_suggestions(
+                        span,
+                        format!(
+                            "if there were a trait named `Example` with associated type `{name}` \
+                             implemented for one of the types, you could use the fully-qualified \
+                             path",
+                        ),
+                        types
+                            .into_iter()
+                            .map(|type_str| format!("<{type_str} as Example>::{name}")),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                (types, traits) => {
+                    let mut suggestions = vec![];
+                    for type_str in types {
+                        for trait_str in traits {
+                            suggestions.push(format!("<{type_str} as {trait_str}>::{name}"));
+                        }
+                    }
+                    err.span_suggestions(
+                        span,
+                        "use fully-qualified syntax",
+                        suggestions,
+                        Applicability::MachineApplicable,
+                    );
+                }
+            }
+        }
+        let reported = err.emit();
+        self.set_tainted_by_errors(reported);
+        reported
+    }
+
     pub(crate) fn complain_about_ambiguous_inherent_assoc_type(
         &self,
         name: Ident,

compiler/rustc_hir_analysis/src/astconv/generics.rs

@@ -409,15 +409,12 @@ pub fn check_generic_arg_count_for_call(
     seg: &hir::PathSegment<'_>,
     is_method_call: IsMethodCall,
 ) -> GenericArgCountResult {
-    let empty_args = hir::GenericArgs::none();
-    let gen_args = seg.args.unwrap_or(&empty_args);
     let gen_pos = match is_method_call {
         IsMethodCall::Yes => GenericArgPosition::MethodCall,
         IsMethodCall::No => GenericArgPosition::Value,
     };
     let has_self = generics.parent.is_none() && generics.has_self;
-
-    check_generic_arg_count(tcx, def_id, seg, generics, gen_args, gen_pos, has_self, seg.infer_args)
+    check_generic_arg_count(tcx, def_id, seg, generics, gen_pos, has_self)
 }
 
 /// Checks that the correct number of generic arguments have been provided.
@@ -428,11 +425,10 @@ pub(crate) fn check_generic_arg_count(
     def_id: DefId,
     seg: &hir::PathSegment<'_>,
     gen_params: &ty::Generics,
-    gen_args: &hir::GenericArgs<'_>,
     gen_pos: GenericArgPosition,
     has_self: bool,
-    infer_args: bool,
 ) -> GenericArgCountResult {
+    let gen_args = seg.args();
     let default_counts = gen_params.own_defaults();
     let param_counts = gen_params.own_counts();
 
@@ -453,7 +449,7 @@ pub(crate) fn check_generic_arg_count(
         .count();
     let named_const_param_count = param_counts.consts - synth_const_param_count;
     let infer_lifetimes =
-        (gen_pos != GenericArgPosition::Type || infer_args) && !gen_args.has_lifetime_params();
+        (gen_pos != GenericArgPosition::Type || seg.infer_args) && !gen_args.has_lifetime_params();
 
     if gen_pos != GenericArgPosition::Type
         && let Some(b) = gen_args.bindings.first()
@@ -586,7 +582,7 @@ pub(crate) fn check_generic_arg_count(
     };
 
     let args_correct = {
-        let expected_min = if infer_args {
+        let expected_min = if seg.infer_args {
             0
         } else {
             param_counts.consts + named_type_param_count

compiler/rustc_hir_analysis/src/astconv/lint.rs

@@ -8,7 +8,7 @@ use rustc_trait_selection::traits::error_reporting::suggestions::NextTypeParamNa
 
 use super::AstConv;
 
-impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
+impl<'tcx> dyn AstConv<'tcx> + '_ {
     /// Make sure that we are in the condition to suggest the blanket implementation.
     pub(super) fn maybe_lint_blanket_trait_impl<G: EmissionGuarantee>(
         &self,