Clean up some things in the name resolver

* Get rid of a typo in a function name
* Rename `currently_processing_generics`: The old name confused me at first since
  I assumed it referred to generic *parameters* when it was in fact referring to
  generic *arguments*. Generics are typically short for generic params.
* Get rid of a few unwraps by properly leveraging slice patterns

compiler/rustc_resolve/src/late.rs

@@ -594,9 +594,9 @@ struct DiagnosticMetadata<'ast> {
     /// The current trait (used to suggest).
     current_item: Option<&'ast Item>,
 
-    /// When processing generics and encountering a type not found, suggest introducing a type
+    /// When processing generic arguments and encountering an unresolved ident not found,
-    /// param.
+    /// suggest introducing a type or const param depending on the context.
-    currently_processing_generics: bool,
+    currently_processing_generic_args: bool,
 
     /// The current enclosing (non-closure) function (used for better errors).
     current_function: Option<(FnKind<'ast>, Span)>,
@@ -1069,7 +1069,7 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
 
     fn visit_generic_arg(&mut self, arg: &'ast GenericArg) {
         debug!("visit_generic_arg({:?})", arg);
-        let prev = replace(&mut self.diagnostic_metadata.currently_processing_generics, true);
+        let prev = replace(&mut self.diagnostic_metadata.currently_processing_generic_args, true);
         match arg {
             GenericArg::Type(ref ty) => {
                 // We parse const arguments as path types as we cannot distinguish them during
@@ -1100,7 +1100,7 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                                 },
                             );
 
-                            self.diagnostic_metadata.currently_processing_generics = prev;
+                            self.diagnostic_metadata.currently_processing_generic_args = prev;
                             return;
                         }
                     }
@@ -1113,7 +1113,7 @@ impl<'a: 'ast, 'ast, 'tcx> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast,
                 self.resolve_anon_const(ct, AnonConstKind::ConstArg(IsRepeatExpr::No))
             }
         }
-        self.diagnostic_metadata.currently_processing_generics = prev;
+        self.diagnostic_metadata.currently_processing_generic_args = prev;
     }
 
     fn visit_assoc_constraint(&mut self, constraint: &'ast AssocConstraint) {

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -452,7 +452,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         }
 
         self.suggest_self_or_self_ref(&mut err, path, span);
-        self.detect_assoct_type_constraint_meant_as_path(&mut err, &base_error);
+        self.detect_assoc_type_constraint_meant_as_path(&mut err, &base_error);
         if self.suggest_self_ty(&mut err, source, path, span)
             || self.suggest_self_value(&mut err, source, path, span)
         {
@@ -491,7 +491,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         (err, candidates)
     }
 
-    fn detect_assoct_type_constraint_meant_as_path(
+    fn detect_assoc_type_constraint_meant_as_path(
         &self,
         err: &mut Diagnostic,
         base_error: &BaseError,
@@ -799,7 +799,9 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
             false,
         ) = (source, res, is_macro)
         {
-            if let Some(bounds @ [_, .., _]) = self.diagnostic_metadata.current_trait_object {
+            if let Some(bounds @ [first_bound, .., last_bound]) =
+                self.diagnostic_metadata.current_trait_object
+            {
                 fallback = true;
                 let spans: Vec<Span> = bounds
                     .iter()
@@ -807,9 +809,9 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                     .filter(|&sp| sp != base_error.span)
                     .collect();
 
-                let start_span = bounds[0].span();
+                let start_span = first_bound.span();
                 // `end_span` is the end of the poly trait ref (Foo + 'baz + Bar><)
-                let end_span = bounds.last().unwrap().span();
+                let end_span = last_bound.span();
                 // `last_bound_span` is the last bound of the poly trait ref (Foo + >'baz< + Bar)
                 let last_bound_span = spans.last().cloned().unwrap();
                 let mut multi_span: MultiSpan = spans.clone().into();
@@ -2419,10 +2421,10 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         let mut iter = ident.chars().map(|c| c.is_uppercase());
         let single_uppercase_char =
             matches!(iter.next(), Some(true)) && matches!(iter.next(), None);
-        if !self.diagnostic_metadata.currently_processing_generics && !single_uppercase_char {
+        if !self.diagnostic_metadata.currently_processing_generic_args && !single_uppercase_char {
             return None;
         }
-        match (self.diagnostic_metadata.current_item, single_uppercase_char, self.diagnostic_metadata.currently_processing_generics) {
+        match (self.diagnostic_metadata.current_item, single_uppercase_char, self.diagnostic_metadata.currently_processing_generic_args) {
             (Some(Item { kind: ItemKind::Fn(..), ident, .. }), _, _) if ident.name == sym::main => {
                 // Ignore `fn main()` as we don't want to suggest `fn main<T>()`
             }