core/model: introduce `Model<'ctx>` abstraction

nac3core/src/codegen/mod.rs

@@ -41,6 +41,7 @@ pub mod extern_fns;
 mod generator;
 pub mod irrt;
 pub mod llvm_intrinsics;
+pub mod model;
 pub mod numpy;
 pub mod stmt;
 

nac3core/src/codegen/model/core.rs

@@ -0,0 +1,92 @@
+use inkwell::{
+    context::Context,
+    types::{AnyTypeEnum, BasicTypeEnum},
+    values::{AnyValue, AnyValueEnum, BasicValueEnum},
+};
+
+use crate::codegen::CodeGenContext;
+
+use super::{slice::ArraySlice, Int, Pointer};
+
+/// A value that belongs to/produced by a [`Model<'ctx>`]
+pub trait ModelValue<'ctx>: Clone + Copy {
+    fn get_llvm_value(&self) -> BasicValueEnum<'ctx>;
+}
+
+// Should have been within [`Model<'ctx>`],
+// but rust object safety requirements made it necessary to
+// split this interface out
+pub trait CanCheckLLVMType<'ctx> {
+    /// Check if `scrutinee` matches the same LLVM type of this [`Model<'ctx>`].
+    ///
+    /// If they don't not match, a human-readable error message is returned.
+    fn check_llvm_type(
+        &self,
+        ctx: &'ctx Context,
+        scrutinee: AnyTypeEnum<'ctx>,
+    ) -> Result<(), String>;
+}
+
+/// A [`Model`] is a type-safe concrete representation of a complex LLVM type.
+pub trait Model<'ctx>: Clone + Copy + CanCheckLLVMType<'ctx> + Sized {
+    /// The values that inhabit this [`Model<'ctx>`].
+    ///
+    /// ...that is the type of wrapper that wraps the LLVM values that inhabit [`Model<'ctx>::get_llvm_type()`].
+    type Value: ModelValue<'ctx>;
+
+    /// Get the [`BasicTypeEnum<'ctx>`] this [`Model<'ctx>`] is representing.
+    fn get_llvm_type(&self, ctx: &'ctx Context) -> BasicTypeEnum<'ctx>;
+
+    /// Cast an [`AnyValueEnum<'ctx>`] into [`Self::Value`].
+    ///
+    /// Panics if `value` cannot pass [`CanCheckLLVMType::check_llvm_type()`].
+    fn review(&self, ctx: &'ctx Context, value: AnyValueEnum<'ctx>) -> Self::Value;
+
+    /// Check if [`Self::Value`] has the correct type described by this [`Model<'ctx>`]
+    ///
+    /// For example:
+    /// ```ignore
+    /// let ctx: &CodeGenContext<'ctx, '_>;
+    /// let my_i32 = IntModel(ctx.ctx.i32_type());
+    /// let my_i64 = IntModel(ctx.ctx.i64_type());
+    /// let value1 = my_i32.constant(3);
+    /// let value2 = my_i64.constant(3);
+    /// // Both value1 and value2 have type `IntModel<'ctx>`!
+    /// // There is no type constraints to tell which value has what int type.
+    /// my_i32.check(value1); // ok
+    /// my_i64.check(value2); // ok
+    ///
+    /// my_i32.check(value2); // PANIC
+    /// my_i64.check(value1); // PANIC
+    /// ```
+    fn check(&self, ctx: &'ctx Context, value: Self::Value) {
+        self.review(ctx, value.get_llvm_value().as_any_value_enum());
+    }
+
+    /// Build an instruction to allocate a value of [`Model::get_llvm_type`].
+    fn alloca(&self, ctx: &CodeGenContext<'ctx, '_>, name: &str) -> Pointer<'ctx, Self> {
+        Pointer {
+            element: *self,
+            value: ctx.builder.build_alloca(self.get_llvm_type(ctx.ctx), name).unwrap(),
+        }
+    }
+
+    /// Build an instruction to allocate an array of [`Model::get_llvm_type`].
+    fn array_alloca(
+        &self,
+        ctx: &CodeGenContext<'ctx, '_>,
+        count: Int<'ctx>,
+        name: &str,
+    ) -> ArraySlice<'ctx, Self> {
+        ArraySlice {
+            num_elements: count,
+            pointer: Pointer {
+                element: *self,
+                value: ctx
+                    .builder
+                    .build_array_alloca(self.get_llvm_type(ctx.ctx), count.0, name)
+                    .unwrap(),
+            },
+        }
+    }
+}

nac3core/src/codegen/model/fixed_int.rs

@@ -0,0 +1,159 @@
+use inkwell::{
+    context::Context,
+    types::{AnyTypeEnum, BasicType, BasicTypeEnum, IntType},
+    values::{AnyValueEnum, BasicValue, BasicValueEnum, IntValue},
+};
+
+use crate::codegen::CodeGenContext;
+
+use super::{
+    core::*,
+    int_util::{check_int_llvm_type, review_int_llvm_value},
+    Int, IntModel,
+};
+
+/// A marker trait to mark singleton struct that describes a particular fixed integer type.
+/// See [`Bool`], [`Byte`], [`Int32`], etc.
+///
+/// The [`Default`] trait is to enable auto-derivations for utilities like
+/// [`FieldBuilder::add_field_auto`]
+pub trait IsFixedInt: Clone + Copy + Default {
+    fn get_int_type(ctx: &Context) -> IntType<'_>;
+    fn get_bit_width() -> u32; // This is required, instead of only relying on get_int_type
+
+    fn constant<'ctx>(&self, ctx: &'ctx Context, value: u64) -> FixedInt<'ctx, Self> {
+        FixedInt { int: *self, value: Self::get_int_type(ctx).const_int(value, false) }
+    }
+}
+
+// Some pre-defined fixed integers
+
+#[derive(Debug, Clone, Copy, Default)]
+pub struct Bool;
+pub type BoolModel = FixedIntModel<Bool>;
+
+impl IsFixedInt for Bool {
+    fn get_int_type(ctx: &Context) -> IntType<'_> {
+        ctx.bool_type()
+    }
+
+    fn get_bit_width() -> u32 {
+        1
+    }
+}
+
+#[derive(Debug, Clone, Copy, Default)]
+pub struct Byte;
+pub type ByteModel = FixedIntModel<Byte>;
+
+impl IsFixedInt for Byte {
+    fn get_int_type(ctx: &Context) -> IntType<'_> {
+        ctx.i8_type()
+    }
+
+    fn get_bit_width() -> u32 {
+        8
+    }
+}
+
+#[derive(Debug, Clone, Copy, Default)]
+pub struct Int32;
+pub type Int32Model = FixedIntModel<Int32>;
+
+impl IsFixedInt for Int32 {
+    fn get_int_type(ctx: &Context) -> IntType<'_> {
+        ctx.i32_type()
+    }
+
+    fn get_bit_width() -> u32 {
+        32
+    }
+}
+
+#[derive(Debug, Clone, Copy, Default)]
+pub struct Int64;
+pub type Int64Model = FixedIntModel<Int64>;
+
+impl IsFixedInt for Int64 {
+    fn get_int_type(ctx: &Context) -> IntType<'_> {
+        ctx.i64_type()
+    }
+
+    fn get_bit_width() -> u32 {
+        64
+    }
+}
+
+/// A model representing a compile-time known [`IntType<'ctx>`].
+///
+/// Also see [`IntModel`], which is less constrained than [`FixedIntModel`],
+/// but enables one to handle [`IntType<'ctx>`] that could be dynamic
+#[derive(Debug, Clone, Copy, Default)]
+pub struct FixedIntModel<T>(pub T);
+
+// FixedIntModel's implementation
+
+impl<'ctx, T: IsFixedInt> CanCheckLLVMType<'ctx> for FixedIntModel<T> {
+    fn check_llvm_type(
+        &self,
+        ctx: &'ctx Context,
+        scrutinee: AnyTypeEnum<'ctx>,
+    ) -> Result<(), String> {
+        check_int_llvm_type(scrutinee, T::get_int_type(ctx))
+    }
+}
+
+impl<'ctx, T: IsFixedInt> Model<'ctx> for FixedIntModel<T> {
+    type Value = FixedInt<'ctx, T>;
+
+    fn get_llvm_type(&self, ctx: &'ctx Context) -> BasicTypeEnum<'ctx> {
+        T::get_int_type(ctx).as_basic_type_enum()
+    }
+
+    fn review(&self, ctx: &'ctx Context, value: AnyValueEnum<'ctx>) -> Self::Value {
+        let value = review_int_llvm_value(value, T::get_int_type(ctx)).unwrap();
+        FixedInt { int: self.0, value }
+    }
+}
+
+impl<T: IsFixedInt> FixedIntModel<T> {
+    pub fn to_int_model(self, ctx: &Context) -> IntModel<'_> {
+        IntModel(T::get_int_type(ctx))
+    }
+}
+
+/// An inhabitant of [`FixedIntModel<'ctx>`]
+#[derive(Debug, Clone, Copy)]
+pub struct FixedInt<'ctx, T: IsFixedInt> {
+    pub int: T,
+    pub value: IntValue<'ctx>,
+}
+
+// FixedInt's Implementation
+
+impl<'ctx, T: IsFixedInt> ModelValue<'ctx> for FixedInt<'ctx, T> {
+    fn get_llvm_value(&self) -> BasicValueEnum<'ctx> {
+        self.value.as_basic_value_enum()
+    }
+}
+
+impl<'ctx, T: IsFixedInt> FixedInt<'ctx, T> {
+    pub fn to_int(self) -> Int<'ctx> {
+        Int(self.value)
+    }
+
+    pub fn signed_cast_to_fixed<R: IsFixedInt>(
+        self,
+        ctx: &CodeGenContext<'ctx, '_>,
+        target_fixed_int: R,
+        name: &str,
+    ) -> FixedInt<'ctx, R> {
+        FixedInt {
+            int: target_fixed_int,
+            value: ctx
+                .builder
+                .build_int_s_extend_or_bit_cast(self.value, R::get_int_type(ctx.ctx), name)
+                .unwrap(),
+        }
+    }
+}

nac3core/src/codegen/model/int.rs

@@ -0,0 +1,97 @@
+use inkwell::{
+    context::Context,
+    types::{AnyType, AnyTypeEnum, BasicType, BasicTypeEnum, IntType},
+    values::{AnyValueEnum, BasicValue, BasicValueEnum, IntValue},
+};
+
+use crate::codegen::CodeGenContext;
+
+use super::{core::*, int_util::check_int_llvm_type, FixedInt, IsFixedInt};
+
+/// A model representing an [`IntType<'ctx>`].
+///
+/// Also see [`FixedIntModel`], which is more constrained than [`IntModel`]
+/// but provides more type-safe mechanisms and even auto-derivation of [`BasicTypeEnum<'ctx>`]
+/// for creating LLVM structures.
+#[derive(Debug, Clone, Copy)]
+pub struct IntModel<'ctx>(pub IntType<'ctx>);
+
+impl<'ctx> CanCheckLLVMType<'ctx> for IntModel<'ctx> {
+    fn check_llvm_type(
+        &self,
+        _ctx: &'ctx Context,
+        scrutinee: AnyTypeEnum<'ctx>,
+    ) -> Result<(), String> {
+        check_int_llvm_type(scrutinee, self.0)
+    }
+}
+
+impl<'ctx> Model<'ctx> for IntModel<'ctx> {
+    type Value = Int<'ctx>;
+
+    fn get_llvm_type(&self, _ctx: &'ctx Context) -> BasicTypeEnum<'ctx> {
+        self.0.as_basic_type_enum()
+    }
+
+    fn review(&self, ctx: &'ctx Context, value: AnyValueEnum<'ctx>) -> Self::Value {
+        let int = value.into_int_value();
+        self.check_llvm_type(ctx, int.get_type().as_any_type_enum()).unwrap();
+        Int(int)
+    }
+}
+
+impl<'ctx> IntModel<'ctx> {
+    /// Create a constant value that inhabits this [`IntModel<'ctx>`].
+    #[must_use]
+    pub fn constant(&self, value: u64) -> Int<'ctx> {
+        Int(self.0.const_int(value, false))
+    }
+
+    /// Check if `other` is fully compatible with this [`IntModel<'ctx>`].
+    ///
+    /// This simply checks if the underlying [`IntType<'ctx>`] has
+    /// the same number of bits.
+    #[must_use]
+    pub fn same_as(&self, other: IntModel<'ctx>) -> bool {
+        // TODO: or `self.0 == other.0` would also work?
+        self.0.get_bit_width() == other.0.get_bit_width()
+    }
+}
+
+/// An inhabitant of an [`IntModel<'ctx>`]
+#[derive(Debug, Clone, Copy)]
+pub struct Int<'ctx>(pub IntValue<'ctx>);
+
+impl<'ctx> ModelValue<'ctx> for Int<'ctx> {
+    fn get_llvm_value(&self) -> BasicValueEnum<'ctx> {
+        self.0.as_basic_value_enum()
+    }
+}
+
+impl<'ctx> Int<'ctx> {
+    #[must_use]
+    pub fn signed_cast_to_int(
+        self,
+        ctx: &CodeGenContext<'ctx, '_>,
+        target_int: IntModel<'ctx>,
+        name: &str,
+    ) -> Int<'ctx> {
+        Int(ctx.builder.build_int_s_extend_or_bit_cast(self.0, target_int.0, name).unwrap())
+    }
+
+    #[must_use]
+    pub fn signed_cast_to_fixed<T: IsFixedInt>(
+        self,
+        ctx: &CodeGenContext<'ctx, '_>,
+        target_fixed: T,
+        name: &str,
+    ) -> FixedInt<'ctx, T> {
+        FixedInt {
+            int: target_fixed,
+            value: ctx
+                .builder
+                .build_int_s_extend_or_bit_cast(self.0, T::get_int_type(ctx.ctx), name)
+                .unwrap(),
+        }
+    }
+}

nac3core/src/codegen/model/int_util.rs

@@ -0,0 +1,39 @@
+use inkwell::{
+    types::{AnyType, AnyTypeEnum, IntType},
+    values::{AnyValueEnum, IntValue},
+};
+
+/// Helper function to check if `scrutinee` is the same as `expected_int_type`
+pub fn check_int_llvm_type<'ctx>(
+    scrutinee: AnyTypeEnum<'ctx>,
+    expected_int_type: IntType<'ctx>,
+) -> Result<(), String> {
+    // Check if llvm_type is int type
+    let AnyTypeEnum::IntType(scrutinee) = scrutinee else {
+        return Err(format!("Expecting an int type but got {scrutinee:?}"));
+    };
+
+    // Check bit width
+    if scrutinee.get_bit_width() != expected_int_type.get_bit_width() {
+        return Err(format!(
+            "Expecting an int type of {}-bit(s) but got int type {}-bit(s)",
+            expected_int_type.get_bit_width(),
+            scrutinee.get_bit_width()
+        ));
+    }
+
+    Ok(())
+}
+
+/// Helper function to cast `scrutinee` is into an [`IntValue<'ctx>`].
+/// The LLVM type of `scrutinee` will be checked with [`check_int_llvm_type`].
+pub fn review_int_llvm_value<'ctx>(
+    value: AnyValueEnum<'ctx>,
+    expected_int_type: IntType<'ctx>,
+) -> Result<IntValue<'ctx>, String> {
+    // Check if value is of int type, error if that is anything else
+    check_int_llvm_type(value.get_type().as_any_type_enum(), expected_int_type)?;
+
+    // Ok, it is must be an int
+    Ok(value.into_int_value())
+}

nac3core/src/codegen/model/mod.rs

@@ -0,0 +1,16 @@
+pub mod core;
+pub mod fixed_int;
+pub mod int;
+mod int_util;
+pub mod opaque;
+pub mod pointer;
+pub mod slice;
+pub mod structure;
+
+pub use core::*;
+pub use fixed_int::*;
+pub use int::*;
+pub use opaque::*;
+pub use pointer::*;
+pub use slice::*;
+pub use structure::*;

nac3core/src/codegen/model/opaque.rs

@@ -0,0 +1,46 @@
+use inkwell::{
+    context::Context,
+    types::{AnyTypeEnum, BasicTypeEnum},
+    values::{AnyValueEnum, BasicValueEnum},
+};
+
+use super::*;
+
+#[derive(Debug, Clone, Copy)]
+pub struct OpaqueModel<'ctx>(pub BasicTypeEnum<'ctx>);
+
+impl<'ctx> CanCheckLLVMType<'ctx> for OpaqueModel<'ctx> {
+    fn check_llvm_type(
+        &self,
+        _ctx: &'ctx Context,
+        scrutinee: AnyTypeEnum<'ctx>,
+    ) -> Result<(), String> {
+        match BasicTypeEnum::try_from(scrutinee) {
+            Ok(_) => Ok(()),
+            Err(_err) => Err(format!("Expecting a BasicTypeEnum, but got {scrutinee:?}")),
+        }
+    }
+}
+
+impl<'ctx> Model<'ctx> for OpaqueModel<'ctx> {
+    type Value = Opaque<'ctx>;
+
+    fn get_llvm_type(&self, _ctx: &'ctx Context) -> BasicTypeEnum<'ctx> {
+        self.0
+    }
+
+    fn review(&self, ctx: &'ctx Context, value: AnyValueEnum<'ctx>) -> Self::Value {
+        self.check_llvm_type(ctx, value.get_type()).unwrap();
+        let value = BasicValueEnum::try_from(value).unwrap(); // Must work
+        Opaque(value)
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct Opaque<'ctx>(pub BasicValueEnum<'ctx>);
+
+impl<'ctx> ModelValue<'ctx> for Opaque<'ctx> {
+    fn get_llvm_value(&self) -> BasicValueEnum<'ctx> {
+        self.0
+    }
+}

nac3core/src/codegen/model/pointer.rs

@@ -0,0 +1,83 @@
+use inkwell::{
+    context::Context,
+    types::{AnyTypeEnum, BasicType, BasicTypeEnum},
+    values::{AnyValue, AnyValueEnum, BasicValue, BasicValueEnum, PointerValue},
+    AddressSpace,
+};
+
+use crate::codegen::CodeGenContext;
+
+use super::{core::*, OpaqueModel};
+
+/// A [`Model<'ctx>`] representing an LLVM [`PointerType<'ctx>`]
+/// with *full* information on the element u
+///
+/// [`self.0`] contains [`Model<'ctx>`] that represents the
+/// LLVM type of element of the [`PointerType<'ctx>`] is pointing at
+/// (like `PointerType<'ctx>::get_element_type()`, but abstracted as a [`Model<'ctx>`]).
+#[derive(Debug, Clone, Copy, Default)]
+pub struct PointerModel<E>(pub E);
+
+impl<'ctx, E: Model<'ctx>> CanCheckLLVMType<'ctx> for PointerModel<E> {
+    fn check_llvm_type(
+        &self,
+        ctx: &'ctx Context,
+        scrutinee: AnyTypeEnum<'ctx>,
+    ) -> Result<(), String> {
+        // Check if scrutinee is even a PointerValue
+        let AnyTypeEnum::PointerType(scrutinee) = scrutinee else {
+            return Err(format!("Expecting a pointer value, but got {scrutinee:?}"));
+        };
+
+        // Check the type of what the pointer is pointing at
+        // TODO: This will be deprecated by inkwell > llvm14 because `get_element_type()` will be gone
+        self.0.check_llvm_type(ctx, scrutinee.get_element_type())?; // TODO: Include backtrace?
+
+        Ok(())
+    }
+}
+
+impl<'ctx, E: Model<'ctx>> Model<'ctx> for PointerModel<E> {
+    type Value = Pointer<'ctx, E>;
+
+    fn get_llvm_type(&self, ctx: &'ctx Context) -> BasicTypeEnum<'ctx> {
+        self.0.get_llvm_type(ctx).ptr_type(AddressSpace::default()).as_basic_type_enum()
+    }
+
+    fn review(&self, ctx: &'ctx Context, value: AnyValueEnum<'ctx>) -> Self::Value {
+        self.check_llvm_type(ctx, value.get_type()).unwrap();
+
+        // TODO: Check get_element_type(). For inkwell LLVM 14 at least...
+        Pointer { element: self.0, value: value.into_pointer_value() }
+    }
+}
+
+/// An inhabitant of [`PointerModel<E>`]
+#[derive(Debug, Clone, Copy)]
+pub struct Pointer<'ctx, E: Model<'ctx>> {
+    pub element: E,
+    pub value: PointerValue<'ctx>,
+}
+
+impl<'ctx, E: Model<'ctx>> ModelValue<'ctx> for Pointer<'ctx, E> {
+    fn get_llvm_value(&self) -> BasicValueEnum<'ctx> {
+        self.value.as_basic_value_enum()
+    }
+}
+
+impl<'ctx, E: Model<'ctx>> Pointer<'ctx, E> {
+    /// Build an instruction to store a value into this pointer
+    pub fn store(&self, ctx: &CodeGenContext<'ctx, '_>, val: E::Value) {
+        ctx.builder.build_store(self.value, val.get_llvm_value()).unwrap();
+    }
+
+    /// Build an instruction to load a value from this pointer
+    pub fn load(&self, ctx: &CodeGenContext<'ctx, '_>, name: &str) -> E::Value {
+        let val = ctx.builder.build_load(self.value, name).unwrap();
+        self.element.review(ctx.ctx, val.as_any_value_enum())
+    }
+
+    pub fn to_opaque(self, ctx: &'ctx Context) -> Pointer<'ctx, OpaqueModel<'ctx>> {
+        Pointer { element: OpaqueModel(self.element.get_llvm_type(ctx)), value: self.value }
+    }
+}

nac3core/src/codegen/model/slice.rs

@@ -0,0 +1,73 @@
+use crate::codegen::{CodeGenContext, CodeGenerator};
+
+use super::{Int, Model, Pointer};
+
+pub struct ArraySlice<'ctx, E: Model<'ctx>> {
+    pub num_elements: Int<'ctx>,
+    pub pointer: Pointer<'ctx, E>,
+}
+
+impl<'ctx, E: Model<'ctx>> ArraySlice<'ctx, E> {
+    pub fn ix_unchecked(
+        &self,
+        ctx: &CodeGenContext<'ctx, '_>,
+        idx: Int<'ctx>,
+        name: &str,
+    ) -> Pointer<'ctx, E> {
+        let element_addr =
+            unsafe { ctx.builder.build_in_bounds_gep(self.pointer.value, &[idx.0], name).unwrap() };
+        Pointer { value: element_addr, element: self.pointer.element }
+    }
+
+    pub fn ix<G: CodeGenerator + ?Sized>(
+        &self,
+        generator: &mut G,
+        ctx: &mut CodeGenContext<'ctx, '_>,
+        idx: Int<'ctx>,
+        name: &str,
+    ) -> Pointer<'ctx, E> {
+        let int_type = self.num_elements.0.get_type(); // NOTE: Weird get_type(), see comment under `trait Ixed`
+
+        assert_eq!(int_type.get_bit_width(), idx.0.get_type().get_bit_width()); // Might as well check bit width to catch bugs
+
+        // TODO: SGE or UGE? or make it defined by the implementee?
+
+        // Check `0 <= index`
+        let lower_bounded = ctx
+            .builder
+            .build_int_compare(
+                inkwell::IntPredicate::SLE,
+                int_type.const_zero(),
+                idx.0,
+                "lower_bounded",
+            )
+            .unwrap();
+
+        // Check `index < num_elements`
+        let upper_bounded = ctx
+            .builder
+            .build_int_compare(
+                inkwell::IntPredicate::SLT,
+                idx.0,
+                self.num_elements.0,
+                "upper_bounded",
+            )
+            .unwrap();
+
+        // Compute `0 <= index && index < num_elements`
+        let bounded = ctx.builder.build_and(lower_bounded, upper_bounded, "bounded").unwrap();
+
+        // Assert `bounded`
+        ctx.make_assert(
+            generator,
+            bounded,
+            "0:IndexError",
+            "nac3core LLVM codegen attempting to access out of bounds array index {0}. Must satisfy 0 <= index < {2}",
+            [ Some(idx.0), Some(self.num_elements.0), None],
+            ctx.current_loc
+        );
+
+        // ...and finally do indexing
+        self.ix_unchecked(ctx, idx, name)
+    }
+}

nac3core/src/codegen/model/structure.rs

@@ -0,0 +1,219 @@
+use inkwell::{
+    context::Context,
+    types::{AnyType, AnyTypeEnum, BasicType, BasicTypeEnum, StructType},
+    values::{AnyValueEnum, BasicValue, BasicValueEnum, StructValue},
+};
+use itertools::{izip, Itertools};
+
+use crate::codegen::CodeGenContext;
+
+use super::{core::CanCheckLLVMType, Model, ModelValue, Pointer};
+
+#[derive(Debug, Clone, Copy)]
+pub struct Field<E> {
+    pub gep_index: u64,
+    pub name: &'static str,
+    pub element: E,
+}
+
+struct FieldLLVM<'ctx> {
+    gep_index: u64,
+    name: &'ctx str,
+    llvm_type: BasicTypeEnum<'ctx>,
+
+    // Only CanCheckLLVMType is needed, dont use `Model<'ctx>`
+    llvm_type_model: Box<dyn CanCheckLLVMType<'ctx> + 'ctx>,
+}
+
+pub struct FieldBuilder<'ctx> {
+    pub ctx: &'ctx Context,
+    gep_index_counter: u64,
+    struct_name: &'ctx str,
+    fields: Vec<FieldLLVM<'ctx>>,
+}
+
+impl<'ctx> FieldBuilder<'ctx> {
+    #[must_use]
+    pub fn new(ctx: &'ctx Context, struct_name: &'ctx str) -> Self {
+        FieldBuilder { ctx, gep_index_counter: 0, struct_name, fields: Vec::new() }
+    }
+
+    fn next_gep_index(&mut self) -> u64 {
+        let index = self.gep_index_counter;
+        self.gep_index_counter += 1;
+        index
+    }
+
+    pub fn add_field<E: Model<'ctx> + 'ctx>(&mut self, name: &'static str, element: E) -> Field<E> {
+        let gep_index = self.next_gep_index();
+
+        self.fields.push(FieldLLVM {
+            gep_index,
+            name,
+            llvm_type: element.get_llvm_type(self.ctx),
+            llvm_type_model: Box::new(element),
+        });
+
+        Field { gep_index, name, element }
+    }
+
+    pub fn add_field_auto<E: Model<'ctx> + Default + 'ctx>(
+        &mut self,
+        name: &'static str,
+    ) -> Field<E> {
+        self.add_field(name, E::default())
+    }
+}
+
+/// A marker trait to mark singleton struct that describes a particular LLVM structure.
+pub trait IsStruct<'ctx>: Clone + Copy {
+    /// The type of the Rust `struct` that holds all the fields of this LLVM struct.
+    type Fields;
+
+    /// A cosmetic name for this struct.
+    /// TODO: Currently unused. To be used in error reporting.
+    fn struct_name(&self) -> &'static str;
+
+    fn build_fields(&self, builder: &mut FieldBuilder<'ctx>) -> Self::Fields;
+
+    fn get_fields(&self, ctx: &'ctx Context) -> Self::Fields {
+        let mut builder = FieldBuilder::new(ctx, self.struct_name());
+        self.build_fields(&mut builder)
+    }
+
+    /// Get the LLVM struct type this [`IsStruct<'ctx>`] is representing.
+    fn get_struct_type(&self, ctx: &'ctx Context) -> StructType<'ctx> {
+        let mut builder = FieldBuilder::new(ctx, self.struct_name());
+        self.build_fields(&mut builder); // Self::Fields is discarded
+
+        let field_types = builder.fields.iter().map(|f| f.llvm_type).collect_vec();
+        ctx.struct_type(&field_types, false)
+    }
+
+    /// Check if `scrutinee` matches the [`StructType<'ctx>`] this [`IsStruct<'ctx>`] is representing.
+    fn check_struct_type(
+        &self,
+        ctx: &'ctx Context,
+        scrutinee: StructType<'ctx>,
+    ) -> Result<(), String> {
+        // Details about scrutinee
+        let scrutinee_field_types = scrutinee.get_field_types();
+
+        // Details about the defined specifications of this struct
+        // We will access them through builder
+        let mut builder = FieldBuilder::new(ctx, self.struct_name());
+        self.build_fields(&mut builder);
+
+        // Check # of fields
+        if builder.fields.len() != scrutinee_field_types.len() {
+            return Err(format!(
+                "Expecting struct to have {} field(s), but scrutinee has {} field(s)",
+                builder.fields.len(),
+                scrutinee_field_types.len()
+            ));
+        }
+
+        // Check the types of each field
+        // TODO: Traceback?
+        for (f, scrutinee_field_type) in izip!(builder.fields, scrutinee_field_types) {
+            f.llvm_type_model.check_llvm_type(ctx, scrutinee_field_type.as_any_type_enum())?;
+        }
+
+        Ok(())
+    }
+}
+
+/// A [`Model<'ctx>`] that represents an LLVM struct.
+///
+/// `self.0` contains a [`IsStruct<'ctx>`] that gives the details of the LLVM struct.
+#[derive(Debug, Clone, Copy, Default)]
+pub struct StructModel<S>(pub S);
+
+impl<'ctx, S: IsStruct<'ctx>> CanCheckLLVMType<'ctx> for StructModel<S> {
+    fn check_llvm_type(
+        &self,
+        ctx: &'ctx Context,
+        scrutinee: AnyTypeEnum<'ctx>,
+    ) -> Result<(), String> {
+        // Check if scrutinee is even a struct type
+        let AnyTypeEnum::StructType(scrutinee) = scrutinee else {
+            return Err(format!("Expecting a struct type, but got {scrutinee:?}"));
+        };
+
+        // Ok. now check the struct type *thoroughly*
+        self.0.check_struct_type(ctx, scrutinee)
+    }
+}
+
+impl<'ctx, S: IsStruct<'ctx>> Model<'ctx> for StructModel<S> {
+    type Value = Struct<'ctx, S>;
+
+    fn get_llvm_type(&self, ctx: &'ctx Context) -> BasicTypeEnum<'ctx> {
+        self.0.get_struct_type(ctx).as_basic_type_enum()
+    }
+
+    fn review(&self, ctx: &'ctx Context, value: AnyValueEnum<'ctx>) -> Self::Value {
+        // Check that `value` is not some bogus values or an incorrect StructValue
+        self.check_llvm_type(ctx, value.get_type()).unwrap();
+
+        Struct { structure: self.0, value: value.into_struct_value() }
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct Struct<'ctx, S> {
+    pub structure: S,
+    pub value: StructValue<'ctx>,
+}
+
+impl<'ctx, S: IsStruct<'ctx>> ModelValue<'ctx> for Struct<'ctx, S> {
+    fn get_llvm_value(&self) -> BasicValueEnum<'ctx> {
+        self.value.as_basic_value_enum()
+    }
+}
+
+impl<'ctx, S: IsStruct<'ctx>> Pointer<'ctx, StructModel<S>> {
+    /// Build an instruction that does `getelementptr` on an LLVM structure referenced by this pointer.
+    ///
+    /// This provides a nice syntax to chain up `getelementptr` in an intuitive and type-safe way:
+    ///
+    /// ```ignore
+    /// let ctx: &CodeGenContext<'ctx, '_>;
+    /// let ndarray: Pointer<'ctx, StructModel<NpArray<'ctx>>>;
+    /// ndarray.gep(ctx, |f| f.ndims).store();
+    /// ```
+    ///
+    /// You might even write chains `gep`, i.e.,
+    /// ```ignore
+    /// my_struct
+    ///     .gep(ctx, |f| f.thing1)
+    ///     .gep(ctx, |f| f.value)
+    ///     .store(ctx, my_value) // Equivalent to `my_struct.thing1.value = my_value`
+    /// ```
+    pub fn gep<E, GetFieldFn>(
+        &self,
+        ctx: &CodeGenContext<'ctx, '_>,
+        get_field: GetFieldFn,
+    ) -> Pointer<'ctx, E>
+    where
+        E: Model<'ctx>,
+        GetFieldFn: FnOnce(S::Fields) -> Field<E>,
+    {
+        let fields = self.element.0.get_fields(ctx.ctx);
+        let field = get_field(fields);
+
+        let llvm_i32 = ctx.ctx.i32_type(); // TODO: I think I'm not supposed to *just* use i32 for GEP like that
+
+        let ptr = unsafe {
+            ctx.builder
+                .build_in_bounds_gep(
+                    self.value,
+                    &[llvm_i32.const_zero(), llvm_i32.const_int(field.gep_index, false)],
+                    field.name,
+                )
+                .unwrap()
+        };
+
+        Pointer { element: field.element, value: ptr }
+    }
+}