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transpile: Refactor, document and correct uses of used() flag #1975

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1ff24a5
transpile: Rename `ExprContext::used` to `is_used` and document it
Rua Aug 15, 2026
5b9770c
transpile: Replace `ExprContext::is_used` and `is_unused` with direct…
Rua Aug 15, 2026
28179a9
transpile: Pass `used()` for conditions of control flow
Rua Aug 16, 2026
be63aa3
transpile: Pass `used()` for macro initializer
Rua Aug 16, 2026
4de31ca
transpile: Pass `used()` correctly for assignment operands
Rua Aug 16, 2026
9809867
transpile: Set `is_used` to `false` by default at the top level
Rua Aug 16, 2026
c6d949c
transpile: Remove `used()` for children of pure expressions
Rua Aug 16, 2026
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8 changes: 4 additions & 4 deletions c2rust-transpile/src/cfg/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1508,7 +1508,7 @@ impl CfgBuilder {

// Condition
let (stmts, val) = translator
.convert_condition(ctx, true, scrutinee)?
.convert_condition(ctx.used(), true, scrutinee)?
.discard_unsafe();
wip.extend(stmts);

Expand Down Expand Up @@ -1581,7 +1581,7 @@ impl CfgBuilder {

// Condition
let (stmts, val) = translator
.convert_condition(ctx, true, condition)?
.convert_condition(ctx.used(), true, condition)?
.discard_unsafe();
let cond_val = translator
.ast_context
Expand Down Expand Up @@ -1660,7 +1660,7 @@ impl CfgBuilder {

// Condition
let (stmts, val) = translator
.convert_condition(ctx, true, condition)?
.convert_condition(ctx.used(), true, condition)?
.discard_unsafe();
let cond_val = translator
.ast_context
Expand Down Expand Up @@ -1715,7 +1715,7 @@ impl CfgBuilder {
// Condition
if let Some(cond) = condition {
let (stmts, val) = translator
.convert_condition(ctx, true, cond)?
.convert_condition(ctx.used(), true, cond)?
.discard_unsafe();
let cond_val = translator
.ast_context
Expand Down
6 changes: 3 additions & 3 deletions c2rust-transpile/src/translator/builtins.rs
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Expand Up @@ -330,7 +330,7 @@ impl<'c> Translation<'c> {
}

"__builtin_va_start" => {
if ctx.is_unused() && args.len() == 2 {
if !ctx.is_used && args.len() == 2 {
if let Some(va_id) = self.match_vastart(args[0]) {
if self.ast_context.get_decl(&va_id).is_some() {
let dst = self.convert_expr(ctx.used(), args[0], None)?;
Expand All @@ -352,7 +352,7 @@ impl<'c> Translation<'c> {
Err(TranslationError::generic("Unsupported va_start"))
}
"__builtin_va_copy" => {
if ctx.is_unused() && args.len() == 2 {
if !ctx.is_used && args.len() == 2 {
if let Some((_dst_va_id, _src_va_id)) = self.match_vacopy(args[0], args[1]) {
let dst = self.convert_expr(ctx.used(), args[0], None)?;
let src = self.convert_expr(ctx.used(), args[1], None)?;
Expand All @@ -370,7 +370,7 @@ impl<'c> Translation<'c> {
Err(TranslationError::generic("Unsupported va_copy"))
}
"__builtin_va_end" => {
if ctx.is_unused() && args.len() == 1 {
if !ctx.is_used && args.len() == 1 {
if let Some(_va_id) = self.match_vaend(args[0]) {
// nothing to do since the translated Rust `va_list` values get `Drop`'ed.
return Ok(WithStmts::new_val(self.panic("va_end stub")));
Expand Down
8 changes: 4 additions & 4 deletions c2rust-transpile/src/translator/literals.rs
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Expand Up @@ -242,7 +242,7 @@ impl<'c> Translation<'c> {

let to_array_element = |id: CExprId| -> TranslationResult<_> {
let val =
self.convert_expr(ctx.used(), id, Some(CQualTypeId::new(element_type_id)))?;
self.convert_expr(ctx, id, Some(CQualTypeId::new(element_type_id)))?;
val.try_map(|x| {
// Array literals require all of their elements to be
// the correct type; they will not use implicit casts to
Expand Down Expand Up @@ -309,7 +309,7 @@ impl<'c> Translation<'c> {
// * `ptr_extra_braces`
// * `array_of_ptrs`
// * `array_of_arrays`
self.convert_expr(ctx.used(), single, expected_type_id)
self.convert_expr(ctx, single, expected_type_id)
}
&[single] if is_zero_literal(single) && n > 1 => {
// This was likely a C array of the form `int x[16] = { 0 }`.
Expand Down Expand Up @@ -348,9 +348,9 @@ impl<'c> Translation<'c> {
}
ref kind if kind.is_scalar() => {
if let Some(&first) = ids.first() {
self.convert_expr(ctx.used(), first, expected_type_id)
self.convert_expr(ctx, first, expected_type_id)
} else {
self.implicit_default_expr(ctx.used(), result_type_id.ctype)
self.implicit_default_expr(ctx, result_type_id.ctype)
}
}
ref t => Err(format_err!("Init list not implemented for {:?}", t).into()),
Expand Down
2 changes: 1 addition & 1 deletion c2rust-transpile/src/translator/macros.rs
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Expand Up @@ -80,7 +80,7 @@ impl<'c> Translation<'c> {
.kind
.get_type()
.ok_or_else(|| format_err!("Invalid expression type"))?;
let expr = self.convert_expr(ctx, id, None)?;
let expr = self.convert_expr(ctx.used(), id, None)?;

// Join ty and cur_ty to the smaller of the two types. If the
// types are not cast-compatible, abort the fold.
Expand Down
58 changes: 36 additions & 22 deletions c2rust-transpile/src/translator/mod.rs
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Expand Up @@ -122,7 +122,23 @@ pub enum ReplaceMode {
/// Options that impact an expression and all of its subexpressions.
#[derive(Copy, Clone, Debug)]
pub struct ExprContext {
used: bool,
/// Whether the result value of the expression is used in a larger expression.
///
/// When the result value is not used in a particular context, only the side effects of the
/// expression matter. The `stmts` field of `WithStmts` should hold any statements with side
/// effects, and the `val` field is expected to be discarded. It should not appear in the final
/// transpiler output, and may be an expression that panics when evaluated.
///
/// `is_used` should be `false` for the top-level expression of an `ExprStmt`, the increment
/// expression of a `for` loop, the `lhs` of a comma operator expression, and other such cases.
/// It should be `true` if an expression is needed to evaluate the side effects of a parent
/// expression, such as the arguments of a function call, the operands of an assignment
/// expression, the expression of a `return` statement, etc.
///
/// If an expression is pure (has no side effects), then it should inherit its `is_used` value
/// from its parent expression: if the parent expression is going to be discarded, then so are
/// all of its pure child expressions.
is_used: bool,

/// In a Rust const context, for example in a static initializer or constant-like macro
/// translation.
Expand All @@ -149,20 +165,18 @@ pub struct ExprContext {

impl ExprContext {
pub fn used(self) -> Self {
ExprContext { used: true, ..self }
ExprContext {
is_used: true,
..self
}
}
pub fn unused(self) -> Self {
ExprContext {
used: false,
is_used: false,
..self
}
}
pub fn is_used(&self) -> bool {
self.used
}
pub fn is_unused(&self) -> bool {
!self.used
}

pub fn decay_ref(self) -> Self {
ExprContext {
decay_ref: DecayRef::Yes,
Expand Down Expand Up @@ -855,7 +869,7 @@ pub fn translate(
) -> (String, Option<DeclMap>, PragmaVec, CrateSet) {
let mut t = Translation::new(ast_context, tcfg, main_file);
let ctx = ExprContext {
used: true,
is_used: false,
is_static: false,
is_const: false,
decay_ref: DecayRef::Default,
Expand Down Expand Up @@ -2455,7 +2469,7 @@ impl<'c> Translation<'c> {

let null_pointer_case =
|ptr: CExprId, is_null: bool| -> TranslationResult<WithStmts<Box<Expr>>> {
let val = self.convert_expr(ctx.used().decay_ref(), ptr, None)?;
let val = self.convert_expr(ctx.decay_ref(), ptr, None)?;
let ptr_type = self
.ast_context
.index_unwrap_parens(ptr)
Expand Down Expand Up @@ -2510,7 +2524,7 @@ impl<'c> Translation<'c> {
// in https://github.com/rust-lang/rust/issues/53772, you cant compare a reference (lhs) to
// a ptr (rhs) (even though the reverse works!). We could also be smarter here and just
// specify Yes for that particular case, given enough analysis.
let val = self.convert_expr(ctx.used().decay_ref(), cond_id, None)?;
let val = self.convert_expr(ctx.decay_ref(), cond_id, None)?;
val.try_map(|e| self.match_bool(ctx, target, ty_id, e))
}
}
Expand Down Expand Up @@ -3066,7 +3080,7 @@ impl<'c> Translation<'c> {

let elts = self.compute_size_of_type(ctx, expected_type_id, result_type_id, elts)?;
return elts.and_then_try(|lhs| {
let len = self.convert_expr(ctx.used().not_static(), len, expected_type_id)?;
let len = self.convert_expr(ctx.not_static(), len, expected_type_id)?;
Ok(len.map(|len| {
let rhs = cast_int(len, "usize", true);
mk().binary_expr(BinOp::Mul(Default::default()), lhs, rhs)
Expand Down Expand Up @@ -3166,10 +3180,10 @@ impl<'c> Translation<'c> {
/// Translate a C expression into a Rust one, possibly collecting side-effecting statements
/// to run before the expression.
///
/// `ctx.is_used()` informs us how the C expression we are translating is used in the C
/// `ctx.is_used` informs us how the C expression we are translating is used in the C
/// program.
///
/// In the case that `ctx.is_unused()`, all side-effecting components will be in the
/// In the case that `!ctx.is_used`, all side-effecting components will be in the
/// `stmts` field of the output and it is expected that the `val` field of the output will be
/// ignored.
///
Expand Down Expand Up @@ -3391,12 +3405,12 @@ impl<'c> Translation<'c> {
}

Conditional(ty, cond, lhs, rhs) => {
let cond = self.convert_condition(ctx, true, cond)?;
let cond = self.convert_condition(ctx.used(), true, cond)?;

let lhs = self.convert_expr(ctx, lhs, Some(override_ty.unwrap_or(ty)))?;
let rhs = self.convert_expr(ctx, rhs, Some(override_ty.unwrap_or(ty)))?;

if ctx.is_unused() {
if !ctx.is_used {
let is_unsafe = lhs.is_unsafe() || rhs.is_unsafe();
let then = mk().block(lhs.into_stmts());
let else_ = mk().block_expr(mk().block(rhs.into_stmts()));
Expand Down Expand Up @@ -3431,9 +3445,9 @@ impl<'c> Translation<'c> {
BinaryConditional(ty, lhs, rhs) => {
let rhs = self.convert_expr(ctx, rhs, None)?;

if ctx.is_unused() {
if !ctx.is_used {
let lhs = self
.convert_condition(ctx, false, lhs)?
.convert_condition(ctx.used(), false, lhs)?
.merge_unsafe(rhs.is_unsafe());

Ok(lhs.and_then(|val| {
Expand Down Expand Up @@ -3746,7 +3760,7 @@ impl<'c> Translation<'c> {
expr: WithStmts<Box<Expr>>,
panic_msg: &str,
) -> WithStmts<Box<Expr>> {
if ctx.is_unused() {
if !ctx.is_used {
// Recall that if `used` is false, the `stmts` field of the output must contain
// all side-effects (and a function call can always have side-effects)
expr.and_then(|expr| {
Expand Down Expand Up @@ -3816,7 +3830,7 @@ impl<'c> Translation<'c> {
match as_semi_break_stmt(&stmt, &lbl) {
Some(val) => {
let block = mk().block_expr(match val {
Some(val) if ctx.is_used() => WithStmts::new(stmts, val).to_block(),
Some(val) if ctx.is_used => WithStmts::new(stmts, val).to_block(),
_ => mk().block(stmts),
});

Expand Down Expand Up @@ -3845,7 +3859,7 @@ impl<'c> Translation<'c> {
))
}
_ => {
if ctx.is_unused() {
if !ctx.is_used {
let val =
self.panic_or_err("Empty statement expression is not supposed to be used");
Ok(WithStmts::new_val(val))
Expand Down
2 changes: 1 addition & 1 deletion c2rust-transpile/src/translator/named_references.rs
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Expand Up @@ -93,7 +93,7 @@ impl<'c> Translation<'c> {

let is_pure = self.ast_context.is_expr_pure(reference);
let read = |write| self.read(reference_ty, write);
let reference = self.convert_expr(ctx.used(), reference, Some(reference_ty))?;
let reference = self.convert_expr(ctx, reference, Some(reference_ty))?;
reference.and_then_try(|reference| {
if is_lvalue(&reference) && (is_pure || !uses_read) {
let rvalue = uses_read.then(|| read(reference.clone())).transpose()?;
Expand Down
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