sv_sim_sim/constraints/build/

aggregate.rs

//! Unpacked struct and fixed-size unpacked array class properties for the
//! [`Builder`] (plan task C14; IEEE 1800-2023 18.4, 18.5.7, 18.8).
//!
//! An aggregate property is stored as one flat value (`state::storage`): a
//! struct's fields concatenated, an array's elements lowest index first. Its
//! *leaves* (an integral or real member or element) are the variables of the
//! solve. A leaf is a [`Window`] of the property's value: it has its own
//! [`VarInfo`] (or [`RealVarInfo`]), keyed by the property and the path of
//! [`Step`]s from it, and the registry reads and writes the leaf through that
//! window.
//!
//! * A leaf is random when the property is declared `rand`/`randc` (18.4: an
//!   array declared `rand` makes every element random; a `rand` unpacked
//!   struct solves every member concurrently) or when a struct member on its
//!   path is. A member's own qualifier overrides the one it would inherit, so a
//!   `randc` member of a `rand` struct is `randc` and a member that is not
//!   `rand` of a non-`rand` struct is a state variable (18.3).
//! * An unpacked union is never random; a `string` or `chandle` member has no
//!   solved value and is skipped. A class handle, queue, dynamic array or
//!   associative array *inside* a random aggregate is
//!   [`BuildError::Unsupported`]: such a leaf would be a random object or a
//!   random-size array nested in a flat value.
//! * `a[i]` with a constant index names one leaf. An index that is a state
//!   variable or loop variable names a *choice* among the leaves
//!   ([`ArrayNode::Pick`]) the registry resolves once the index is known; an
//!   index that is a random variable is [`BuildError::BadArray`] (18.5.7.1).
//!   An index outside the array is an error only if the constraint it is in
//!   is live: "users have to explicitly exclude these indices using
//!   predicates".
//! * `A.sum()` and the other reduction methods over a fixed-size array unroll
//!   over its elements (`arrays`); `foreach` over one unrolls too.

use super::arrays::{ArrayShape, array_shape, const_i64};
use super::real::{real_kind, state_ty, strip_conversions};
use super::*;
use crate::constraints::ir::{ArrayNode, Assignment, eval};
use crate::constraints::real_domain::{FloatKind as Fk, RealExpr};
use crate::state::storage::{field_slice, storage_width};

/// One step from an aggregate property to a leaf of it.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum Step {
    /// The element at this declared index of a fixed-size array.
    Index(i64),
    /// The member at this position (declaration order) of an unpacked struct.
    Field(u32),
}

/// A leaf declared as a random variable, in the namespace it lives in.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Declared {
    /// An integral leaf: an id of [`ConstraintSet::vars`].
    Int(u32),
    /// A `real`/`shortreal` leaf: an id of [`ConstraintSet::real_vars`].
    Real(u32),
}

/// One step of an expression naming a leaf, before indices are evaluated.
#[derive(Clone, Copy)]
pub(super) enum RawStep<'d> {
    /// `[expr]`, to be built.
    Index(Expression<'d>),
    /// `[constant]`.
    Const(i64),
    /// `.member`, by position.
    Field(u32),
}

/// An expression naming a leaf of an aggregate property of this class.
#[derive(Clone)]
pub(super) struct Chain<'d> {
    /// The class property.
    pub root: Symbol<'d>,
    /// The selects and member accesses from it, outermost first (none for the
    /// property itself).
    pub steps: Vec<RawStep<'d>>,
}

/// What `e` names, if it is a class property of this class or a member or
/// element of one: `s`, `this.s`, `s.m`, `a[i]`, `s.m[i].n`, ... (anything
/// reached through an object handle, a local or a union is `None`).
pub(super) fn aggregate_chain<'d>(
    e: Expression<'d>,
    forced: &HashSet<SymbolId>,
) -> Option<Chain<'d>> {
    let e = strip_conversions(e);
    let kind = e.kind();
    // Every other kind of expression names no leaf of a property: `None`.
    if kind == ExpressionKind::NamedValue {
        let sym = e.referenced_symbol()?;
        return (sym.kind() == SymbolKind::ClassProperty || forced.contains(&sym.uid())).then(
            || Chain {
                root: sym,
                steps: Vec::new(),
            },
        );
    }
    if kind == ExpressionKind::MemberAccess {
        let base = e.left()?;
        let member = e.member_symbol()?;
        let is_this = strip_conversions(base).kind() == ExpressionKind::NamedValue
            && strip_conversions(base)
                .referenced_symbol()
                .is_some_and(|s| s.name() == "this");
        if is_this {
            return (member.kind() == SymbolKind::ClassProperty).then(|| Chain {
                root: member,
                steps: Vec::new(),
            });
        }
        let bt = base.expr_type()?.canonical();
        if !bt.is_unpacked_struct() {
            return None;
        }
        let index = bt.fields().position(|f| f.name() == member.name())?;
        let mut chain = aggregate_chain(base, forced)?;
        chain.steps.push(RawStep::Field(u32::try_from(index).ok()?));
        return Some(chain);
    }
    if kind == ExpressionKind::ElementSelect {
        let base = e.select_value()?;
        if array_shape(base.expr_type()?) != Some(ArrayShape::Fixed) {
            return None;
        }
        let mut chain = aggregate_chain(base, forced)?;
        chain.steps.push(RawStep::Index(e.selector()?));
        return Some(chain);
    }
    None
}

/// Whether `t` is an unpacked struct or a fixed-size unpacked array: a type
/// whose leaves are variables ([`Builder::declare_rand_aggregate`]).
pub fn is_aggregate_type(t: Type<'_>) -> bool {
    let c = t.canonical();
    c.is_unpacked_struct() || array_shape(c) == Some(ArrayShape::Fixed)
}

/// Whether the class property `sym` has any random leaf: it is declared
/// `rand`/`randc`, or an unpacked-struct member at any depth is. Such a
/// property has a `rand_mode` (18.8) and is declared with
/// [`Builder::declare_rand_aggregate`] (or [`Builder::declare_rand`] when it is
/// not an aggregate).
pub fn property_is_random(sym: Symbol<'_>) -> bool {
    if sym.rand_mode() != RandMode::None {
        return true;
    }
    sym.value_type().is_some_and(has_random_member)
}

fn has_random_member(t: Type<'_>) -> bool {
    let c = t.canonical();
    if c.is_unpacked_struct() {
        return c.fields().any(|f| {
            f.field_rand_mode() != RandMode::None || f.value_type().is_some_and(has_random_member)
        });
    }
    array_shape(c) == Some(ArrayShape::Fixed) && c.element_type().is_some_and(has_random_member)
}

/// What kind of value a leaf is.
#[derive(Clone, Copy, Debug)]
pub(super) enum LeafKind {
    /// An integral leaf.
    Int(Ty),
    /// A `real` or `shortreal` leaf.
    Real(Fk),
    /// A virtual interface: never randomizable.
    Vif,
    /// A `string` or `chandle`: stored, but with no solved value.
    Skipped,
    /// A class handle, queue, dynamic or associative array, or a union.
    Other,
}

/// A leaf of an aggregate property.
#[derive(Clone)]
pub(super) struct LeafRef<'d> {
    /// The property's key.
    pub root: SymbolId,
    /// `s.f[1]`: the property's name and the steps to the leaf.
    pub name: String,
    /// The steps from the property.
    pub path: Vec<Step>,
    /// Where the leaf sits in the property's value.
    pub window: Window,
    /// The effective `rand`/`randc` mode (none for a state leaf).
    pub mode: RandMode,
    /// The leaf's value kind.
    pub kind: LeafKind,
    /// The leaf's declared type.
    pub vt: Type<'d>,
}

impl LeafRef<'_> {
    pub(super) fn randc(&self) -> bool {
        self.mode == RandMode::RandC
    }
}

/// A resolved leaf expression: one leaf, or a run-time choice among several.
pub(super) enum Sel<'d> {
    Leaf(LeafRef<'d>),
    Pick {
        index: Expr,
        low: i64,
        elems: Vec<Sel<'d>>,
    },
}

/// A position inside an aggregate property while its steps are followed.
#[derive(Clone)]
struct Cursor<'d> {
    root: SymbolId,
    /// The whole property's width.
    whole: u64,
    ty: Type<'d>,
    mode: RandMode,
    path: Vec<Step>,
    name: String,
    /// The LSB of the current value within the property's value.
    bit: u64,
}

fn missing(what: impl Into<String>) -> BuildError {
    BuildError::Missing { what: what.into() }
}

impl<'d> Cursor<'d> {
    fn of(root: Symbol<'d>) -> Result<Cursor<'d>, BuildError> {
        let ty = root
            .value_type()
            .ok_or_else(|| missing(format!("value type for `{}`", root.name())))?;
        let whole = storage_width(&ty).ok_or_else(|| {
            missing(format!(
                "stored width of `{}` of type `{}`",
                root.name(),
                ty.to_sv_string()
            ))
        })?;
        Ok(Cursor {
            root: root.uid(),
            whole,
            ty,
            mode: root.rand_mode(),
            path: Vec::new(),
            name: root.name().to_string(),
            bit: 0,
        })
    }

    /// Member number `i` of the struct this is at.
    fn field(&self, i: u32) -> Result<Cursor<'d>, BuildError> {
        let t = self.ty.canonical();
        if !t.is_unpacked_struct() {
            return Err(missing(format!(
                "`{}` is not an unpacked struct",
                self.name
            )));
        }
        let f = t
            .fields()
            .nth(i as usize)
            .ok_or_else(|| missing(format!("member {i} of `{}`", self.name)))?;
        let (lsb, _) = field_slice(&t, i)
            .ok_or_else(|| missing(format!("stored position of member {i} of `{}`", self.name)))?;
        let ty = f
            .value_type()
            .ok_or_else(|| missing(format!("type of `{}.{}`", self.name, f.name())))?;
        let mut path = self.path.clone();
        path.push(Step::Field(i));
        Ok(Cursor {
            root: self.root.clone(),
            whole: self.whole,
            ty,
            // A member's own qualifier replaces the one it would inherit.
            mode: match f.field_rand_mode() {
                RandMode::None => self.mode,
                own => own,
            },
            path,
            name: format!("{}.{}", self.name, f.name()),
            bit: self.bit + lsb,
        })
    }

    /// The declared bounds `(lo, hi)` and the element type of the fixed-size
    /// array this is at.
    fn dim(&self) -> Result<(i64, i64, Type<'d>), BuildError> {
        let t = self.ty.canonical();
        match array_shape(t) {
            Some(ArrayShape::Fixed) => {}
            Some(ArrayShape::Collection | ArrayShape::Assoc) => {
                return Err(unsupported(
                    format!(
                        "`{}`, whose size is only known at run time (a dynamic array, queue or \
                         associative array), as an element of an aggregate",
                        self.name
                    ),
                    "C14",
                ));
            }
            None => {
                return Err(BuildError::OutOfRangeSelect {
                    what: format!("`{}` is not an array, so it has no elements", self.name),
                });
            }
        }
        let r = t.fixed_unpacked_array_range();
        let elem = t
            .element_type()
            .ok_or_else(|| missing(format!("element type of the array `{}`", self.name)))?;
        Ok((
            i64::from(r.left).min(i64::from(r.right)),
            i64::from(r.left).max(i64::from(r.right)),
            elem,
        ))
    }

    /// The element at declared index `i` of the array this is at.
    fn index(&self, i: i64) -> Result<Cursor<'d>, BuildError> {
        let (lo, hi, elem) = self.dim()?;
        if !(lo..=hi).contains(&i) {
            let r = self.ty.canonical().fixed_unpacked_array_range();
            return Err(BuildError::OutOfRangeSelect {
                what: format!("`{}` index {i} outside [{}:{}]", self.name, r.left, r.right),
            });
        }
        let ew = storage_width(&elem)
            .ok_or_else(|| missing(format!("stored width of an element of `{}`", self.name)))?;
        let mut path = self.path.clone();
        path.push(Step::Index(i));
        Ok(Cursor {
            root: self.root.clone(),
            whole: self.whole,
            ty: elem,
            mode: self.mode,
            path,
            name: format!("{}[{i}]", self.name),
            bit: self.bit + (i - lo) as u64 * ew,
        })
    }

    fn leaf(self) -> Result<LeafRef<'d>, BuildError> {
        let width = storage_width(&self.ty)
            .ok_or_else(|| missing(format!("stored width of `{}`", self.name)))?;
        let narrow = |v: u64| {
            u32::try_from(v).map_err(|_| {
                missing(format!(
                    "a window of `{}` within 32-bit offsets (the property is {} bits)",
                    self.name, self.whole
                ))
            })
        };
        let window = Window {
            bit: narrow(self.bit)?,
            width: narrow(width)?,
            field_width: narrow(self.whole)?,
        };
        let c = self.ty.canonical();
        let kind = if let Some(k) = real_kind(Some(self.ty)) {
            LeafKind::Real(k)
        } else if c.is_virtual_interface() {
            LeafKind::Vif
        } else if c.is_integral() {
            LeafKind::Int(ty_of(Some(self.ty), &format!("`{}`", self.name))?)
        } else if c.is_string() || c.is_chandle() {
            LeafKind::Skipped
        } else {
            LeafKind::Other
        };
        Ok(LeafRef {
            root: self.root,
            name: self.name,
            path: self.path,
            window,
            mode: self.mode,
            kind,
            vt: self.ty,
        })
    }

    /// Every random leaf at or below this, in declaration order.
    fn random_leaves(self, out: &mut Vec<LeafRef<'d>>) -> Result<(), BuildError> {
        let t = self.ty.canonical();
        if t.is_unpacked_struct() {
            for i in 0..t.fields().count() {
                self.field(i as u32)?.random_leaves(out)?;
            }
            return Ok(());
        }
        if array_shape(t) == Some(ArrayShape::Fixed) {
            let (lo, hi, _) = self.dim()?;
            for i in lo..=hi {
                self.index(i)?.random_leaves(out)?;
            }
            return Ok(());
        }
        if self.mode == RandMode::None || t.is_unpacked_union() {
            return Ok(());
        }
        let leaf = self.leaf()?;
        match leaf.kind {
            LeafKind::Int(_) | LeafKind::Real(_) => out.push(leaf),
            LeafKind::Skipped => {}
            LeafKind::Vif => return Err(BuildError::VirtualInterfaceRand { name: leaf.name }),
            LeafKind::Other => {
                return Err(unsupported(
                    format!(
                        "the random member `{}` of type `{}` inside an unpacked struct or array \
                         (a class handle, queue, dynamic array or associative array nested in a \
                         flat aggregate cannot be randomized)",
                        leaf.name,
                        leaf.vt.to_sv_string()
                    ),
                    "C14",
                ));
            }
        }
        Ok(())
    }
}

impl Builder<'_> {
    /// The cursor at the root of `sym`; a variable declared by
    /// [`Self::declare_aggregate_forced`] is random whatever its declaration.
    fn cursor_of<'d>(&self, sym: Symbol<'d>) -> Result<Cursor<'d>, BuildError> {
        let mut cur = Cursor::of(sym)?;
        if self.forced_roots.contains(&cur.root) {
            cur.mode = RandMode::Rand;
        }
        Ok(cur)
    }

    /// Declares every leaf of the unpacked-struct or fixed-size array variable
    /// `sym` random for this call whether or not it is declared `rand` (LRM
    /// 18.12: the arguments of `std::randomize` are random; 18.11 for a
    /// property), and returns their ids. A member's own `rand`/`randc`
    /// qualifier still decides its cyclic nature (18.4). The variable may then
    /// be named in the constraints like a property (`a[i]`, `a.sum()`,
    /// `foreach`, `s.m`).
    pub fn declare_aggregate_forced(
        &mut self,
        sym: Symbol<'_>,
    ) -> Result<Vec<Declared>, BuildError> {
        self.forced_roots.insert(sym.uid());
        self.declare_rand_aggregate(sym)
    }

    /// Declares every random leaf of the unpacked-struct or fixed-size
    /// unpacked-array property `sym` as a random variable, in declaration
    /// order, and returns their ids (idempotent). `sym` need not be `rand`
    /// itself: a struct with a `rand` member has random leaves (LRM 18.4).
    pub fn declare_rand_aggregate(&mut self, sym: Symbol<'_>) -> Result<Vec<Declared>, BuildError> {
        let cur = self.cursor_of(sym)?;
        if !is_aggregate_type(cur.ty) {
            return Err(missing(format!(
                "an unpacked struct or fixed-size array type for `{}` (use `declare_rand`)",
                sym.name()
            )));
        }
        let mut leaves = Vec::new();
        cur.random_leaves(&mut leaves)?;
        leaves.iter().map(|l| self.declare_leaf(l)).collect()
    }

    /// Declares element `idx` (declared indices, outermost dimension first) of
    /// the `rand` or `randc` unpacked-array property `array` as a random
    /// variable and returns its id (idempotent). It is named `a[2]` or `m[1][2]`.
    pub fn declare_rand_element(
        &mut self,
        array: Symbol<'_>,
        idx: &[i64],
    ) -> Result<u32, BuildError> {
        let mut cur = Cursor::of(array)?;
        for &i in idx {
            cur = cur.index(i)?;
        }
        if is_aggregate_type(cur.ty) {
            return Err(BuildError::OutOfRangeSelect {
                what: format!(
                    "`{}` is still an aggregate: name one index per unpacked dimension",
                    cur.name
                ),
            });
        }
        if cur.mode == RandMode::None {
            return Err(BuildError::NotRand { name: cur.name });
        }
        if idx.is_empty() {
            return Err(BuildError::OutOfRangeSelect {
                what: format!(
                    "`{}` is not an array element (use `declare_rand`)",
                    cur.name
                ),
            });
        }
        let leaf = cur.leaf()?;
        match self.declare_leaf(&leaf)? {
            Declared::Int(id) => Ok(id),
            Declared::Real(_) => Err(BuildError::RealConstraint {
                what: format!(
                    "`{}` is real: it is a real random variable, not an integral element",
                    leaf.name
                ),
            }),
        }
    }

    /// Declares the random leaf `leaf` (idempotent).
    fn declare_leaf(&mut self, leaf: &LeafRef<'_>) -> Result<Declared, BuildError> {
        let key = (leaf.root.clone(), leaf.path.clone());
        match leaf.kind {
            LeafKind::Int(_) => {
                if let Some(&id) = self.elem_ix.get(&key) {
                    return Ok(Declared::Int(id));
                }
                let id = self.new_rand_var(
                    leaf.root.clone(),
                    leaf.name.clone(),
                    leaf.vt,
                    leaf.randc(),
                    leaf.path.clone(),
                    Some(leaf.window),
                )?;
                self.elem_ix.insert(key, id);
                Ok(Declared::Int(id))
            }
            LeafKind::Real(kind) => {
                if let Some(&id) = self.real_leaf_ix.get(&key) {
                    return Ok(Declared::Real(id));
                }
                if leaf.randc() {
                    return Err(BuildError::RealConstraint {
                        what: format!(
                            "`randc` variable `{}` is real; `randc` is for integral types only \
                             (IEEE 1800-2023 18.4.2)",
                            leaf.name
                        ),
                    });
                }
                let id = self.real_vars.len() as u32;
                self.real_vars.push(RealVarInfo {
                    symbol: leaf.root.clone(),
                    name: leaf.name.clone(),
                    kind,
                    window: Some(leaf.window),
                });
                self.real_leaf_ix.insert(key, id);
                Ok(Declared::Real(id))
            }
            LeafKind::Vif | LeafKind::Skipped | LeafKind::Other => Err(missing(format!(
                "a randomizable value for `{}` of type `{}`",
                leaf.name,
                leaf.vt.to_sv_string()
            ))),
        }
    }

    /// Follows `chain` to its leaf, or to the run-time choice among leaves an
    /// index that is not a constant makes.
    fn resolve_chain<'d>(&mut self, chain: &Chain<'d>) -> Result<Sel<'d>, BuildError> {
        let cur = self.cursor_of(chain.root)?;
        self.walk(cur, &chain.steps)
    }

    fn walk<'d>(&mut self, cur: Cursor<'d>, steps: &[RawStep<'d>]) -> Result<Sel<'d>, BuildError> {
        let Some((first, rest)) = steps.split_first() else {
            return Ok(Sel::Leaf(cur.leaf()?));
        };
        match *first {
            RawStep::Field(i) => self.walk(cur.field(i)?, rest),
            RawStep::Const(i) => self.walk(cur.index(i)?, rest),
            RawStep::Index(e) => {
                let ix = self.expr(e)?;
                if let Some(i) = Self::constant_index(&ix) {
                    let (lo, hi, _) = cur.dim()?;
                    if (lo..=hi).contains(&i) {
                        return self.walk(cur.index(i)?, rest);
                    }
                }
                self.not_in_function(
                    "an element of an unpacked array property at a computed index",
                )?;
                self.arrays_allowed("an element of an unpacked array property")?;
                let mut has_var = false;
                ix.for_each_var(&mut |_, _| has_var = true);
                if has_var {
                    return Err(BuildError::BadArray {
                        what: format!(
                            "a random variable in the index of the array `{}`: an index is a \
                             constant, a state variable, a loop variable or the size (IEEE \
                             1800-2023 18.5.7.1)",
                            cur.name
                        ),
                    });
                }
                // Every element is a candidate; the registry keeps the one the
                // index names (or reports an index outside the array if the
                // constraint is live).
                let (lo, hi, _) = cur.dim()?;
                let mut elems = Vec::new();
                for i in lo..=hi {
                    elems.push(self.walk(cur.index(i)?, rest)?);
                }
                Ok(Sel::Pick {
                    index: ix,
                    low: lo,
                    elems,
                })
            }
        }
    }

    /// The value of `ix` if it involves no variable, state or array node.
    pub(super) fn constant_index(ix: &Expr) -> Option<i64> {
        if ix.any_node(&mut |n| matches!(n, Expr::Var(..) | Expr::State(..) | Expr::Array(_))) {
            return None;
        }
        let v = eval(ix, &Assignment::default()).ok()?;
        const_i64(ix.ty(), &v)
    }

    /// The leaf `chain` names, and its id if it is a declared random variable;
    /// `what` names the construct for a run-time index.
    pub(super) fn declared_leaf<'d>(
        &mut self,
        chain: Chain<'d>,
        what: &str,
    ) -> Result<(LeafRef<'d>, Option<Declared>), BuildError> {
        let Sel::Leaf(leaf) = self.resolve_chain(&chain)? else {
            return Err(unsupported(
                format!("{what} that selects an element at a computed index"),
                "C14",
            ));
        };
        let key = (leaf.root.clone(), leaf.path.clone());
        let declared = match leaf.kind {
            LeafKind::Int(_) => self.elem_ix.get(&key).map(|&id| Declared::Int(id)),
            LeafKind::Real(_) => self.real_leaf_ix.get(&key).map(|&id| Declared::Real(id)),
            LeafKind::Vif | LeafKind::Skipped | LeafKind::Other => {
                return Err(BuildError::NonIntegral {
                    what: format!("`{}` of type `{}`", leaf.name, leaf.vt.to_sv_string()),
                });
            }
        };
        Ok((leaf, declared))
    }

    /// The error for a random leaf nobody declared.
    fn undeclared(leaf: &LeafRef<'_>) -> BuildError {
        unsupported(
            format!(
                "the random variable `{}`, a leaf of an aggregate property that was not declared \
                 random (`declare_rand_aggregate`)",
                leaf.name
            ),
            "C14",
        )
    }

    /// The state variable of the non-random leaf `leaf`, registered on first use.
    fn leaf_state(&mut self, leaf: &LeafRef<'_>, ty: Ty) -> Expr {
        let key = (leaf.root.clone(), leaf.path.clone());
        if let Some(&id) = self.leaf_state_ix.get(&key) {
            return Expr::State(id, self.states[id as usize].ty);
        }
        let id = self.states.len() as u32;
        self.states.push(StateInfo {
            root_bit: 0,
            symbol: leaf.root.clone(),
            name: leaf.name.clone(),
            ty,
            via: Vec::new(),
            window: Some(leaf.window),
            class_of: false,
        });
        self.leaf_state_ix.insert(key, id);
        Expr::State(id, ty)
    }

    /// The expression for one integral leaf: its random variable, or its state.
    fn leaf_expr(&mut self, leaf: &LeafRef<'_>) -> Result<Expr, BuildError> {
        let LeafKind::Int(ty) = leaf.kind else {
            return Err(BuildError::NonIntegral {
                what: format!("`{}` of type `{}`", leaf.name, leaf.vt.to_sv_string()),
            });
        };
        if leaf.mode == RandMode::None {
            return Ok(self.leaf_state(leaf, ty));
        }
        let key = (leaf.root.clone(), leaf.path.clone());
        let &id = self
            .elem_ix
            .get(&key)
            .ok_or_else(|| Self::undeclared(leaf))?;
        if self.frame.is_some() {
            return Err(self.rand_read_in_function(&leaf.name));
        }
        Ok(Expr::Var(id, self.vars[id as usize].ty))
    }

    fn sel_expr(&mut self, sel: Sel<'_>, ty: Ty) -> Result<Expr, BuildError> {
        match sel {
            Sel::Leaf(leaf) => {
                let x = self.leaf_expr(&leaf)?;
                Ok(if x.ty() == ty {
                    x
                } else {
                    Expr::Resize(Box::new(x), ty)
                })
            }
            Sel::Pick { index, low, elems } => {
                let elems = elems
                    .into_iter()
                    .map(|s| self.sel_expr(s, ty))
                    .collect::<Result<Vec<_>, _>>()?;
                Ok(Expr::Array(ArrayNode::Pick {
                    index: Box::new(index),
                    low,
                    elems,
                    ty,
                }))
            }
        }
    }

    /// The integral value `chain` names, of type `ty`.
    pub(super) fn aggregate_value(&mut self, chain: Chain<'_>, ty: Ty) -> Result<Expr, BuildError> {
        let sel = self.resolve_chain(&chain)?;
        self.sel_expr(sel, ty)
    }

    /// The bit-stream (LRM 6.24.3, 11.4.14) of the aggregate property `chain`
    /// names: its integral leaves concatenated, the first struct member or the
    /// array element at the left bound the most significant. A real, string or
    /// handle leaf is an error (no bit pattern is solved for it).
    pub(super) fn aggregate_bitstream(&mut self, chain: Chain<'_>) -> Result<Expr, BuildError> {
        let mut parts = Vec::new();
        let mut cur = self.cursor_of(chain.root)?;
        // Walk the chain's own steps first (constant steps only: a computed
        // index would pick among leaves, which a stream cannot).
        let mut rest: &[RawStep<'_>] = &chain.steps;
        while let Some((first, tail)) = rest.split_first() {
            cur = match *first {
                RawStep::Field(i) => cur.field(i)?,
                RawStep::Const(i) => cur.index(i)?,
                RawStep::Index(e) => {
                    let ix = self.expr(e)?;
                    let i = Self::constant_index(&ix).ok_or_else(|| {
                        unsupported("a bit-stream cast of an element at a computed index", "C2")
                    })?;
                    cur.index(i)?
                }
            };
            rest = tail;
        }
        self.stream_leaves(cur, &mut parts)?;
        let w = parts.iter().map(|p| p.ty().width).sum();
        Ok(Expr::Concat(parts, Ty::unsigned(w)))
    }

    fn stream_leaves(&mut self, cur: Cursor<'_>, out: &mut Vec<Expr>) -> Result<(), BuildError> {
        let t = cur.ty.canonical();
        if t.is_unpacked_struct() {
            for i in 0..t.fields().count() {
                self.stream_leaves(cur.field(i as u32)?, out)?;
            }
        } else if array_shape(t) == Some(ArrayShape::Fixed) {
            let r = t.fixed_unpacked_array_range();
            let (l, h) = (i64::from(r.left), i64::from(r.right));
            let idx: Vec<i64> = if l <= h {
                (l..=h).collect()
            } else {
                (h..=l).rev().collect()
            };
            for i in idx {
                self.stream_leaves(cur.index(i)?, out)?;
            }
        } else {
            let leaf = cur.leaf()?;
            let LeafKind::Int(ty) = leaf.kind else {
                return Err(BuildError::NonIntegral {
                    what: format!(
                        "`{}` of type `{}` in a bit-stream cast",
                        leaf.name,
                        leaf.vt.to_sv_string()
                    ),
                });
            };
            let x = self.leaf_expr(&leaf)?;
            out.push(Expr::Resize(Box::new(x), Ty::unsigned(ty.width)));
        }
        Ok(())
    }

    /// Element `i` (a declared index) of the fixed-size array `arr` names, of
    /// type `ty`.
    pub(super) fn aggregate_element(
        &mut self,
        arr: Expression<'_>,
        i: i64,
        ty: Ty,
    ) -> Result<Expr, BuildError> {
        let mut chain = aggregate_chain(arr, &self.forced_roots).ok_or_else(|| {
            unsupported(
                "a method over an unpacked array that is not a property (or a member of a struct \
                 property) of this class",
                "C14",
            )
        })?;
        chain.steps.push(RawStep::Const(i));
        self.aggregate_value(chain, ty)
    }

    /// The real value `chain` names, of grid type `kind`.
    pub(super) fn real_aggregate(
        &mut self,
        chain: Chain<'_>,
        kind: Fk,
    ) -> Result<RealExpr, BuildError> {
        let leaf = match self.resolve_chain(&chain)? {
            Sel::Leaf(leaf) => leaf,
            Sel::Pick { .. } => {
                return Err(BuildError::RealConstraint {
                    what: "a real element of an array at a computed index (only an integral \
                           element can be chosen by a state or loop variable)"
                        .into(),
                });
            }
        };
        if !matches!(leaf.kind, LeafKind::Real(_)) {
            return Err(missing(format!("a real value for `{}`", leaf.name)));
        }
        if leaf.mode == RandMode::None {
            let st = self.leaf_state(&leaf, state_ty(kind));
            return self.real_state(st, kind);
        }
        let key = (leaf.root.clone(), leaf.path.clone());
        let &id = self
            .real_leaf_ix
            .get(&key)
            .ok_or_else(|| Self::undeclared(&leaf))?;
        Ok(RealExpr::Var(id))
    }
}