//! A classic liveness analysis based on dataflow over the AST. Computes, //! for each local variable in a function, whether that variable is live //! at a given point. Program execution points are identified by their //! IDs. //! //! # Basic idea //! //! The basic model is that each local variable is assigned an index. We //! represent sets of local variables using a vector indexed by this //! index. The value in the vector is either 0, indicating the variable //! is dead, or the ID of an expression that uses the variable. //! //! We conceptually walk over the AST in reverse execution order. If we //! find a use of a variable, we add it to the set of live variables. If //! we find an assignment to a variable, we remove it from the set of live //! variables. When we have to merge two flows, we take the union of //! those two flows -- if the variable is live on both paths, we simply //! pick one ID. In the event of loops, we continue doing this until a //! fixed point is reached. //! //! ## Checking initialization //! //! At the function entry point, all variables must be dead. If this is //! not the case, we can report an error using the ID found in the set of //! live variables, which identifies a use of the variable which is not //! dominated by an assignment. //! //! ## Checking moves //! //! After each explicit move, the variable must be dead. //! //! ## Computing last uses //! //! Any use of the variable where the variable is dead afterwards is a //! last use. //! //! # Implementation details //! //! The actual implementation contains two (nested) walks over the AST. //! The outer walk has the job of building up the ir_maps instance for the //! enclosing function. On the way down the tree, it identifies those AST //! nodes and variable IDs that will be needed for the liveness analysis //! and assigns them contiguous IDs. The liveness ID for an AST node is //! called a `live_node` (it's a newtype'd `u32`) and the ID for a variable //! is called a `variable` (another newtype'd `u32`). //! //! On the way back up the tree, as we are about to exit from a function //! declaration we allocate a `liveness` instance. Now that we know //! precisely how many nodes and variables we need, we can allocate all //! the various arrays that we will need to precisely the right size. We then //! perform the actual propagation on the `liveness` instance. //! //! This propagation is encoded in the various `propagate_through_*()` //! methods. It effectively does a reverse walk of the AST; whenever we //! reach a loop node, we iterate until a fixed point is reached. //! //! ## The `RWU` struct //! //! At each live node `N`, we track three pieces of information for each //! variable `V` (these are encapsulated in the `RWU` struct): //! //! - `reader`: the `LiveNode` ID of some node which will read the value //! that `V` holds on entry to `N`. Formally: a node `M` such //! that there exists a path `P` from `N` to `M` where `P` does not //! write `V`. If the `reader` is `invalid_node()`, then the current //! value will never be read (the variable is dead, essentially). //! //! - `writer`: the `LiveNode` ID of some node which will write the //! variable `V` and which is reachable from `N`. Formally: a node `M` //! such that there exists a path `P` from `N` to `M` and `M` writes //! `V`. If the `writer` is `invalid_node()`, then there is no writer //! of `V` that follows `N`. //! //! - `used`: a boolean value indicating whether `V` is *used*. We //! distinguish a *read* from a *use* in that a *use* is some read that //! is not just used to generate a new value. For example, `x += 1` is //! a read but not a use. This is used to generate better warnings. //! //! ## Special Variables //! //! We generate various special variables for various, well, special purposes. //! These are described in the `specials` struct: //! //! - `exit_ln`: a live node that is generated to represent every 'exit' from //! the function, whether it be by explicit return, panic, or other means. //! //! - `fallthrough_ln`: a live node that represents a fallthrough //! //! - `clean_exit_var`: a synthetic variable that is only 'read' from the //! fallthrough node. It is only live if the function could converge //! via means other than an explicit `return` expression. That is, it is //! only dead if the end of the function's block can never be reached. //! It is the responsibility of typeck to ensure that there are no //! `return` expressions in a function declared as diverging. use self::LiveNodeKind::*; use self::VarKind::*; use rustc::hir::map::Map; use rustc::ty::query::Providers; use rustc::ty::{self, TyCtxt}; use rustc_ast::ast; use rustc_data_structures::fx::FxIndexMap; use rustc_errors::Applicability; use rustc_hir as hir; use rustc_hir::def::*; use rustc_hir::def_id::DefId; use rustc_hir::intravisit::{self, FnKind, NestedVisitorMap, Visitor}; use rustc_hir::{Expr, HirId, HirIdMap, HirIdSet, Node}; use rustc_session::lint; use rustc_span::symbol::sym; use rustc_span::Span; use std::collections::VecDeque; use std::io; use std::io::prelude::*; use std::rc::Rc; use std::{fmt, u32}; #[derive(Copy, Clone, PartialEq)] struct Variable(u32); #[derive(Copy, Clone, PartialEq)] struct LiveNode(u32); impl Variable { fn get(&self) -> usize { self.0 as usize } } impl LiveNode { fn get(&self) -> usize { self.0 as usize } } #[derive(Copy, Clone, PartialEq, Debug)] enum LiveNodeKind { UpvarNode(Span), ExprNode(Span), VarDefNode(Span), ExitNode, } fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_>) -> String { let sm = tcx.sess.source_map(); match lnk { UpvarNode(s) => format!("Upvar node [{}]", sm.span_to_string(s)), ExprNode(s) => format!("Expr node [{}]", sm.span_to_string(s)), VarDefNode(s) => format!("Var def node [{}]", sm.span_to_string(s)), ExitNode => "Exit node".to_owned(), } } impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> { type Map = Map<'tcx>; fn nested_visit_map(&mut self) -> NestedVisitorMap { NestedVisitorMap::OnlyBodies(self.tcx.hir()) } fn visit_fn( &mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl<'tcx>, b: hir::BodyId, s: Span, id: HirId, ) { visit_fn(self, fk, fd, b, s, id); } fn visit_local(&mut self, l: &'tcx hir::Local<'tcx>) { visit_local(self, l); } fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) { visit_expr(self, ex); } fn visit_arm(&mut self, a: &'tcx hir::Arm<'tcx>) { visit_arm(self, a); } } fn check_mod_liveness(tcx: TyCtxt<'_>, module_def_id: DefId) { tcx.hir().visit_item_likes_in_module( module_def_id, &mut IrMaps::new(tcx, module_def_id).as_deep_visitor(), ); } pub fn provide(providers: &mut Providers<'_>) { *providers = Providers { check_mod_liveness, ..*providers }; } impl fmt::Debug for LiveNode { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "ln({})", self.get()) } } impl fmt::Debug for Variable { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "v({})", self.get()) } } // ______________________________________________________________________ // Creating ir_maps // // This is the first pass and the one that drives the main // computation. It walks up and down the IR once. On the way down, // we count for each function the number of variables as well as // liveness nodes. A liveness node is basically an expression or // capture clause that does something of interest: either it has // interesting control flow or it uses/defines a local variable. // // On the way back up, at each function node we create liveness sets // (we now know precisely how big to make our various vectors and so // forth) and then do the data-flow propagation to compute the set // of live variables at each program point. // // Finally, we run back over the IR one last time and, using the // computed liveness, check various safety conditions. For example, // there must be no live nodes at the definition site for a variable // unless it has an initializer. Similarly, each non-mutable local // variable must not be assigned if there is some successor // assignment. And so forth. impl LiveNode { fn is_valid(&self) -> bool { self.0 != u32::MAX } } fn invalid_node() -> LiveNode { LiveNode(u32::MAX) } struct CaptureInfo { ln: LiveNode, var_hid: HirId, } #[derive(Copy, Clone, Debug)] struct LocalInfo { id: HirId, name: ast::Name, is_shorthand: bool, } #[derive(Copy, Clone, Debug)] enum VarKind { Param(HirId, ast::Name), Local(LocalInfo), CleanExit, } struct IrMaps<'tcx> { tcx: TyCtxt<'tcx>, body_owner: DefId, num_live_nodes: usize, num_vars: usize, live_node_map: HirIdMap, variable_map: HirIdMap, capture_info_map: HirIdMap>>, var_kinds: Vec, lnks: Vec, } impl IrMaps<'tcx> { fn new(tcx: TyCtxt<'tcx>, body_owner: DefId) -> IrMaps<'tcx> { IrMaps { tcx, body_owner, num_live_nodes: 0, num_vars: 0, live_node_map: HirIdMap::default(), variable_map: HirIdMap::default(), capture_info_map: Default::default(), var_kinds: Vec::new(), lnks: Vec::new(), } } fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode { let ln = LiveNode(self.num_live_nodes as u32); self.lnks.push(lnk); self.num_live_nodes += 1; debug!("{:?} is of kind {}", ln, live_node_kind_to_string(lnk, self.tcx)); ln } fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) { let ln = self.add_live_node(lnk); self.live_node_map.insert(hir_id, ln); debug!("{:?} is node {:?}", ln, hir_id); } fn add_variable(&mut self, vk: VarKind) -> Variable { let v = Variable(self.num_vars as u32); self.var_kinds.push(vk); self.num_vars += 1; match vk { Local(LocalInfo { id: node_id, .. }) | Param(node_id, _) => { self.variable_map.insert(node_id, v); } CleanExit => {} } debug!("{:?} is {:?}", v, vk); v } fn variable(&self, hir_id: HirId, span: Span) -> Variable { match self.variable_map.get(&hir_id) { Some(&var) => var, None => { span_bug!(span, "no variable registered for id {:?}", hir_id); } } } fn variable_name(&self, var: Variable) -> String { match self.var_kinds[var.get()] { Local(LocalInfo { name, .. }) | Param(_, name) => name.to_string(), CleanExit => "".to_owned(), } } fn variable_is_shorthand(&self, var: Variable) -> bool { match self.var_kinds[var.get()] { Local(LocalInfo { is_shorthand, .. }) => is_shorthand, Param(..) | CleanExit => false, } } fn set_captures(&mut self, hir_id: HirId, cs: Vec) { self.capture_info_map.insert(hir_id, Rc::new(cs)); } fn lnk(&self, ln: LiveNode) -> LiveNodeKind { self.lnks[ln.get()] } } fn visit_fn<'tcx>( ir: &mut IrMaps<'tcx>, fk: FnKind<'tcx>, decl: &'tcx hir::FnDecl<'tcx>, body_id: hir::BodyId, sp: Span, id: hir::HirId, ) { debug!("visit_fn"); // swap in a new set of IR maps for this function body: let def_id = ir.tcx.hir().local_def_id(id); let mut fn_maps = IrMaps::new(ir.tcx, def_id); // Don't run unused pass for #[derive()] if let FnKind::Method(..) = fk { let parent = ir.tcx.hir().get_parent_item(id); if let Some(Node::Item(i)) = ir.tcx.hir().find(parent) { if i.attrs.iter().any(|a| a.check_name(sym::automatically_derived)) { return; } } } debug!("creating fn_maps: {:p}", &fn_maps); let body = ir.tcx.hir().body(body_id); for param in body.params { let is_shorthand = match param.pat.kind { rustc_hir::PatKind::Struct(..) => true, _ => false, }; param.pat.each_binding(|_bm, hir_id, _x, ident| { debug!("adding parameters {:?}", hir_id); let var = if is_shorthand { Local(LocalInfo { id: hir_id, name: ident.name, is_shorthand: true }) } else { Param(hir_id, ident.name) }; fn_maps.add_variable(var); }) } // gather up the various local variables, significant expressions, // and so forth: intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id); // compute liveness let mut lsets = Liveness::new(&mut fn_maps, def_id); let entry_ln = lsets.compute(&body.value); // check for various error conditions lsets.visit_body(body); lsets.warn_about_unused_args(body, entry_ln); } fn add_from_pat(ir: &mut IrMaps<'_>, pat: &hir::Pat<'_>) { // For struct patterns, take note of which fields used shorthand // (`x` rather than `x: x`). let mut shorthand_field_ids = HirIdSet::default(); let mut pats = VecDeque::new(); pats.push_back(pat); while let Some(pat) = pats.pop_front() { use rustc_hir::PatKind::*; match &pat.kind { Binding(.., inner_pat) => { pats.extend(inner_pat.iter()); } Struct(_, fields, _) => { let ids = fields.iter().filter(|f| f.is_shorthand).map(|f| f.pat.hir_id); shorthand_field_ids.extend(ids); } Ref(inner_pat, _) | Box(inner_pat) => { pats.push_back(inner_pat); } TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => { pats.extend(inner_pats.iter()); } Slice(pre_pats, inner_pat, post_pats) => { pats.extend(pre_pats.iter()); pats.extend(inner_pat.iter()); pats.extend(post_pats.iter()); } _ => {} } } pat.each_binding(|_, hir_id, _, ident| { ir.add_live_node_for_node(hir_id, VarDefNode(ident.span)); ir.add_variable(Local(LocalInfo { id: hir_id, name: ident.name, is_shorthand: shorthand_field_ids.contains(&hir_id), })); }); } fn visit_local<'tcx>(ir: &mut IrMaps<'tcx>, local: &'tcx hir::Local<'tcx>) { add_from_pat(ir, &local.pat); intravisit::walk_local(ir, local); } fn visit_arm<'tcx>(ir: &mut IrMaps<'tcx>, arm: &'tcx hir::Arm<'tcx>) { add_from_pat(ir, &arm.pat); intravisit::walk_arm(ir, arm); } fn visit_expr<'tcx>(ir: &mut IrMaps<'tcx>, expr: &'tcx Expr<'tcx>) { match expr.kind { // live nodes required for uses or definitions of variables: hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => { debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res); if let Res::Local(var_hir_id) = path.res { let upvars = ir.tcx.upvars(ir.body_owner); if !upvars.map_or(false, |upvars| upvars.contains_key(&var_hir_id)) { ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span)); } } intravisit::walk_expr(ir, expr); } hir::ExprKind::Closure(..) => { // Interesting control flow (for loops can contain labeled // breaks or continues) ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span)); // Make a live_node for each captured variable, with the span // being the location that the variable is used. This results // in better error messages than just pointing at the closure // construction site. let mut call_caps = Vec::new(); let closure_def_id = ir.tcx.hir().local_def_id(expr.hir_id); if let Some(upvars) = ir.tcx.upvars(closure_def_id) { let parent_upvars = ir.tcx.upvars(ir.body_owner); call_caps.extend(upvars.iter().filter_map(|(&var_id, upvar)| { let has_parent = parent_upvars.map_or(false, |upvars| upvars.contains_key(&var_id)); if !has_parent { let upvar_ln = ir.add_live_node(UpvarNode(upvar.span)); Some(CaptureInfo { ln: upvar_ln, var_hid: var_id }) } else { None } })); } ir.set_captures(expr.hir_id, call_caps); let old_body_owner = ir.body_owner; ir.body_owner = closure_def_id; intravisit::walk_expr(ir, expr); ir.body_owner = old_body_owner; } // live nodes required for interesting control flow: hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => { ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span)); intravisit::walk_expr(ir, expr); } hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => { ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span)); intravisit::walk_expr(ir, expr); } // otherwise, live nodes are not required: hir::ExprKind::Index(..) | hir::ExprKind::Field(..) | hir::ExprKind::Array(..) | hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) | hir::ExprKind::Tup(..) | hir::ExprKind::Binary(..) | hir::ExprKind::AddrOf(..) | hir::ExprKind::Cast(..) | hir::ExprKind::DropTemps(..) | hir::ExprKind::Unary(..) | hir::ExprKind::Break(..) | hir::ExprKind::Continue(_) | hir::ExprKind::Lit(_) | hir::ExprKind::Ret(..) | hir::ExprKind::Block(..) | hir::ExprKind::Assign(..) | hir::ExprKind::AssignOp(..) | hir::ExprKind::Struct(..) | hir::ExprKind::Repeat(..) | hir::ExprKind::InlineAsm(..) | hir::ExprKind::Box(..) | hir::ExprKind::Yield(..) | hir::ExprKind::Type(..) | hir::ExprKind::Err | hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => { intravisit::walk_expr(ir, expr); } } } // ______________________________________________________________________ // Computing liveness sets // // Actually we compute just a bit more than just liveness, but we use // the same basic propagation framework in all cases. #[derive(Clone, Copy)] struct RWU { reader: LiveNode, writer: LiveNode, used: bool, } /// Conceptually, this is like a `Vec`. But the number of `RWU`s can get /// very large, so it uses a more compact representation that takes advantage /// of the fact that when the number of `RWU`s is large, most of them have an /// invalid reader and an invalid writer. struct RWUTable { /// Each entry in `packed_rwus` is either INV_INV_FALSE, INV_INV_TRUE, or /// an index into `unpacked_rwus`. In the common cases, this compacts the /// 65 bits of data into 32; in the uncommon cases, it expands the 65 bits /// in 96. /// /// More compact representations are possible -- e.g., use only 2 bits per /// packed `RWU` and make the secondary table a HashMap that maps from /// indices to `RWU`s -- but this one strikes a good balance between size /// and speed. packed_rwus: Vec, unpacked_rwus: Vec, } // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: false }`. const INV_INV_FALSE: u32 = u32::MAX; // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: true }`. const INV_INV_TRUE: u32 = u32::MAX - 1; impl RWUTable { fn new(num_rwus: usize) -> RWUTable { Self { packed_rwus: vec![INV_INV_FALSE; num_rwus], unpacked_rwus: vec![] } } fn get(&self, idx: usize) -> RWU { let packed_rwu = self.packed_rwus[idx]; match packed_rwu { INV_INV_FALSE => RWU { reader: invalid_node(), writer: invalid_node(), used: false }, INV_INV_TRUE => RWU { reader: invalid_node(), writer: invalid_node(), used: true }, _ => self.unpacked_rwus[packed_rwu as usize], } } fn get_reader(&self, idx: usize) -> LiveNode { let packed_rwu = self.packed_rwus[idx]; match packed_rwu { INV_INV_FALSE | INV_INV_TRUE => invalid_node(), _ => self.unpacked_rwus[packed_rwu as usize].reader, } } fn get_writer(&self, idx: usize) -> LiveNode { let packed_rwu = self.packed_rwus[idx]; match packed_rwu { INV_INV_FALSE | INV_INV_TRUE => invalid_node(), _ => self.unpacked_rwus[packed_rwu as usize].writer, } } fn get_used(&self, idx: usize) -> bool { let packed_rwu = self.packed_rwus[idx]; match packed_rwu { INV_INV_FALSE => false, INV_INV_TRUE => true, _ => self.unpacked_rwus[packed_rwu as usize].used, } } #[inline] fn copy_packed(&mut self, dst_idx: usize, src_idx: usize) { self.packed_rwus[dst_idx] = self.packed_rwus[src_idx]; } fn assign_unpacked(&mut self, idx: usize, rwu: RWU) { if rwu.reader == invalid_node() && rwu.writer == invalid_node() { // When we overwrite an indexing entry in `self.packed_rwus` with // `INV_INV_{TRUE,FALSE}` we don't remove the corresponding entry // from `self.unpacked_rwus`; it's not worth the effort, and we // can't have entries shifting around anyway. self.packed_rwus[idx] = if rwu.used { INV_INV_TRUE } else { INV_INV_FALSE } } else { // Add a new RWU to `unpacked_rwus` and make `packed_rwus[idx]` // point to it. self.packed_rwus[idx] = self.unpacked_rwus.len() as u32; self.unpacked_rwus.push(rwu); } } fn assign_inv_inv(&mut self, idx: usize) { self.packed_rwus[idx] = if self.get_used(idx) { INV_INV_TRUE } else { INV_INV_FALSE }; } } #[derive(Copy, Clone)] struct Specials { exit_ln: LiveNode, fallthrough_ln: LiveNode, clean_exit_var: Variable, } const ACC_READ: u32 = 1; const ACC_WRITE: u32 = 2; const ACC_USE: u32 = 4; struct Liveness<'a, 'tcx> { ir: &'a mut IrMaps<'tcx>, tables: &'a ty::TypeckTables<'tcx>, param_env: ty::ParamEnv<'tcx>, s: Specials, successors: Vec, rwu_table: RWUTable, // mappings from loop node ID to LiveNode // ("break" label should map to loop node ID, // it probably doesn't now) break_ln: HirIdMap, cont_ln: HirIdMap, } impl<'a, 'tcx> Liveness<'a, 'tcx> { fn new(ir: &'a mut IrMaps<'tcx>, def_id: DefId) -> Liveness<'a, 'tcx> { // Special nodes and variables: // - exit_ln represents the end of the fn, either by return or panic // - implicit_ret_var is a pseudo-variable that represents // an implicit return let specials = Specials { exit_ln: ir.add_live_node(ExitNode), fallthrough_ln: ir.add_live_node(ExitNode), clean_exit_var: ir.add_variable(CleanExit), }; let tables = ir.tcx.typeck_tables_of(def_id); let param_env = ir.tcx.param_env(def_id); let num_live_nodes = ir.num_live_nodes; let num_vars = ir.num_vars; Liveness { ir, tables, param_env, s: specials, successors: vec![invalid_node(); num_live_nodes], rwu_table: RWUTable::new(num_live_nodes * num_vars), break_ln: Default::default(), cont_ln: Default::default(), } } fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode { match self.ir.live_node_map.get(&hir_id) { Some(&ln) => ln, None => { // This must be a mismatch between the ir_map construction // above and the propagation code below; the two sets of // code have to agree about which AST nodes are worth // creating liveness nodes for. span_bug!(span, "no live node registered for node {:?}", hir_id); } } } fn variable(&self, hir_id: HirId, span: Span) -> Variable { self.ir.variable(hir_id, span) } fn define_bindings_in_pat(&mut self, pat: &hir::Pat<'_>, mut succ: LiveNode) -> LiveNode { // In an or-pattern, only consider the first pattern; any later patterns // must have the same bindings, and we also consider the first pattern // to be the "authoritative" set of ids. pat.each_binding_or_first(&mut |_, hir_id, pat_sp, ident| { let ln = self.live_node(hir_id, pat_sp); let var = self.variable(hir_id, ident.span); self.init_from_succ(ln, succ); self.define(ln, var); succ = ln; }); succ } fn idx(&self, ln: LiveNode, var: Variable) -> usize { ln.get() * self.ir.num_vars + var.get() } fn live_on_entry(&self, ln: LiveNode, var: Variable) -> Option { assert!(ln.is_valid()); let reader = self.rwu_table.get_reader(self.idx(ln, var)); if reader.is_valid() { Some(self.ir.lnk(reader)) } else { None } } // Is this variable live on entry to any of its successor nodes? fn live_on_exit(&self, ln: LiveNode, var: Variable) -> Option { let successor = self.successors[ln.get()]; self.live_on_entry(successor, var) } fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool { assert!(ln.is_valid()); self.rwu_table.get_used(self.idx(ln, var)) } fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> Option { assert!(ln.is_valid()); let writer = self.rwu_table.get_writer(self.idx(ln, var)); if writer.is_valid() { Some(self.ir.lnk(writer)) } else { None } } fn assigned_on_exit(&self, ln: LiveNode, var: Variable) -> Option { let successor = self.successors[ln.get()]; self.assigned_on_entry(successor, var) } fn indices2(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize), { let node_base_idx = self.idx(ln, Variable(0)); let succ_base_idx = self.idx(succ_ln, Variable(0)); for var_idx in 0..self.ir.num_vars { op(self, node_base_idx + var_idx, succ_base_idx + var_idx); } } fn write_vars(&self, wr: &mut dyn Write, ln: LiveNode, mut test: F) -> io::Result<()> where F: FnMut(usize) -> LiveNode, { let node_base_idx = self.idx(ln, Variable(0)); for var_idx in 0..self.ir.num_vars { let idx = node_base_idx + var_idx; if test(idx).is_valid() { write!(wr, " {:?}", Variable(var_idx as u32))?; } } Ok(()) } #[allow(unused_must_use)] fn ln_str(&self, ln: LiveNode) -> String { let mut wr = Vec::new(); { let wr = &mut wr as &mut dyn Write; write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln)); self.write_vars(wr, ln, |idx| self.rwu_table.get_reader(idx)); write!(wr, " writes"); self.write_vars(wr, ln, |idx| self.rwu_table.get_writer(idx)); write!(wr, " precedes {:?}]", self.successors[ln.get()]); } String::from_utf8(wr).unwrap() } fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) { self.successors[ln.get()] = succ_ln; // It is not necessary to initialize the RWUs here because they are all // set to INV_INV_FALSE when they are created, and the sets only grow // during iterations. } fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) { // more efficient version of init_empty() / merge_from_succ() self.successors[ln.get()] = succ_ln; self.indices2(ln, succ_ln, |this, idx, succ_idx| { this.rwu_table.copy_packed(idx, succ_idx); }); debug!("init_from_succ(ln={}, succ={})", self.ln_str(ln), self.ln_str(succ_ln)); } fn merge_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode, first_merge: bool) -> bool { if ln == succ_ln { return false; } let mut any_changed = false; self.indices2(ln, succ_ln, |this, idx, succ_idx| { // This is a special case, pulled out from the code below, where we // don't have to do anything. It occurs about 60-70% of the time. if this.rwu_table.packed_rwus[succ_idx] == INV_INV_FALSE { return; } let mut changed = false; let mut rwu = this.rwu_table.get(idx); let succ_rwu = this.rwu_table.get(succ_idx); if succ_rwu.reader.is_valid() && !rwu.reader.is_valid() { rwu.reader = succ_rwu.reader; changed = true } if succ_rwu.writer.is_valid() && !rwu.writer.is_valid() { rwu.writer = succ_rwu.writer; changed = true } if succ_rwu.used && !rwu.used { rwu.used = true; changed = true; } if changed { this.rwu_table.assign_unpacked(idx, rwu); any_changed = true; } }); debug!( "merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})", ln, self.ln_str(succ_ln), first_merge, any_changed ); any_changed } // Indicates that a local variable was *defined*; we know that no // uses of the variable can precede the definition (resolve checks // this) so we just clear out all the data. fn define(&mut self, writer: LiveNode, var: Variable) { let idx = self.idx(writer, var); self.rwu_table.assign_inv_inv(idx); debug!("{:?} defines {:?} (idx={}): {}", writer, var, idx, self.ln_str(writer)); } // Either read, write, or both depending on the acc bitset fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) { debug!("{:?} accesses[{:x}] {:?}: {}", ln, acc, var, self.ln_str(ln)); let idx = self.idx(ln, var); let mut rwu = self.rwu_table.get(idx); if (acc & ACC_WRITE) != 0 { rwu.reader = invalid_node(); rwu.writer = ln; } // Important: if we both read/write, must do read second // or else the write will override. if (acc & ACC_READ) != 0 { rwu.reader = ln; } if (acc & ACC_USE) != 0 { rwu.used = true; } self.rwu_table.assign_unpacked(idx, rwu); } fn compute(&mut self, body: &hir::Expr<'_>) -> LiveNode { debug!("compute: using id for body, {:?}", body); // the fallthrough exit is only for those cases where we do not // explicitly return: let s = self.s; self.init_from_succ(s.fallthrough_ln, s.exit_ln); self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ); let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln); // hack to skip the loop unless debug! is enabled: debug!( "^^ liveness computation results for body {} (entry={:?})", { for ln_idx in 0..self.ir.num_live_nodes { debug!("{:?}", self.ln_str(LiveNode(ln_idx as u32))); } body.hir_id }, entry_ln ); entry_ln } fn propagate_through_block(&mut self, blk: &hir::Block<'_>, succ: LiveNode) -> LiveNode { if blk.targeted_by_break { self.break_ln.insert(blk.hir_id, succ); } let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ); blk.stmts.iter().rev().fold(succ, |succ, stmt| self.propagate_through_stmt(stmt, succ)) } fn propagate_through_stmt(&mut self, stmt: &hir::Stmt<'_>, succ: LiveNode) -> LiveNode { match stmt.kind { hir::StmtKind::Local(ref local) => { // Note: we mark the variable as defined regardless of whether // there is an initializer. Initially I had thought to only mark // the live variable as defined if it was initialized, and then we // could check for uninit variables just by scanning what is live // at the start of the function. But that doesn't work so well for // immutable variables defined in a loop: // loop { let x; x = 5; } // because the "assignment" loops back around and generates an error. // // So now we just check that variables defined w/o an // initializer are not live at the point of their // initialization, which is mildly more complex than checking // once at the func header but otherwise equivalent. let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ); self.define_bindings_in_pat(&local.pat, succ) } hir::StmtKind::Item(..) => succ, hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => { self.propagate_through_expr(&expr, succ) } } } fn propagate_through_exprs(&mut self, exprs: &[Expr<'_>], succ: LiveNode) -> LiveNode { exprs.iter().rev().fold(succ, |succ, expr| self.propagate_through_expr(&expr, succ)) } fn propagate_through_opt_expr( &mut self, opt_expr: Option<&Expr<'_>>, succ: LiveNode, ) -> LiveNode { opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ)) } fn propagate_through_expr(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode { debug!("propagate_through_expr: {:?}", expr); match expr.kind { // Interesting cases with control flow or which gen/kill hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => { self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE) } hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ), hir::ExprKind::Closure(..) => { debug!("{:?} is an ExprKind::Closure", expr); // the construction of a closure itself is not important, // but we have to consider the closed over variables. let caps = self .ir .capture_info_map .get(&expr.hir_id) .cloned() .unwrap_or_else(|| span_bug!(expr.span, "no registered caps")); caps.iter().rev().fold(succ, |succ, cap| { self.init_from_succ(cap.ln, succ); let var = self.variable(cap.var_hid, expr.span); self.acc(cap.ln, var, ACC_READ | ACC_USE); cap.ln }) } // Note that labels have been resolved, so we don't need to look // at the label ident hir::ExprKind::Loop(ref blk, _, _) => self.propagate_through_loop(expr, &blk, succ), hir::ExprKind::Match(ref e, arms, _) => { // // (e) // | // v // (expr) // / | \ // | | | // v v v // (..arms..) // | | | // v v v // ( succ ) // // let ln = self.live_node(expr.hir_id, expr.span); self.init_empty(ln, succ); let mut first_merge = true; for arm in arms { let body_succ = self.propagate_through_expr(&arm.body, succ); let guard_succ = self.propagate_through_opt_expr( arm.guard.as_ref().map(|hir::Guard::If(e)| *e), body_succ, ); let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ); self.merge_from_succ(ln, arm_succ, first_merge); first_merge = false; } self.propagate_through_expr(&e, ln) } hir::ExprKind::Ret(ref o_e) => { // ignore succ and subst exit_ln: let exit_ln = self.s.exit_ln; self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln) } hir::ExprKind::Break(label, ref opt_expr) => { // Find which label this break jumps to let target = match label.target_id { Ok(hir_id) => self.break_ln.get(&hir_id), Err(err) => span_bug!(expr.span, "loop scope error: {}", err), } .cloned(); // Now that we know the label we're going to, // look it up in the break loop nodes table match target { Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b), None => { // FIXME: This should have been checked earlier. Once this is fixed, // replace with `delay_span_bug`. (#62480) self.ir .tcx .sess .struct_span_err(expr.span, "`break` to unknown label") .emit(); rustc_errors::FatalError.raise() } } } hir::ExprKind::Continue(label) => { // Find which label this expr continues to let sc = label .target_id .unwrap_or_else(|err| span_bug!(expr.span, "loop scope error: {}", err)); // Now that we know the label we're going to, // look it up in the continue loop nodes table self.cont_ln .get(&sc) .cloned() .unwrap_or_else(|| span_bug!(expr.span, "continue to unknown label")) } hir::ExprKind::Assign(ref l, ref r, _) => { // see comment on places in // propagate_through_place_components() let succ = self.write_place(&l, succ, ACC_WRITE); let succ = self.propagate_through_place_components(&l, succ); self.propagate_through_expr(&r, succ) } hir::ExprKind::AssignOp(_, ref l, ref r) => { // an overloaded assign op is like a method call if self.tables.is_method_call(expr) { let succ = self.propagate_through_expr(&l, succ); self.propagate_through_expr(&r, succ) } else { // see comment on places in // propagate_through_place_components() let succ = self.write_place(&l, succ, ACC_WRITE | ACC_READ); let succ = self.propagate_through_expr(&r, succ); self.propagate_through_place_components(&l, succ) } } // Uninteresting cases: just propagate in rev exec order hir::ExprKind::Array(ref exprs) => self.propagate_through_exprs(exprs, succ), hir::ExprKind::Struct(_, ref fields, ref with_expr) => { let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ); fields .iter() .rev() .fold(succ, |succ, field| self.propagate_through_expr(&field.expr, succ)) } hir::ExprKind::Call(ref f, ref args) => { let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id(); let succ = if self.ir.tcx.is_ty_uninhabited_from( m, self.tables.expr_ty(expr), self.param_env, ) { self.s.exit_ln } else { succ }; let succ = self.propagate_through_exprs(args, succ); self.propagate_through_expr(&f, succ) } hir::ExprKind::MethodCall(.., ref args) => { let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id(); let succ = if self.ir.tcx.is_ty_uninhabited_from( m, self.tables.expr_ty(expr), self.param_env, ) { self.s.exit_ln } else { succ }; self.propagate_through_exprs(args, succ) } hir::ExprKind::Tup(ref exprs) => self.propagate_through_exprs(exprs, succ), hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => { let r_succ = self.propagate_through_expr(&r, succ); let ln = self.live_node(expr.hir_id, expr.span); self.init_from_succ(ln, succ); self.merge_from_succ(ln, r_succ, false); self.propagate_through_expr(&l, ln) } hir::ExprKind::Index(ref l, ref r) | hir::ExprKind::Binary(_, ref l, ref r) => { let r_succ = self.propagate_through_expr(&r, succ); self.propagate_through_expr(&l, r_succ) } hir::ExprKind::Box(ref e) | hir::ExprKind::AddrOf(_, _, ref e) | hir::ExprKind::Cast(ref e, _) | hir::ExprKind::Type(ref e, _) | hir::ExprKind::DropTemps(ref e) | hir::ExprKind::Unary(_, ref e) | hir::ExprKind::Yield(ref e, _) | hir::ExprKind::Repeat(ref e, _) => self.propagate_through_expr(&e, succ), hir::ExprKind::InlineAsm(ref asm) => { let ia = &asm.inner; let outputs = asm.outputs_exprs; let inputs = asm.inputs_exprs; let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| { // see comment on places // in propagate_through_place_components() if o.is_indirect { self.propagate_through_expr(output, succ) } else { let acc = if o.is_rw { ACC_WRITE | ACC_READ } else { ACC_WRITE }; let succ = self.write_place(output, succ, acc); self.propagate_through_place_components(output, succ) } }); // Inputs are executed first. Propagate last because of rev order self.propagate_through_exprs(inputs, succ) } hir::ExprKind::Lit(..) | hir::ExprKind::Err | hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => succ, // Note that labels have been resolved, so we don't need to look // at the label ident hir::ExprKind::Block(ref blk, _) => self.propagate_through_block(&blk, succ), } } fn propagate_through_place_components(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode { // # Places // // In general, the full flow graph structure for an // assignment/move/etc can be handled in one of two ways, // depending on whether what is being assigned is a "tracked // value" or not. A tracked value is basically a local // variable or argument. // // The two kinds of graphs are: // // Tracked place Untracked place // ----------------------++----------------------- // || // | || | // v || v // (rvalue) || (rvalue) // | || | // v || v // (write of place) || (place components) // | || | // v || v // (succ) || (succ) // || // ----------------------++----------------------- // // I will cover the two cases in turn: // // # Tracked places // // A tracked place is a local variable/argument `x`. In // these cases, the link_node where the write occurs is linked // to node id of `x`. The `write_place()` routine generates // the contents of this node. There are no subcomponents to // consider. // // # Non-tracked places // // These are places like `x[5]` or `x.f`. In that case, we // basically ignore the value which is written to but generate // reads for the components---`x` in these two examples. The // components reads are generated by // `propagate_through_place_components()` (this fn). // // # Illegal places // // It is still possible to observe assignments to non-places; // these errors are detected in the later pass borrowck. We // just ignore such cases and treat them as reads. match expr.kind { hir::ExprKind::Path(_) => succ, hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ), _ => self.propagate_through_expr(expr, succ), } } // see comment on propagate_through_place() fn write_place(&mut self, expr: &Expr<'_>, succ: LiveNode, acc: u32) -> LiveNode { match expr.kind { hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => { self.access_path(expr.hir_id, path, succ, acc) } // We do not track other places, so just propagate through // to their subcomponents. Also, it may happen that // non-places occur here, because those are detected in the // later pass borrowck. _ => succ, } } fn access_var( &mut self, hir_id: HirId, var_hid: HirId, succ: LiveNode, acc: u32, span: Span, ) -> LiveNode { let ln = self.live_node(hir_id, span); if acc != 0 { self.init_from_succ(ln, succ); let var = self.variable(var_hid, span); self.acc(ln, var, acc); } ln } fn access_path( &mut self, hir_id: HirId, path: &hir::Path<'_>, succ: LiveNode, acc: u32, ) -> LiveNode { match path.res { Res::Local(hid) => { let upvars = self.ir.tcx.upvars(self.ir.body_owner); if !upvars.map_or(false, |upvars| upvars.contains_key(&hid)) { self.access_var(hir_id, hid, succ, acc, path.span) } else { succ } } _ => succ, } } fn propagate_through_loop( &mut self, expr: &Expr<'_>, body: &hir::Block<'_>, succ: LiveNode, ) -> LiveNode { /* We model control flow like this: (expr) <-+ | | v | (body) --+ Note that a `continue` expression targeting the `loop` will have a successor of `expr`. Meanwhile, a `break` expression will have a successor of `succ`. */ // first iteration: let mut first_merge = true; let ln = self.live_node(expr.hir_id, expr.span); self.init_empty(ln, succ); debug!("propagate_through_loop: using id for loop body {} {:?}", expr.hir_id, body); self.break_ln.insert(expr.hir_id, succ); self.cont_ln.insert(expr.hir_id, ln); let body_ln = self.propagate_through_block(body, ln); // repeat until fixed point is reached: while self.merge_from_succ(ln, body_ln, first_merge) { first_merge = false; assert_eq!(body_ln, self.propagate_through_block(body, ln)); } ln } } // _______________________________________________________________________ // Checking for error conditions impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> { type Map = intravisit::ErasedMap<'tcx>; fn nested_visit_map(&mut self) -> NestedVisitorMap { NestedVisitorMap::None } fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) { self.check_unused_vars_in_pat(&local.pat, None, |spans, hir_id, ln, var| { if local.init.is_some() { self.warn_about_dead_assign(spans, hir_id, ln, var); } }); intravisit::walk_local(self, local); } fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) { check_expr(self, ex); } fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) { self.check_unused_vars_in_pat(&arm.pat, None, |_, _, _, _| {}); intravisit::walk_arm(self, arm); } } fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) { match expr.kind { hir::ExprKind::Assign(ref l, ..) => { this.check_place(&l); } hir::ExprKind::AssignOp(_, ref l, _) => { if !this.tables.is_method_call(expr) { this.check_place(&l); } } hir::ExprKind::InlineAsm(ref asm) => { for input in asm.inputs_exprs { this.visit_expr(input); } // Output operands must be places for (o, output) in asm.inner.outputs.iter().zip(asm.outputs_exprs) { if !o.is_indirect { this.check_place(output); } this.visit_expr(output); } } // no correctness conditions related to liveness hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) | hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) | hir::ExprKind::Index(..) | hir::ExprKind::Field(..) | hir::ExprKind::Array(..) | hir::ExprKind::Tup(..) | hir::ExprKind::Binary(..) | hir::ExprKind::Cast(..) | hir::ExprKind::DropTemps(..) | hir::ExprKind::Unary(..) | hir::ExprKind::Ret(..) | hir::ExprKind::Break(..) | hir::ExprKind::Continue(..) | hir::ExprKind::Lit(_) | hir::ExprKind::Block(..) | hir::ExprKind::AddrOf(..) | hir::ExprKind::Struct(..) | hir::ExprKind::Repeat(..) | hir::ExprKind::Closure(..) | hir::ExprKind::Path(_) | hir::ExprKind::Yield(..) | hir::ExprKind::Box(..) | hir::ExprKind::Type(..) | hir::ExprKind::Err => {} } intravisit::walk_expr(this, expr); } impl<'tcx> Liveness<'_, 'tcx> { fn check_place(&mut self, expr: &'tcx Expr<'tcx>) { match expr.kind { hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => { if let Res::Local(var_hid) = path.res { let upvars = self.ir.tcx.upvars(self.ir.body_owner); if !upvars.map_or(false, |upvars| upvars.contains_key(&var_hid)) { // Assignment to an immutable variable or argument: only legal // if there is no later assignment. If this local is actually // mutable, then check for a reassignment to flag the mutability // as being used. let ln = self.live_node(expr.hir_id, expr.span); let var = self.variable(var_hid, expr.span); self.warn_about_dead_assign(vec![expr.span], expr.hir_id, ln, var); } } } _ => { // For other kinds of places, no checks are required, // and any embedded expressions are actually rvalues intravisit::walk_expr(self, expr); } } } fn should_warn(&self, var: Variable) -> Option { let name = self.ir.variable_name(var); if name.is_empty() || name.as_bytes()[0] == b'_' { None } else { Some(name) } } fn warn_about_unused_args(&self, body: &hir::Body<'_>, entry_ln: LiveNode) { for p in body.params { self.check_unused_vars_in_pat(&p.pat, Some(entry_ln), |spans, hir_id, ln, var| { if self.live_on_entry(ln, var).is_none() { self.report_dead_assign(hir_id, spans, var, true); } }); } } fn check_unused_vars_in_pat( &self, pat: &hir::Pat<'_>, entry_ln: Option, on_used_on_entry: impl Fn(Vec, HirId, LiveNode, Variable), ) { // In an or-pattern, only consider the variable; any later patterns must have the same // bindings, and we also consider the first pattern to be the "authoritative" set of ids. // However, we should take the spans of variables with the same name from the later // patterns so the suggestions to prefix with underscores will apply to those too. let mut vars: FxIndexMap)> = <_>::default(); pat.each_binding(|_, hir_id, pat_sp, ident| { let ln = entry_ln.unwrap_or_else(|| self.live_node(hir_id, pat_sp)); let var = self.variable(hir_id, ident.span); vars.entry(self.ir.variable_name(var)) .and_modify(|(.., spans)| spans.push(ident.span)) .or_insert_with(|| (ln, var, hir_id, vec![ident.span])); }); for (_, (ln, var, id, spans)) in vars { if self.used_on_entry(ln, var) { on_used_on_entry(spans, id, ln, var); } else { self.report_unused(spans, id, ln, var); } } } fn report_unused(&self, spans: Vec, hir_id: HirId, ln: LiveNode, var: Variable) { if let Some(name) = self.should_warn(var).filter(|name| name != "self") { // annoying: for parameters in funcs like `fn(x: i32) // {ret}`, there is only one node, so asking about // assigned_on_exit() is not meaningful. let is_assigned = if ln == self.s.exit_ln { false } else { self.assigned_on_exit(ln, var).is_some() }; if is_assigned { self.ir.tcx.struct_span_lint_hir( lint::builtin::UNUSED_VARIABLES, hir_id, spans, |lint| { lint.build(&format!("variable `{}` is assigned to, but never used", name)) .note(&format!("consider using `_{}` instead", name)) .emit(); }, ) } else { self.ir.tcx.struct_span_lint_hir( lint::builtin::UNUSED_VARIABLES, hir_id, spans.clone(), |lint| { let mut err = lint.build(&format!("unused variable: `{}`", name)); if self.ir.variable_is_shorthand(var) { if let Node::Binding(pat) = self.ir.tcx.hir().get(hir_id) { // Handle `ref` and `ref mut`. let spans = spans .iter() .map(|_span| (pat.span, format!("{}: _", name))) .collect(); err.multipart_suggestion( "try ignoring the field", spans, Applicability::MachineApplicable, ); } } else { err.multipart_suggestion( "if this is intentional, prefix it with an underscore", spans.iter().map(|span| (*span, format!("_{}", name))).collect(), Applicability::MachineApplicable, ); } err.emit() }, ); } } } fn warn_about_dead_assign(&self, spans: Vec, hir_id: HirId, ln: LiveNode, var: Variable) { if self.live_on_exit(ln, var).is_none() { self.report_dead_assign(hir_id, spans, var, false); } } fn report_dead_assign(&self, hir_id: HirId, spans: Vec, var: Variable, is_param: bool) { if let Some(name) = self.should_warn(var) { if is_param { self.ir.tcx.struct_span_lint_hir( lint::builtin::UNUSED_ASSIGNMENTS, hir_id, spans, |lint| { lint.build(&format!("value passed to `{}` is never read", name)) .help("maybe it is overwritten before being read?") .emit(); }, ) } else { self.ir.tcx.struct_span_lint_hir( lint::builtin::UNUSED_ASSIGNMENTS, hir_id, spans, |lint| { lint.build(&format!("value assigned to `{}` is never read", name)) .help("maybe it is overwritten before being read?") .emit(); }, ) } } } }