Suggest replacing typeof(...) with an actual type

This commit suggests replacing typeof(...) with an actual type of "...",
for example in case of `typeof(1)` we suggest replacing it with `i32`.

If the expression
- Is not const (`{ let a = 1; let _: typeof(a); }`)
- Can't be found (`let _: typeof(this_variable_does_not_exist)`)
- Or has non-suggestable type (closure, generator, error, etc)
we don't suggest anything.

Author: WaffleLapkin

compiler/rustc_error_messages/locales/en-US/diagnostics.ftl

@@ -62,6 +62,7 @@ typeck-functional-record-update-on-non-struct =
 
 typeck-typeof-reserved-keyword-used =
     `typeof` is a reserved keyword but unimplemented
+    .suggestion = consider replacing `typeof(...)` with an actual type
     .label = reserved keyword
 
 typeck-return-stmt-outside-of-fn-body =

compiler/rustc_typeck/src/astconv/mod.rs

@@ -2462,8 +2462,16 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 self.normalize_ty(ast_ty.span, array_ty)
             }
             hir::TyKind::Typeof(ref e) => {
-                tcx.sess.emit_err(TypeofReservedKeywordUsed { span: ast_ty.span });
-                tcx.type_of(tcx.hir().local_def_id(e.hir_id))
+                let ty = tcx.type_of(tcx.hir().local_def_id(e.hir_id));
+                let span = ast_ty.span;
+                tcx.sess.emit_err(TypeofReservedKeywordUsed {
+                    span,
+                    ty,
+                    opt_sugg: Some((span, Applicability::MachineApplicable))
+                        .filter(|_| ty.is_suggestable()),
+                });
+
+                ty
             }
             hir::TyKind::Infer => {
                 // Infer also appears as the type of arguments or return

compiler/rustc_typeck/src/errors.rs

@@ -1,5 +1,7 @@
 //! Errors emitted by typeck.
+use rustc_errors::Applicability;
 use rustc_macros::SessionDiagnostic;
+use rustc_middle::ty::Ty;
 use rustc_span::{symbol::Ident, Span, Symbol};
 
 #[derive(SessionDiagnostic)]
@@ -127,10 +129,13 @@ pub struct FunctionalRecordUpdateOnNonStruct {
 
 #[derive(SessionDiagnostic)]
 #[error(code = "E0516", slug = "typeck-typeof-reserved-keyword-used")]
-pub struct TypeofReservedKeywordUsed {
+pub struct TypeofReservedKeywordUsed<'tcx> {
+    pub ty: Ty<'tcx>,
     #[primary_span]
     #[label]
     pub span: Span,
+    #[suggestion_verbose(message = "suggestion", code = "{ty}")]
+    pub opt_sugg: Option<(Span, Applicability)>,
 }
 
 #[derive(SessionDiagnostic)]