diff --git a/Cargo.lock b/Cargo.lock
index 93b622627b4..69f96bcbe63 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -103,9 +103,9 @@ dependencies = [
 
 [[package]]
 name = "anyhow"
-version = "1.0.60"
+version = "1.0.65"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c794e162a5eff65c72ef524dfe393eb923c354e350bb78b9c7383df13f3bc142"
+checksum = "98161a4e3e2184da77bb14f02184cdd111e83bbbcc9979dfee3c44b9a85f5602"
 
 [[package]]
 name = "array_tool"
@@ -1362,9 +1362,9 @@ dependencies = [
 
 [[package]]
 name = "fs-err"
-version = "2.5.0"
+version = "2.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bcd1163ae48bda72a20ae26d66a04d3094135cadab911cff418ae5e33f253431"
+checksum = "64db3e262960f0662f43a6366788d5f10f7f244b8f7d7d987f560baf5ded5c50"
 
 [[package]]
 name = "fs_extra"
@@ -1894,6 +1894,12 @@ dependencies = [
 [[package]]
 name = "jsondoclint"
 version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "fs-err",
+ "rustdoc-json-types",
+ "serde_json",
+]
 
 [[package]]
 name = "jsonpath_lib"
@@ -4449,9 +4455,9 @@ dependencies = [
 
 [[package]]
 name = "serde_json"
-version = "1.0.83"
+version = "1.0.85"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "38dd04e3c8279e75b31ef29dbdceebfe5ad89f4d0937213c53f7d49d01b3d5a7"
+checksum = "e55a28e3aaef9d5ce0506d0a14dbba8054ddc7e499ef522dd8b26859ec9d4a44"
 dependencies = [
  "indexmap",
  "itoa",
diff --git a/src/tools/jsondoclint/Cargo.toml b/src/tools/jsondoclint/Cargo.toml
index 42bb77fde41..84a6c7f96c4 100644
--- a/src/tools/jsondoclint/Cargo.toml
+++ b/src/tools/jsondoclint/Cargo.toml
@@ -6,3 +6,7 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+anyhow = "1.0.62"
+fs-err = "2.8.1"
+rustdoc-json-types = { version = "0.1.0", path = "../../rustdoc-json-types" }
+serde_json = "1.0.85"
diff --git a/src/tools/jsondoclint/src/item_kind.rs b/src/tools/jsondoclint/src/item_kind.rs
new file mode 100644
index 00000000000..7c6c0b54379
--- /dev/null
+++ b/src/tools/jsondoclint/src/item_kind.rs
@@ -0,0 +1,29 @@
+use rustdoc_json_types::ItemEnum;
+
+pub(crate) fn can_appear_in_mod(kind: &ItemEnum) -> bool {
+    match kind {
+        ItemEnum::Module(_) => true,
+        ItemEnum::ExternCrate { .. } => true,
+        ItemEnum::Import(_) => true,
+        ItemEnum::Union(_) => true,
+        ItemEnum::Struct(_) => true,
+        ItemEnum::StructField(_) => false, // Only in structs or variants
+        ItemEnum::Enum(_) => true,
+        ItemEnum::Variant(_) => false, // Only in enums
+        ItemEnum::Function(_) => true,
+        ItemEnum::Trait(_) => true,
+        ItemEnum::TraitAlias(_) => true,
+        ItemEnum::Method(_) => false, // Only in traits
+        ItemEnum::Impl(_) => true,
+        ItemEnum::Typedef(_) => true,
+        ItemEnum::OpaqueTy(_) => todo!("IDK"), // On
+        ItemEnum::Constant(_) => true,
+        ItemEnum::Static(_) => true,
+        ItemEnum::ForeignType => todo!("IDK"),
+        ItemEnum::Macro(_) => true,
+        ItemEnum::ProcMacro(_) => true,
+        ItemEnum::PrimitiveType(_) => todo!("IDK"),
+        ItemEnum::AssocConst { .. } => false, // Trait Only
+        ItemEnum::AssocType { .. } => false,  // Trait only
+    }
+}
diff --git a/src/tools/jsondoclint/src/main.rs b/src/tools/jsondoclint/src/main.rs
index f328e4d9d04..b98601f7d89 100644
--- a/src/tools/jsondoclint/src/main.rs
+++ b/src/tools/jsondoclint/src/main.rs
@@ -1 +1,33 @@
-fn main() {}
+use std::env;
+
+use anyhow::{anyhow, bail, Result};
+use fs_err as fs;
+use rustdoc_json_types::{Crate, Id, FORMAT_VERSION};
+
+pub(crate) mod item_kind;
+mod validator;
+
+#[derive(Debug)]
+struct Error {
+    message: String,
+    id: Id,
+}
+
+fn main() -> Result<()> {
+    let path = env::args().nth(1).ok_or_else(|| anyhow!("no path given"))?;
+    let contents = fs::read_to_string(path)?;
+    let krate: Crate = serde_json::from_str(&contents)?;
+    assert_eq!(krate.format_version, FORMAT_VERSION);
+
+    let mut validator = validator::Validator::new(&krate);
+    validator.check_crate();
+
+    if !validator.errs.is_empty() {
+        for err in validator.errs {
+            eprintln!("`{}`: `{}`", err.id.0, err.message);
+        }
+        bail!("Errors validating json");
+    }
+
+    Ok(())
+}
diff --git a/src/tools/jsondoclint/src/validator.rs b/src/tools/jsondoclint/src/validator.rs
new file mode 100644
index 00000000000..2bb63bc3a44
--- /dev/null
+++ b/src/tools/jsondoclint/src/validator.rs
@@ -0,0 +1,391 @@
+use std::collections::HashSet;
+use std::hash::Hash;
+
+use rustdoc_json_types::{
+    Constant, Crate, DynTrait, Enum, FnDecl, Function, FunctionPointer, GenericArg, GenericArgs,
+    GenericBound, GenericParamDef, Generics, Id, Impl, Import, ItemEnum, Method, Module, OpaqueTy,
+    Path, ProcMacro, Static, Struct, StructKind, Term, Trait, TraitAlias, Type, TypeBinding,
+    TypeBindingKind, Typedef, Union, Variant, WherePredicate,
+};
+
+use crate::{item_kind::can_appear_in_mod, Error};
+
+#[derive(Debug)]
+pub struct Validator<'a> {
+    pub(crate) errs: Vec<Error>,
+    krate: &'a Crate,
+    seen_ids: HashSet<&'a Id>,
+    todo: HashSet<&'a Id>,
+}
+
+fn set_remove<T: Hash + Eq + Clone>(set: &mut HashSet<T>) -> Option<T> {
+    if let Some(id) = set.iter().next() {
+        let id = id.clone();
+        set.take(&id)
+    } else {
+        None
+    }
+}
+
+impl<'a> Validator<'a> {
+    pub fn new(krate: &'a Crate) -> Self {
+        Self { krate, errs: Vec::new(), seen_ids: HashSet::new(), todo: HashSet::new() }
+    }
+
+    pub fn check_crate(&mut self) {
+        let root = &self.krate.root;
+        self.add_mod_id(root);
+        while let Some(id) = set_remove(&mut self.todo) {
+            self.seen_ids.insert(id);
+            self.check_item(id);
+        }
+    }
+
+    fn check_item(&mut self, id: &'a Id) {
+        let item = &self.krate.index[id];
+        match &item.inner {
+            ItemEnum::Import(x) => self.check_import(x),
+            ItemEnum::Union(x) => self.check_union(x),
+            ItemEnum::Struct(x) => self.check_struct(x),
+            ItemEnum::StructField(x) => self.check_struct_field(x),
+            ItemEnum::Enum(x) => self.check_enum(x),
+            ItemEnum::Variant(x) => self.check_variant(x),
+            ItemEnum::Function(x) => self.check_function(x),
+            ItemEnum::Trait(x) => self.check_trait(x),
+            ItemEnum::TraitAlias(x) => self.check_trait_alias(x),
+            ItemEnum::Method(x) => self.check_method(x),
+            ItemEnum::Impl(x) => self.check_impl(x),
+            ItemEnum::Typedef(x) => self.check_typedef(x),
+            ItemEnum::OpaqueTy(x) => self.check_opaque_ty(x),
+            ItemEnum::Constant(x) => self.check_constant(x),
+            ItemEnum::Static(x) => self.check_static(x),
+            ItemEnum::ForeignType => todo!(),
+            ItemEnum::Macro(x) => self.check_macro(x),
+            ItemEnum::ProcMacro(x) => self.check_proc_macro(x),
+            ItemEnum::PrimitiveType(x) => self.check_primitive_type(x),
+            ItemEnum::Module(x) => self.check_module(x),
+
+            ItemEnum::ExternCrate { .. } => todo!(),
+            ItemEnum::AssocConst { .. } => todo!(),
+            ItemEnum::AssocType { .. } => todo!(),
+        }
+    }
+
+    // Core checkers
+    fn check_module(&mut self, module: &'a Module) {
+        module.items.iter().for_each(|i| self.add_mod_item_id(i));
+    }
+
+    fn check_import(&mut self, x: &'a Import) {
+        if x.glob {
+            self.add_mod_id(x.id.as_ref().unwrap());
+        } else if let Some(id) = &x.id {
+            self.add_mod_item_id(id);
+        }
+    }
+
+    fn check_union(&mut self, x: &'a Union) {
+        self.check_generics(&x.generics);
+        x.fields.iter().for_each(|i| self.add_field_id(i));
+        x.impls.iter().for_each(|i| self.add_impl_id(i));
+    }
+
+    fn check_struct(&mut self, x: &'a Struct) {
+        self.check_generics(&x.generics);
+        match &x.kind {
+            StructKind::Unit => {}
+            StructKind::Tuple(fields) => fields.iter().flatten().for_each(|f| self.add_field_id(f)),
+            StructKind::Plain { fields, fields_stripped: _ } => {
+                fields.iter().for_each(|f| self.add_field_id(f))
+            }
+        }
+        x.impls.iter().for_each(|i| self.add_impl_id(i));
+    }
+
+    fn check_struct_field(&mut self, x: &'a Type) {
+        self.check_type(x);
+    }
+
+    fn check_enum(&mut self, x: &'a Enum) {
+        self.check_generics(&x.generics);
+        x.variants.iter().for_each(|i| self.add_variant_id(i));
+        x.impls.iter().for_each(|i| self.add_impl_id(i));
+    }
+
+    fn check_variant(&mut self, x: &'a Variant) {
+        match x {
+            Variant::Plain(_discriminant) => {} // TODO: Check discriminant value parses
+            Variant::Tuple(tys) => tys.iter().flatten().for_each(|t| self.add_field_id(t)),
+            Variant::Struct { fields, fields_stripped: _ } => {
+                fields.iter().for_each(|f| self.add_field_id(f))
+            }
+        }
+    }
+
+    fn check_function(&mut self, x: &'a Function) {
+        self.check_generics(&x.generics);
+        self.check_fn_decl(&x.decl);
+    }
+
+    fn check_trait(&mut self, x: &'a Trait) {
+        self.check_generics(&x.generics);
+        x.items.iter().for_each(|i| self.add_trait_item_id(i));
+        x.bounds.iter().for_each(|i| self.check_generic_bound(i));
+        x.implementations.iter().for_each(|i| self.add_impl_id(i));
+    }
+
+    fn check_trait_alias(&mut self, x: &'a TraitAlias) {
+        self.check_generics(&x.generics);
+        x.params.iter().for_each(|i| self.check_generic_bound(i));
+    }
+
+    fn check_method(&mut self, x: &'a Method) {
+        self.check_fn_decl(&x.decl);
+        self.check_generics(&x.generics);
+    }
+
+    fn check_impl(&mut self, x: &'a Impl) {
+        self.check_generics(&x.generics);
+        if let Some(path) = &x.trait_ {
+            self.check_path(path); // TODO: Check is trait.
+        }
+        self.check_type(&x.for_);
+        x.items.iter().for_each(|i| self.add_trait_item_id(i));
+        if let Some(blanket_impl) = &x.blanket_impl {
+            self.check_type(blanket_impl)
+        }
+    }
+
+    fn check_typedef(&mut self, x: &'a Typedef) {
+        self.check_generics(&x.generics);
+        self.check_type(&x.type_);
+    }
+
+    fn check_opaque_ty(&mut self, x: &'a OpaqueTy) {
+        x.bounds.iter().for_each(|b| self.check_generic_bound(b));
+        self.check_generics(&x.generics);
+    }
+
+    fn check_constant(&mut self, x: &'a Constant) {
+        self.check_type(&x.type_);
+    }
+
+    fn check_static(&mut self, x: &'a Static) {
+        self.check_type(&x.type_);
+    }
+
+    fn check_macro(&mut self, _: &'a str) {
+        // TODO
+    }
+
+    fn check_proc_macro(&mut self, _: &'a ProcMacro) {
+        // TODO
+    }
+
+    fn check_primitive_type(&mut self, _: &'a str) {
+        // TODO
+    }
+
+    fn check_generics(&mut self, x: &'a Generics) {
+        x.params.iter().for_each(|p| self.check_generic_param_def(p));
+        x.where_predicates.iter().for_each(|w| self.check_where_predicate(w));
+    }
+
+    fn check_type(&mut self, x: &'a Type) {
+        match x {
+            Type::ResolvedPath(path) => self.check_path(path),
+            Type::DynTrait(dyn_trait) => self.check_dyn_trait(dyn_trait),
+            Type::Generic(_) => {}
+            Type::Primitive(_) => {}
+            Type::FunctionPointer(fp) => self.check_function_pointer(&**fp),
+            Type::Tuple(tys) => tys.iter().for_each(|ty| self.check_type(ty)),
+            Type::Slice(inner) => self.check_type(&**inner),
+            Type::Array { type_, len: _ } => self.check_type(&**type_),
+            Type::ImplTrait(bounds) => bounds.iter().for_each(|b| self.check_generic_bound(b)),
+            Type::Infer => {}
+            Type::RawPointer { mutable: _, type_ } => self.check_type(&**type_),
+            Type::BorrowedRef { lifetime: _, mutable: _, type_ } => self.check_type(&**type_),
+            Type::QualifiedPath { name: _, args, self_type, trait_ } => {
+                self.check_generic_args(&**args);
+                self.check_type(&**self_type);
+                self.check_path(trait_);
+            }
+        }
+    }
+
+    fn check_fn_decl(&mut self, x: &'a FnDecl) {
+        x.inputs.iter().for_each(|(_name, ty)| self.check_type(ty));
+        if let Some(output) = &x.output {
+            self.check_type(output);
+        }
+    }
+
+    fn check_generic_bound(&mut self, x: &'a GenericBound) {
+        match x {
+            GenericBound::TraitBound { trait_, generic_params, modifier: _ } => {
+                self.check_path(trait_);
+                generic_params.iter().for_each(|gpd| self.check_generic_param_def(gpd));
+            }
+            GenericBound::Outlives(_) => todo!(),
+        }
+    }
+
+    fn check_path(&mut self, x: &'a Path) {
+        self.add_id(&x.id); // TODO: What kinds are allowed here.
+        if let Some(args) = &x.args {
+            self.check_generic_args(&**args);
+        }
+    }
+
+    fn check_generic_args(&mut self, x: &'a GenericArgs) {
+        match x {
+            GenericArgs::AngleBracketed { args, bindings } => {
+                args.iter().for_each(|arg| self.check_generic_arg(arg));
+                bindings.iter().for_each(|bind| self.check_type_binding(bind));
+            }
+            GenericArgs::Parenthesized { inputs, output } => {
+                inputs.iter().for_each(|ty| self.check_type(ty));
+                if let Some(o) = output {
+                    self.check_type(o);
+                }
+            }
+        }
+    }
+
+    fn check_generic_param_def(&mut self, gpd: &'a GenericParamDef) {
+        match &gpd.kind {
+            rustdoc_json_types::GenericParamDefKind::Lifetime { outlives: _ } => {}
+            rustdoc_json_types::GenericParamDefKind::Type { bounds, default, synthetic: _ } => {
+                bounds.iter().for_each(|b| self.check_generic_bound(b));
+                if let Some(ty) = default {
+                    self.check_type(ty);
+                }
+            }
+            rustdoc_json_types::GenericParamDefKind::Const { type_, default: _ } => {
+                self.check_type(type_)
+            }
+        }
+    }
+
+    fn check_generic_arg(&mut self, arg: &'a GenericArg) {
+        match arg {
+            GenericArg::Lifetime(_) => {}
+            GenericArg::Type(ty) => self.check_type(ty),
+            GenericArg::Const(c) => self.check_constant(c),
+            GenericArg::Infer => {}
+        }
+    }
+
+    fn check_type_binding(&mut self, bind: &'a TypeBinding) {
+        self.check_generic_args(&bind.args);
+        match &bind.binding {
+            TypeBindingKind::Equality(term) => self.check_term(term),
+            TypeBindingKind::Constraint(bounds) => {
+                bounds.iter().for_each(|b| self.check_generic_bound(b))
+            }
+        }
+    }
+
+    fn check_term(&mut self, term: &'a Term) {
+        match term {
+            Term::Type(ty) => self.check_type(ty),
+            Term::Constant(con) => self.check_constant(con),
+        }
+    }
+
+    fn check_where_predicate(&mut self, w: &'a WherePredicate) {
+        match w {
+            WherePredicate::BoundPredicate { type_, bounds, generic_params } => {
+                self.check_type(type_);
+                bounds.iter().for_each(|b| self.check_generic_bound(b));
+                generic_params.iter().for_each(|gpd| self.check_generic_param_def(gpd));
+            }
+            WherePredicate::RegionPredicate { lifetime: _, bounds } => {
+                bounds.iter().for_each(|b| self.check_generic_bound(b));
+            }
+            WherePredicate::EqPredicate { lhs, rhs } => {
+                self.check_type(lhs);
+                self.check_term(rhs);
+            }
+        }
+    }
+
+    fn check_dyn_trait(&mut self, dyn_trait: &'a DynTrait) {
+        for pt in &dyn_trait.traits {
+            self.check_path(&pt.trait_);
+            pt.generic_params.iter().for_each(|gpd| self.check_generic_param_def(gpd));
+        }
+    }
+
+    fn check_function_pointer(&mut self, fp: &'a FunctionPointer) {
+        self.check_fn_decl(&fp.decl);
+        fp.generic_params.iter().for_each(|gpd| self.check_generic_param_def(gpd));
+    }
+
+    // Aux functions
+    fn add_id(&mut self, id: &'a Id) {
+        if !self.seen_ids.contains(id) {
+            self.todo.insert(id);
+        }
+    }
+
+    fn add_field_id(&mut self, id: &'a Id) {
+        let item = &self.krate.index[id];
+        if let ItemEnum::StructField(_) = item.inner {
+            self.add_id(id);
+        } else {
+            self.fail(id, "Expecting field");
+        }
+    }
+
+    fn add_mod_id(&mut self, id: &'a Id) {
+        let item = &self.krate.index[id];
+        if let ItemEnum::Module(_) = item.inner {
+            self.add_id(id);
+        } else {
+            self.fail(id, "Expecting module");
+        }
+    }
+
+    fn add_impl_id(&mut self, id: &'a Id) {
+        let item = &self.krate.index[id];
+        if let ItemEnum::StructField(_) = item.inner {
+            self.add_id(id);
+        } else {
+            self.fail(id, "Expecting impl");
+        }
+    }
+
+    fn add_variant_id(&mut self, id: &'a Id) {
+        let item = &self.krate.index[id];
+        if let ItemEnum::StructField(_) = item.inner {
+            self.add_id(id);
+        } else {
+            self.fail(id, "Expecting variant");
+        }
+    }
+
+    /// Add an Id that appeared in a trait
+    fn add_trait_item_id(&mut self, id: &'a Id) {
+        let item = &self.krate.index[id];
+        if !can_appear_in_mod(&item.inner) {
+            self.fail(id, "Expecting item that can appear in trait");
+        } else {
+            self.add_id(id);
+        }
+    }
+
+    /// Add an Id that appeared in a mod
+    fn add_mod_item_id(&mut self, id: &'a Id) {
+        let item = &self.krate.index[id];
+        if can_appear_in_mod(&item.inner) {
+            self.add_id(id);
+        } else {
+            self.fail(id, "Expecting item that can appear in trait");
+        }
+    }
+
+    fn fail(&mut self, id: &Id, msg: &str) {
+        self.errs.push(Error { id: id.clone(), message: msg.to_string() });
+    }
+}