syn-canon 0.5.1

//! Target-independent checked integer literal evaluation.

use alloc::boxed::Box;
use alloc::format;
use alloc::vec::Vec;
use syn::spanned::Spanned;
use syn::visit_mut::{self, VisitMut};

use crate::alpha::Renamer;
use crate::scope::{IntWidth, PrimTy, primitive_suffix};

const MAX_DEPTH: usize = 64;

#[derive(Clone, Copy, PartialEq, Eq)]
struct FixedInt {
    width: IntWidth,
    signed: bool,
}

impl FixedInt {
    fn suffix(self) -> &'static str {
        PrimTy::Int {
            width: self.width,
            signed: self.signed,
        }
        .suffix()
    }
}

#[derive(Clone, Copy)]
struct Integer {
    ty: FixedInt,
    bits: u128,
}

impl Integer {
    fn width(self) -> u32 {
        self.ty.width.bits()
    }

    fn signed(self) -> bool {
        self.ty.signed
    }

    fn mask(self) -> u128 {
        u128::MAX >> (128 - self.width())
    }

    fn signed_value(self) -> i128 {
        let sign = 1u128 << (self.width() - 1);
        let bits = if self.bits & sign == 0 {
            self.bits
        } else {
            self.bits | !self.mask()
        };
        i128::from_le_bytes(bits.to_le_bytes())
    }

    fn checked_signed(self, value: i128) -> Option<Self> {
        let limit = i128::MAX >> (128 - self.width());
        if value < -limit - 1 || value > limit {
            return None;
        }
        Some(Self {
            bits: u128::from_le_bytes(value.to_le_bytes()) & self.mask(),
            ..self
        })
    }

    fn checked_unsigned(self, value: u128) -> Option<Self> {
        (value <= self.mask()).then_some(Self {
            bits: value,
            ..self
        })
    }

    fn shift(self, right: Self, left: bool) -> Option<Self> {
        if right.signed() && right.signed_value() < 0 {
            return None;
        }
        let count = u32::try_from(right.bits).ok()?;
        if count >= self.width() {
            return None;
        }
        let bits = if left {
            self.bits.checked_shl(count)? & self.mask()
        } else if self.signed() {
            u128::from_le_bytes(self.signed_value().checked_shr(count)?.to_le_bytes()) & self.mask()
        } else {
            self.bits.checked_shr(count)?
        };
        Some(Self { bits, ..self })
    }

    fn binary(self, right: Self, op: BinaryOp) -> Option<Self> {
        let op = match op {
            BinaryOp::Shl => return self.shift(right, true),
            BinaryOp::Shr => return self.shift(right, false),
            BinaryOp::BitAnd => {
                return Some(Self {
                    bits: self.bits & right.bits,
                    ..self
                });
            }
            BinaryOp::BitOr => {
                return Some(Self {
                    bits: self.bits | right.bits,
                    ..self
                });
            }
            BinaryOp::BitXor => {
                return Some(Self {
                    bits: self.bits ^ right.bits,
                    ..self
                });
            }
            BinaryOp::Math(op) => op,
        };
        if self.signed() {
            let (left, right) = (self.signed_value(), right.signed_value());
            let minimum = -(i128::MAX >> (128 - self.width())) - 1;
            if matches!(op, MathOp::Div | MathOp::Rem) && left == minimum && right == -1 {
                return None;
            }
            let value = match op {
                MathOp::Add => left.checked_add(right),
                MathOp::Sub => left.checked_sub(right),
                MathOp::Mul => left.checked_mul(right),
                MathOp::Div => left.checked_div(right),
                MathOp::Rem => left.checked_rem(right),
            }?;
            self.checked_signed(value)
        } else {
            let value = match op {
                MathOp::Add => self.bits.checked_add(right.bits),
                MathOp::Sub => self.bits.checked_sub(right.bits),
                MathOp::Mul => self.bits.checked_mul(right.bits),
                MathOp::Div => self.bits.checked_div(right.bits),
                MathOp::Rem => self.bits.checked_rem(right.bits),
            }?;
            self.checked_unsigned(value)
        }
    }

    fn expression(self, span: proc_macro2::Span) -> syn::Expr {
        let negative = self.signed() && self.signed_value() < 0;
        let magnitude = if negative {
            self.signed_value().unsigned_abs()
        } else {
            self.bits
        };
        let literal = syn::Expr::Lit(syn::ExprLit {
            attrs: Vec::new(),
            lit: syn::Lit::Int(syn::LitInt::new(
                &format!("{magnitude}{}", self.ty.suffix()),
                span,
            )),
        });
        if negative {
            syn::Expr::Unary(syn::ExprUnary {
                attrs: Vec::new(),
                op: syn::UnOp::Neg(syn::token::Minus { spans: [span] }),
                expr: Box::new(literal),
            })
        } else {
            literal
        }
    }
}

#[derive(Clone, Copy)]
enum MathOp {
    Add,
    Sub,
    Mul,
    Div,
    Rem,
}

#[derive(Clone, Copy)]
enum BinaryOp {
    Math(MathOp),
    BitAnd,
    BitOr,
    BitXor,
    Shl,
    Shr,
}

fn arithmetic(op: syn::BinOp) -> Option<BinaryOp> {
    Some(match op {
        syn::BinOp::Add(_) => BinaryOp::Math(MathOp::Add),
        syn::BinOp::Sub(_) => BinaryOp::Math(MathOp::Sub),
        syn::BinOp::Mul(_) => BinaryOp::Math(MathOp::Mul),
        syn::BinOp::Div(_) => BinaryOp::Math(MathOp::Div),
        syn::BinOp::Rem(_) => BinaryOp::Math(MathOp::Rem),
        syn::BinOp::BitAnd(_) => BinaryOp::BitAnd,
        syn::BinOp::BitOr(_) => BinaryOp::BitOr,
        syn::BinOp::BitXor(_) => BinaryOp::BitXor,
        syn::BinOp::Shl(_) => BinaryOp::Shl,
        syn::BinOp::Shr(_) => BinaryOp::Shr,
        _ => return None,
    })
}

fn scalar_const(block: &syn::Block) -> Option<&syn::Expr> {
    match block.stmts.as_slice() {
        [syn::Stmt::Expr(expr, None)] => Some(expr),
        _ => None,
    }
}

enum Node<'a> {
    Literal(&'a syn::LitInt),
    Nested(&'a syn::Expr),
    Unary(&'a syn::ExprUnary),
    Binary(&'a syn::ExprBinary),
}

fn node(expr: &syn::Expr) -> Option<Node<'_>> {
    match expr {
        syn::Expr::Lit(syn::ExprLit {
            lit: syn::Lit::Int(literal),
            ..
        }) => Some(Node::Literal(literal)),
        syn::Expr::Paren(paren) => Some(Node::Nested(&paren.expr)),
        syn::Expr::Group(group) => Some(Node::Nested(&group.expr)),
        syn::Expr::Const(constant) => scalar_const(&constant.block).map(Node::Nested),
        syn::Expr::Unary(unary) => Some(Node::Unary(unary)),
        syn::Expr::Binary(binary) => Some(Node::Binary(binary)),
        _ => None,
    }
}

fn closed(expr: &syn::Expr, depth: usize) -> bool {
    if depth >= MAX_DEPTH {
        return true;
    }
    match node(expr) {
        Some(Node::Literal(_)) => true,
        Some(Node::Nested(expr)) => closed(expr, depth + 1),
        Some(Node::Unary(unary)) if matches!(unary.op, syn::UnOp::Neg(_) | syn::UnOp::Not(_)) => {
            closed(&unary.expr, depth + 1)
        }
        Some(Node::Binary(binary)) if arithmetic(binary.op).is_some() => {
            closed(&binary.left, depth + 1) && closed(&binary.right, depth + 1)
        }
        _ => false,
    }
}

/// The value of an unsuffixed literal expression that every integer type evaluates alike,
/// every literal, intermediate and result held to `0..=127` so nothing overflows `i8` or `u8`.
fn type_free_value(expr: &syn::Expr, depth: usize) -> Option<u8> {
    if depth >= MAX_DEPTH || !crate::drift::expr_attrs(expr).is_empty() {
        return None;
    }
    let value = match node(expr)? {
        Node::Literal(literal) if literal.suffix().is_empty() => {
            literal.base10_parse::<u8>().ok()?
        }
        Node::Nested(expr) => type_free_value(expr, depth + 1)?,
        Node::Binary(binary) => {
            let left = type_free_value(&binary.left, depth + 1)?;
            let right = type_free_value(&binary.right, depth + 1)?;
            match arithmetic(binary.op)? {
                BinaryOp::Math(MathOp::Add) => left.checked_add(right)?,
                BinaryOp::Math(MathOp::Sub) => left.checked_sub(right)?,
                BinaryOp::Math(MathOp::Mul) => left.checked_mul(right)?,
                BinaryOp::Math(MathOp::Div) => left.checked_div(right)?,
                BinaryOp::Math(MathOp::Rem) => left.checked_rem(right)?,
                BinaryOp::BitAnd => left & right,
                BinaryOp::BitOr => left | right,
                BinaryOp::BitXor => left ^ right,
                BinaryOp::Shl if right < 8 => u8::try_from(u16::from(left) << right).ok()?,
                BinaryOp::Shr if right < 8 => left >> right,
                BinaryOp::Shl | BinaryOp::Shr => return None,
            }
        }
        Node::Literal(_) | Node::Unary(_) => return None,
    };
    (value <= 127).then_some(value)
}

fn integer_type(ty: PrimTy) -> Option<FixedInt> {
    match ty {
        PrimTy::Int { width, signed } => Some(FixedInt { width, signed }),
        PrimTy::Bool => None,
    }
}

/// A folded literal expression of a proven scalar type.
pub(crate) fn literal_expr(ty: PrimTy, bits: u128, span: proc_macro2::Span) -> syn::Expr {
    if ty == PrimTy::Bool {
        return syn::Expr::Lit(syn::ExprLit {
            attrs: Vec::new(),
            lit: syn::Lit::Bool(syn::LitBool::new(bits != 0, span)),
        });
    }
    let ty = integer_type(ty).expect("an integer type");
    Integer { ty, bits }.expression(span)
}

#[derive(Clone, Copy)]
enum TypeProof {
    Unsuffixed,
    Fixed(FixedInt),
}

impl TypeProof {
    fn fixed(self) -> Option<FixedInt> {
        match self {
            Self::Unsuffixed => None,
            Self::Fixed(ty) => Some(ty),
        }
    }
}

fn proven_type(expr: &syn::Expr, expected: Option<FixedInt>, depth: usize) -> Option<TypeProof> {
    if depth >= MAX_DEPTH || !crate::drift::expr_attrs(expr).is_empty() {
        return None;
    }
    match node(expr)? {
        Node::Literal(literal) => {
            if literal.suffix().is_empty() {
                Some(expected.map_or(TypeProof::Unsuffixed, TypeProof::Fixed))
            } else {
                let ty = integer_type(primitive_suffix(literal.suffix())?)?;
                if expected.is_some_and(|expected| expected != ty) {
                    return None;
                }
                Some(TypeProof::Fixed(ty))
            }
        }
        Node::Nested(expr) => proven_type(expr, expected, depth + 1),
        Node::Unary(unary) => proven_type(&unary.expr, expected, depth + 1),
        Node::Binary(binary) => {
            let left = proven_type(&binary.left, expected, depth + 1)?;
            if matches!(binary.op, syn::BinOp::Shl(_) | syn::BinOp::Shr(_)) {
                proven_type(&binary.right, None, depth + 1)?.fixed()?;
                return Some(left);
            }
            let right = proven_type(&binary.right, expected, depth + 1)?;
            match (left, right) {
                (TypeProof::Fixed(left), TypeProof::Fixed(right)) if left != right => None,
                (TypeProof::Fixed(ty), _) | (_, TypeProof::Fixed(ty)) => Some(TypeProof::Fixed(ty)),
                (TypeProof::Unsuffixed, TypeProof::Unsuffixed) => Some(TypeProof::Unsuffixed),
            }
        }
    }
}

fn literal(expr: &syn::Expr) -> Option<&syn::LitInt> {
    match expr {
        syn::Expr::Lit(syn::ExprLit {
            lit: syn::Lit::Int(literal),
            ..
        }) => Some(literal),
        syn::Expr::Group(group) => literal(&group.expr),
        _ => None,
    }
}

pub(crate) fn signed_literal(expr: &syn::Expr) -> Option<(PrimTy, u128)> {
    if !matches!(expr, syn::Expr::Unary(unary) if matches!(unary.op, syn::UnOp::Neg(_)) && literal(&unary.expr).is_some())
    {
        return None;
    }
    let ty = proven_type(expr, None, 0)?.fixed()?;
    let value = evaluate(expr, ty, 0)?;
    Some((
        PrimTy::Int {
            width: ty.width,
            signed: ty.signed,
        },
        value.bits,
    ))
}

fn evaluate(expr: &syn::Expr, ty: FixedInt, depth: usize) -> Option<Integer> {
    let model = Integer { ty, bits: 0 };
    match node(expr)? {
        Node::Literal(literal) => {
            let magnitude = literal.base10_parse::<u128>().ok()?;
            if model.signed() {
                model.checked_signed(i128::try_from(magnitude).ok()?)
            } else {
                model.checked_unsigned(magnitude)
            }
        }
        Node::Nested(expr) => evaluate(expr, ty, depth + 1),
        Node::Unary(unary) => {
            if matches!(unary.op, syn::UnOp::Neg(_))
                && model.signed()
                && let Some(literal) = literal(&unary.expr)
            {
                let magnitude = literal.base10_parse::<u128>().ok()?;
                let limit = 1u128 << (model.width() - 1);
                if magnitude > limit {
                    return None;
                }
                return Some(Integer {
                    bits: magnitude.wrapping_neg() & model.mask(),
                    ..model
                });
            }
            let value = evaluate(&unary.expr, ty, depth + 1)?;
            match unary.op {
                syn::UnOp::Neg(_) if value.signed() => {
                    value.checked_signed(value.signed_value().checked_neg()?)
                }
                syn::UnOp::Not(_) => Some(Integer {
                    bits: !value.bits & value.mask(),
                    ..value
                }),
                _ => None,
            }
        }
        Node::Binary(binary) => {
            let left = evaluate(&binary.left, ty, depth + 1)?;
            let right_ty = if matches!(binary.op, syn::BinOp::Shl(_) | syn::BinOp::Shr(_)) {
                proven_type(&binary.right, None, depth + 1)?.fixed()?
            } else {
                ty
            };
            left.binary(
                evaluate(&binary.right, right_ty, depth + 1)?,
                arithmetic(binary.op)?,
            )
        }
    }
}

/// The suffixed literal a byte literal, a cast of a literal or a primitive limit spells: `b'a'`
/// reads as `97u8`, an unsuffixed integer literal cast to a proven integer type as that type's
/// literal, which the cast gives the literal anyway, an ASCII character or byte cast to one as
/// its code, and `u8::MAX` or `i32::MIN` as the value. A pointer-sized target takes values every
/// pointer width holds.
fn literal_form(renamer: &Renamer<'_>, expr: &syn::Expr) -> Option<syn::Expr> {
    let span = expr.span();
    match expr {
        syn::Expr::Lit(syn::ExprLit {
            lit: syn::Lit::Byte(byte),
            ..
        }) if byte.suffix().is_empty() => Some(literal_expr(
            PrimTy::Int {
                width: IntWidth::W8,
                signed: false,
            },
            u128::from(byte.value()),
            span,
        )),
        syn::Expr::Cast(cast) => {
            let syn::Expr::Lit(syn::ExprLit { attrs, lit }) = &*cast.expr else {
                return None;
            };
            let value = match lit {
                syn::Lit::Int(int) if attrs.is_empty() && int.suffix().is_empty() => {
                    int.base10_parse::<u128>().ok()?
                }
                syn::Lit::Char(c)
                    if attrs.is_empty() && c.suffix().is_empty() && c.value().is_ascii() =>
                {
                    u128::from(u32::from(c.value()))
                }
                syn::Lit::Byte(byte)
                    if attrs.is_empty() && byte.suffix().is_empty() && byte.value().is_ascii() =>
                {
                    u128::from(byte.value())
                }
                _ => return None,
            };
            let syn::Type::Path(path) = &*cast.ty else {
                return None;
            };
            let name = path.path.get_ident().filter(|_| path.qself.is_none())?;
            if let Some(signed) = renamer.pointer_sized(name) {
                let limit = if signed {
                    i16::MAX.unsigned_abs()
                } else {
                    u16::MAX
                };
                if value > u128::from(limit) {
                    return None;
                }
                let suffix = if signed { "isize" } else { "usize" };
                return Some(syn::Expr::Lit(syn::ExprLit {
                    attrs: Vec::new(),
                    lit: syn::Lit::Int(syn::LitInt::new(&format!("{value}{suffix}"), span)),
                }));
            }
            let ty = integer_type(renamer.type_name_prim(name)?)?;
            let model = Integer { ty, bits: 0 };
            let value = if ty.signed {
                model.checked_signed(i128::try_from(value).ok()?)
            } else {
                model.checked_unsigned(value)
            }?;
            Some(value.expression(span))
        }
        syn::Expr::Path(path) if path.qself.is_none() && path.path.leading_colon.is_none() => {
            let mut segments = path.path.segments.iter();
            let (Some(ty), Some(limit), None) = (segments.next(), segments.next(), segments.next())
            else {
                return None;
            };
            if !ty.arguments.is_none() || !limit.arguments.is_none() {
                return None;
            }
            let prim @ PrimTy::Int { width, signed } = renamer.type_name_prim(&ty.ident)? else {
                return None;
            };
            let all = u128::MAX >> (128 - width.bits());
            let bits = if limit.ident == "MAX" {
                if signed { all >> 1 } else { all }
            } else if limit.ident == "MIN" {
                if signed { 1 << (width.bits() - 1) } else { 0 }
            } else {
                return None;
            };
            Some(literal_expr(prim, bits, span))
        }
        _ => None,
    }
}

/// Fold the literal arithmetic of `expr`, whose declared type is `expected` when
/// `declared`, an unproven declared type keeping type-free folds out of its root.
pub(crate) fn normalize_expr(
    renamer: &Renamer<'_>,
    expr: &mut syn::Expr,
    expected: Option<PrimTy>,
    declared: bool,
) {
    Normalizer {
        renamer,
        expected: expected.and_then(integer_type),
        returns: None,
        type_free: !declared,
        contextual: true,
        depth: 0,
    }
    .visit_expr_mut(expr);
}

/// Fold the literal arithmetic of `block`. A function body whose declared return type is a
/// proven fixed-width integer types its tail and its `return` values by it.
pub(crate) fn normalize_block(renamer: &Renamer<'_>, block: &mut syn::Block) {
    let returns = renamer.body_return(block).and_then(integer_type);
    let mut normalizer = Normalizer {
        renamer,
        expected: None,
        returns,
        type_free: true,
        contextual: true,
        depth: 0,
    };
    if returns.is_some() {
        normalizer.tail_block(block, returns);
    } else {
        normalizer.visit_block_mut(block);
    }
}

struct Normalizer<'a, 'env> {
    renamer: &'a Renamer<'env>,
    expected: Option<FixedInt>,
    /// The type of the enclosing function's `return` values, outside closures and async blocks.
    returns: Option<FixedInt>,
    /// Whether a closed expression here may fold to a value every integer type reads alike.
    type_free: bool,
    contextual: bool,
    depth: usize,
}

impl VisitMut for Normalizer<'_, '_> {
    fn visit_expr_mut(&mut self, expr: &mut syn::Expr) {
        if self.depth >= MAX_DEPTH || !crate::drift::expr_attrs(expr).is_empty() {
            return;
        }
        if let Some(literal) = literal_form(self.renamer, expr) {
            *expr = literal;
            return;
        }
        if closed(expr, 0) {
            if let Some(ty) = proven_type(expr, self.expected, 0).and_then(TypeProof::fixed)
                && let Some(value) = evaluate(expr, ty, 0)
            {
                if matches!(expr, syn::Expr::Lit(syn::ExprLit { lit: syn::Lit::Int(lit), .. }) if lit.suffix() == ty.suffix())
                {
                    return;
                }
                *expr = value.expression(expr.span());
            } else if self.type_free
                && literal(expr).is_none()
                && let Some(value) = type_free_value(expr, 0)
            {
                *expr = syn::Expr::Lit(syn::ExprLit {
                    attrs: Vec::new(),
                    lit: syn::Lit::Int(syn::LitInt::new(&format!("{value}"), expr.span())),
                });
            }
            return;
        }
        let expected = self.expected.take();
        let type_free = core::mem::replace(&mut self.type_free, true);
        self.depth += 1;
        if let syn::Expr::If(branch) = expr
            && expected.is_some()
            && let Some((_, otherwise)) = &mut branch.else_branch
        {
            self.visit_expr_mut(&mut branch.cond);
            self.tail_block(&mut branch.then_branch, expected);
            self.expected = expected;
            self.visit_expr_mut(otherwise);
        } else if let syn::Expr::Block(block) = expr
            && expected.is_some()
            && block.label.is_none()
        {
            self.tail_block(&mut block.block, expected);
        } else if let syn::Expr::Return(r#return) = expr
            && let Some(value) = &mut r#return.expr
            && self.returns.is_some()
        {
            self.expected = self.returns;
            self.visit_expr_mut(value);
        } else if matches!(expr, syn::Expr::Closure(_) | syn::Expr::Async(_)) {
            let returns = self.returns.take();
            visit_mut::visit_expr_mut(self, expr);
            self.returns = returns;
        } else {
            visit_mut::visit_expr_mut(self, expr);
        }
        self.depth -= 1;
        self.expected = expected;
        self.type_free = type_free;
    }

    fn visit_block_mut(&mut self, block: &mut syn::Block) {
        if self.depth >= MAX_DEPTH || crate::schedule::unknown_macro_reaches(self.renamer, block) {
            return;
        }
        let contextual = self.contextual;
        self.contextual &= !block.stmts.iter().any(|stmt| {
            matches!(stmt, syn::Stmt::Item(syn::Item::Use(_)))
                || (self.depth != 0 && matches!(stmt, syn::Stmt::Item(_)))
        });
        self.depth += 1;
        for stmt in &mut block.stmts {
            self.visit_stmt_mut(stmt);
        }
        self.depth -= 1;
        self.contextual = contextual;
    }

    fn visit_local_mut(&mut self, local: &mut syn::Local) {
        if !local.attrs.is_empty() {
            return;
        }
        let mut annotation = None;
        if let Some(init) = &mut local.init {
            let expected = self.expected;
            annotation = match &local.pat {
                syn::Pat::Type(typed) if self.contextual => self.renamer.param_prim(&typed.ty),
                _ => None,
            };
            self.expected = annotation.and_then(integer_type);
            if !matches!(local.pat, syn::Pat::Type(_))
                || self.expected.is_some()
                || !closed(&init.expr, 0)
            {
                self.visit_expr_mut(&mut init.expr);
            }
            self.expected = expected;
            if let Some((_, diverge)) = &mut init.diverge {
                self.visit_expr_mut(diverge);
            }
        }
        drop_stated_annotation(local, annotation);
        self.visit_pat_mut(&mut local.pat);
    }

    fn visit_type_mut(&mut self, ty: &mut syn::Type) {
        if crate::drift::type_attrs(ty).is_empty() {
            visit_mut::visit_type_mut(self, ty);
        }
    }

    fn visit_generic_argument_mut(&mut self, arg: &mut syn::GenericArgument) {
        visit_mut::visit_generic_argument_mut(self, arg);
        if let syn::GenericArgument::Const(expr) = arg {
            crate::drift::fold_scalar_argument_block(expr);
        }
    }

    fn visit_item_mut(&mut self, _: &mut syn::Item) {}
    fn visit_macro_mut(&mut self, _: &mut syn::Macro) {}
    fn visit_attribute_mut(&mut self, _: &mut syn::Attribute) {}
}

impl Normalizer<'_, '_> {
    /// Visit `block` with its tail expression expecting `expected`, the type of the block's
    /// value, which an unsuffixed integer literal tail then takes.
    fn tail_block(&mut self, block: &mut syn::Block, expected: Option<FixedInt>) {
        self.visit_block_mut(block);
        if let Some(syn::Stmt::Expr(tail, None)) = block.stmts.last_mut() {
            self.expected = expected;
            self.visit_expr_mut(tail);
            self.expected = None;
        }
    }
}

/// Drop the annotation of a `let` binding one identifier whose initializer is an integer
/// literal of the annotated fixed-width type, so the binding reads like `let v = 5u32;`.
fn drop_stated_annotation(local: &mut syn::Local, annotation: Option<PrimTy>) {
    let (Some(annotation @ PrimTy::Int { .. }), Some(init)) = (annotation, &local.init) else {
        return;
    };
    let stated = match &*init.expr {
        syn::Expr::Lit(_) => literal(&init.expr)
            .filter(|literal| !literal.suffix().is_empty())
            .and_then(|_| proven_type(&init.expr, None, 0)?.fixed())
            .map(|ty| PrimTy::Int {
                width: ty.width,
                signed: ty.signed,
            }),
        _ => signed_literal(&init.expr).map(|(prim, _)| prim),
    };
    if init.diverge.is_some() || stated != Some(annotation) {
        return;
    }
    if let syn::Pat::Type(typed) = &mut local.pat
        && typed.attrs.is_empty()
        && matches!(typed.pat.as_ref(), syn::Pat::Ident(ident)
            if ident.attrs.is_empty() && ident.by_ref.is_none() && ident.subpat.is_none())
    {
        let binding = core::mem::replace(
            &mut *typed.pat,
            syn::Pat::Verbatim(proc_macro2::TokenStream::new()),
        );
        local.pat = binding;
    }
}