//! 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 `None`, 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 `None`, 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 nodes and variables //! //! We generate various special nodes for various, well, special purposes. //! These are described in the `Liveness` struct. use self::LiveNodeKind::*; use self::VarKind::*; use rustc_ast::InlineAsmOptions; use rustc_data_structures::fx::FxIndexMap; use rustc_errors::Applicability; use rustc_hir as hir; use rustc_hir::def::*; use rustc_hir::def_id::LocalDefId; use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor}; use rustc_hir::{Expr, HirId, HirIdMap, HirIdSet}; use rustc_index::vec::IndexVec; use rustc_middle::hir::map::Map; use rustc_middle::ty::query::Providers; use rustc_middle::ty::{self, DefIdTree, TyCtxt}; use rustc_session::lint; use rustc_span::symbol::{kw, sym, Symbol}; use rustc_span::Span; use std::collections::VecDeque; use std::io; use std::io::prelude::*; use std::rc::Rc; mod rwu_table; rustc_index::newtype_index! { pub struct Variable { DEBUG_FORMAT = "v({})", } } rustc_index::newtype_index! { pub struct LiveNode { DEBUG_FORMAT = "ln({})", } } #[derive(Copy, Clone, PartialEq, Debug)] enum LiveNodeKind { UpvarNode(Span), ExprNode(Span), VarDefNode(Span), ClosureNode, 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)), ClosureNode => "Closure node".to_owned(), ExitNode => "Exit node".to_owned(), } } fn check_mod_liveness(tcx: TyCtxt<'_>, module_def_id: LocalDefId) { tcx.hir().visit_item_likes_in_module(module_def_id, &mut IrMaps::new(tcx).as_deep_visitor()); } pub fn provide(providers: &mut Providers) { *providers = Providers { check_mod_liveness, ..*providers }; } // ______________________________________________________________________ // 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. struct CaptureInfo { ln: LiveNode, var_hid: HirId, } #[derive(Copy, Clone, Debug)] struct LocalInfo { id: HirId, name: Symbol, is_shorthand: bool, } #[derive(Copy, Clone, Debug)] enum VarKind { Param(HirId, Symbol), Local(LocalInfo), Upvar(HirId, Symbol), } struct IrMaps<'tcx> { tcx: TyCtxt<'tcx>, live_node_map: HirIdMap, variable_map: HirIdMap, capture_info_map: HirIdMap>>, var_kinds: IndexVec, lnks: IndexVec, } impl IrMaps<'tcx> { fn new(tcx: TyCtxt<'tcx>) -> IrMaps<'tcx> { IrMaps { tcx, live_node_map: HirIdMap::default(), variable_map: HirIdMap::default(), capture_info_map: Default::default(), var_kinds: IndexVec::new(), lnks: IndexVec::new(), } } fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode { let ln = self.lnks.push(lnk); 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 = self.var_kinds.push(vk); match vk { Local(LocalInfo { id: node_id, .. }) | Param(node_id, _) | Upvar(node_id, _) => { self.variable_map.insert(node_id, v); } } 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) -> Symbol { match self.var_kinds[var] { Local(LocalInfo { name, .. }) | Param(_, name) | Upvar(_, name) => name, } } fn variable_is_shorthand(&self, var: Variable) -> bool { match self.var_kinds[var] { Local(LocalInfo { is_shorthand, .. }) => is_shorthand, Param(..) | Upvar(..) => false, } } fn set_captures(&mut self, hir_id: HirId, cs: Vec) { self.capture_info_map.insert(hir_id, Rc::new(cs)); } fn add_from_pat(&mut self, pat: &hir::Pat<'tcx>) { // 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| { self.add_live_node_for_node(hir_id, VarDefNode(ident.span)); self.add_variable(Local(LocalInfo { id: hir_id, name: ident.name, is_shorthand: shorthand_field_ids.contains(&hir_id), })); }); } } 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_body(&mut self, body: &'tcx hir::Body<'tcx>) { debug!("visit_body {:?}", body.id()); // swap in a new set of IR maps for this body let mut maps = IrMaps::new(self.tcx); let hir_id = maps.tcx.hir().body_owner(body.id()); let local_def_id = maps.tcx.hir().local_def_id(hir_id); let def_id = local_def_id.to_def_id(); // Don't run unused pass for #[derive()] if let Some(parent) = self.tcx.parent(def_id) { if let DefKind::Impl = self.tcx.def_kind(parent.expect_local()) { if self.tcx.has_attr(parent, sym::automatically_derived) { return; } } } if let Some(captures) = maps.tcx.typeck(local_def_id).closure_captures.get(&def_id) { for &var_hir_id in captures.keys() { let var_name = maps.tcx.hir().name(var_hir_id); maps.add_variable(Upvar(var_hir_id, var_name)); } } // gather up the various local variables, significant expressions, // and so forth: intravisit::walk_body(&mut maps, body); // compute liveness let mut lsets = Liveness::new(&mut maps, local_def_id); let entry_ln = lsets.compute(&body, hir_id); lsets.log_liveness(entry_ln, body.id().hir_id); // check for various error conditions lsets.visit_body(body); lsets.warn_about_unused_upvars(entry_ln); lsets.warn_about_unused_args(body, entry_ln); } fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) { self.add_from_pat(&local.pat); intravisit::walk_local(self, local); } fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) { self.add_from_pat(&arm.pat); if let Some(hir::Guard::IfLet(ref pat, _)) = arm.guard { self.add_from_pat(pat); } intravisit::walk_arm(self, arm); } fn visit_param(&mut self, param: &'tcx hir::Param<'tcx>) { let is_shorthand = matches!(param.pat.kind, rustc_hir::PatKind::Struct(..)); param.pat.each_binding(|_bm, hir_id, _x, ident| { let var = if is_shorthand { Local(LocalInfo { id: hir_id, name: ident.name, is_shorthand: true }) } else { Param(hir_id, ident.name) }; self.add_variable(var); }); intravisit::walk_param(self, param); } fn visit_expr(&mut self, 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 { self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span)); } intravisit::walk_expr(self, expr); } hir::ExprKind::Closure(..) => { // Interesting control flow (for loops can contain labeled // breaks or continues) self.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 = self.tcx.hir().local_def_id(expr.hir_id); if let Some(captures) = self .tcx .typeck(closure_def_id) .closure_captures .get(&closure_def_id.to_def_id()) { // If closure captures is Some, upvars_mentioned must also be Some let upvars = self.tcx.upvars_mentioned(closure_def_id).unwrap(); call_caps.extend(captures.keys().map(|var_id| { let upvar = upvars[var_id]; let upvar_ln = self.add_live_node(UpvarNode(upvar.span)); CaptureInfo { ln: upvar_ln, var_hid: *var_id } })); } self.set_captures(expr.hir_id, call_caps); intravisit::walk_expr(self, expr); } // live nodes required for interesting control flow: hir::ExprKind::If(..) | hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => { self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span)); intravisit::walk_expr(self, expr); } hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => { self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span)); intravisit::walk_expr(self, 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::ConstBlock(..) | hir::ExprKind::Ret(..) | hir::ExprKind::Block(..) | hir::ExprKind::Assign(..) | hir::ExprKind::AssignOp(..) | hir::ExprKind::Struct(..) | hir::ExprKind::Repeat(..) | hir::ExprKind::InlineAsm(..) | hir::ExprKind::LlvmInlineAsm(..) | hir::ExprKind::Box(..) | hir::ExprKind::Yield(..) | hir::ExprKind::Type(..) | hir::ExprKind::Err | hir::ExprKind::Path(hir::QPath::TypeRelative(..)) | hir::ExprKind::Path(hir::QPath::LangItem(..)) => { intravisit::walk_expr(self, 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. const ACC_READ: u32 = 1; const ACC_WRITE: u32 = 2; const ACC_USE: u32 = 4; struct Liveness<'a, 'tcx> { ir: &'a mut IrMaps<'tcx>, body_owner: LocalDefId, typeck_results: &'a ty::TypeckResults<'tcx>, param_env: ty::ParamEnv<'tcx>, upvars: Option<&'tcx FxIndexMap>, closure_captures: Option<&'tcx FxIndexMap>, successors: IndexVec>, rwu_table: rwu_table::RWUTable, /// A live node representing a point of execution before closure entry & /// after closure exit. Used to calculate liveness of captured variables /// through calls to the same closure. Used for Fn & FnMut closures only. closure_ln: LiveNode, /// A live node representing every 'exit' from the function, whether it be /// by explicit return, panic, or other means. exit_ln: LiveNode, // 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>, body_owner: LocalDefId) -> Liveness<'a, 'tcx> { let typeck_results = ir.tcx.typeck(body_owner); let param_env = ir.tcx.param_env(body_owner); let upvars = ir.tcx.upvars_mentioned(body_owner); let closure_captures = typeck_results.closure_captures.get(&body_owner.to_def_id()); let closure_ln = ir.add_live_node(ClosureNode); let exit_ln = ir.add_live_node(ExitNode); let num_live_nodes = ir.lnks.len(); let num_vars = ir.var_kinds.len(); Liveness { ir, body_owner, typeck_results, param_env, upvars, closure_captures, successors: IndexVec::from_elem_n(None, num_live_nodes), rwu_table: rwu_table::RWUTable::new(num_live_nodes, num_vars), closure_ln, exit_ln, 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 live_on_entry(&self, ln: LiveNode, var: Variable) -> bool { self.rwu_table.get_reader(ln, var) } // Is this variable live on entry to any of its successor nodes? fn live_on_exit(&self, ln: LiveNode, var: Variable) -> bool { let successor = self.successors[ln].unwrap(); self.live_on_entry(successor, var) } fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool { self.rwu_table.get_used(ln, var) } fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> bool { self.rwu_table.get_writer(ln, var) } fn assigned_on_exit(&self, ln: LiveNode, var: Variable) -> bool { let successor = self.successors[ln].unwrap(); self.assigned_on_entry(successor, var) } fn write_vars(&self, wr: &mut dyn Write, mut test: F) -> io::Result<()> where F: FnMut(Variable) -> bool, { for var_idx in 0..self.ir.var_kinds.len() { let var = Variable::from(var_idx); if test(var) { write!(wr, " {:?}", var)?; } } 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, "[{:?} of kind {:?} reads", ln, self.ir.lnks[ln]); self.write_vars(wr, |var| self.rwu_table.get_reader(ln, var)); write!(wr, " writes"); self.write_vars(wr, |var| self.rwu_table.get_writer(ln, var)); write!(wr, " uses"); self.write_vars(wr, |var| self.rwu_table.get_used(ln, var)); write!(wr, " precedes {:?}]", self.successors[ln]); } String::from_utf8(wr).unwrap() } fn log_liveness(&self, entry_ln: LiveNode, hir_id: hir::HirId) { // hack to skip the loop unless debug! is enabled: debug!( "^^ liveness computation results for body {} (entry={:?})", { for ln_idx in 0..self.ir.lnks.len() { debug!("{:?}", self.ln_str(LiveNode::from(ln_idx))); } hir_id }, entry_ln ); } fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) { self.successors[ln] = Some(succ_ln); // It is not necessary to initialize the RWUs here because they are all // empty when 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] = Some(succ_ln); self.rwu_table.copy(ln, succ_ln); 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) -> bool { if ln == succ_ln { return false; } let changed = self.rwu_table.union(ln, succ_ln); debug!("merge_from_succ(ln={:?}, succ={}, changed={})", ln, self.ln_str(succ_ln), changed); 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 used = self.rwu_table.get_used(writer, var); self.rwu_table.set(writer, var, rwu_table::RWU { reader: false, writer: false, used }); debug!("{:?} defines {:?}: {}", writer, var, 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 mut rwu = self.rwu_table.get(ln, var); if (acc & ACC_WRITE) != 0 { rwu.reader = false; rwu.writer = true; } // Important: if we both read/write, must do read second // or else the write will override. if (acc & ACC_READ) != 0 { rwu.reader = true; } if (acc & ACC_USE) != 0 { rwu.used = true; } self.rwu_table.set(ln, var, rwu); } fn compute(&mut self, body: &hir::Body<'_>, hir_id: HirId) -> LiveNode { debug!("compute: for body {:?}", body.id().hir_id); // # Liveness of captured variables // // When computing the liveness for captured variables we take into // account how variable is captured (ByRef vs ByValue) and what is the // closure kind (Generator / FnOnce vs Fn / FnMut). // // Variables captured by reference are assumed to be used on the exit // from the closure. // // In FnOnce closures, variables captured by value are known to be dead // on exit since it is impossible to call the closure again. // // In Fn / FnMut closures, variables captured by value are live on exit // if they are live on the entry to the closure, since only the closure // itself can access them on subsequent calls. if let Some(closure_captures) = self.closure_captures { // Mark upvars captured by reference as used after closure exits. // Since closure_captures is Some, upvars must exists too. let upvars = self.upvars.unwrap(); for (&var_hir_id, upvar_id) in closure_captures { let upvar = upvars[&var_hir_id]; match self.typeck_results.upvar_capture(*upvar_id) { ty::UpvarCapture::ByRef(_) => { let var = self.variable(var_hir_id, upvar.span); self.acc(self.exit_ln, var, ACC_READ | ACC_USE); } ty::UpvarCapture::ByValue(_) => {} } } } let succ = self.propagate_through_expr(&body.value, self.exit_ln); if self.closure_captures.is_none() { // Either not a closure, or closure without any captured variables. // No need to determine liveness of captured variables, since there // are none. return succ; } let ty = self.typeck_results.node_type(hir_id); match ty.kind() { ty::Closure(_def_id, substs) => match substs.as_closure().kind() { ty::ClosureKind::Fn => {} ty::ClosureKind::FnMut => {} ty::ClosureKind::FnOnce => return succ, }, ty::Generator(..) => return succ, _ => { span_bug!( body.value.span, "{} has upvars so it should have a closure type: {:?}", hir_id, ty ); } }; // Propagate through calls to the closure. loop { self.init_from_succ(self.closure_ln, succ); for param in body.params { param.pat.each_binding(|_bm, hir_id, _x, ident| { let var = self.variable(hir_id, ident.span); self.define(self.closure_ln, var); }) } if !self.merge_from_succ(self.exit_ln, self.closure_ln) { break; } assert_eq!(succ, self.propagate_through_expr(&body.value, self.exit_ln)); } succ } 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_deref(), 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_deref(), 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::If(ref cond, ref then, ref else_opt) => { // // (cond) // | // v // (expr) // / \ // | | // v v // (then)(els) // | | // v v // ( succ ) // let else_ln = self.propagate_through_opt_expr(else_opt.as_ref().map(|e| &**e), succ); let then_ln = self.propagate_through_expr(&then, succ); let ln = self.live_node(expr.hir_id, expr.span); self.init_from_succ(ln, else_ln); self.merge_from_succ(ln, then_ln); self.propagate_through_expr(&cond, ln) } 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); for arm in arms { let body_succ = self.propagate_through_expr(&arm.body, succ); let guard_succ = arm.guard.as_ref().map_or(body_succ, |g| match g { hir::Guard::If(e) => self.propagate_through_expr(e, body_succ), hir::Guard::IfLet(pat, e) => { let let_bind = self.define_bindings_in_pat(pat, body_succ); self.propagate_through_expr(e, let_bind) } }); let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ); self.merge_from_succ(ln, arm_succ); } self.propagate_through_expr(&e, ln) } hir::ExprKind::Ret(ref o_e) => { // Ignore succ and subst exit_ln. self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), self.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 => span_bug!(expr.span, "`break` to unknown label"), } } 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.typeck_results.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.typeck_results.expr_ty(expr), self.param_env, ) { self.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.typeck_results.expr_ty(expr), self.param_env, ) { self.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); 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) => { // Handle non-returning asm let mut succ = if asm.options.contains(InlineAsmOptions::NORETURN) { self.exit_ln } else { succ }; // Do a first pass for writing outputs only for (op, _op_sp) in asm.operands.iter().rev() { match op { hir::InlineAsmOperand::In { .. } | hir::InlineAsmOperand::Const { .. } | hir::InlineAsmOperand::Sym { .. } => {} hir::InlineAsmOperand::Out { expr, .. } => { if let Some(expr) = expr { succ = self.write_place(expr, succ, ACC_WRITE); } } hir::InlineAsmOperand::InOut { expr, .. } => { succ = self.write_place(expr, succ, ACC_READ | ACC_WRITE | ACC_USE); } hir::InlineAsmOperand::SplitInOut { out_expr, .. } => { if let Some(expr) = out_expr { succ = self.write_place(expr, succ, ACC_WRITE); } } } } // Then do a second pass for inputs let mut succ = succ; for (op, _op_sp) in asm.operands.iter().rev() { match op { hir::InlineAsmOperand::In { expr, .. } | hir::InlineAsmOperand::Const { expr, .. } | hir::InlineAsmOperand::Sym { expr, .. } => { succ = self.propagate_through_expr(expr, succ) } hir::InlineAsmOperand::Out { expr, .. } => { if let Some(expr) = expr { succ = self.propagate_through_place_components(expr, succ); } } hir::InlineAsmOperand::InOut { expr, .. } => { succ = self.propagate_through_place_components(expr, succ); } hir::InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => { if let Some(expr) = out_expr { succ = self.propagate_through_place_components(expr, succ); } succ = self.propagate_through_expr(in_expr, succ); } } } succ } hir::ExprKind::LlvmInlineAsm(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::ConstBlock(..) | hir::ExprKind::Err | hir::ExprKind::Path(hir::QPath::TypeRelative(..)) | hir::ExprKind::Path(hir::QPath::LangItem(..)) => 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 in_upvars = self.upvars.map_or(false, |u| u.contains_key(&hid)); let in_captures = self.closure_captures.map_or(false, |c| c.contains_key(&hid)); match (in_upvars, in_captures) { (false, _) | (true, true) => self.access_var(hir_id, hid, succ, acc, path.span), (true, false) => { // This case is possible when with RFC-2229, a wild pattern // is used within a closure. // eg: `let _ = x`. The closure doesn't capture x here, // even though it's mentioned in the closure. 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 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) { 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.typeck_results.is_method_call(expr) { this.check_place(&l); } } hir::ExprKind::InlineAsm(ref asm) => { for (op, _op_sp) in asm.operands { match op { hir::InlineAsmOperand::Out { expr, .. } => { if let Some(expr) = expr { this.check_place(expr); } } hir::InlineAsmOperand::InOut { expr, .. } => { this.check_place(expr); } hir::InlineAsmOperand::SplitInOut { out_expr, .. } => { if let Some(out_expr) = out_expr { this.check_place(out_expr); } } _ => {} } } } hir::ExprKind::LlvmInlineAsm(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::If(..) | hir::ExprKind::DropTemps(..) | hir::ExprKind::Unary(..) | hir::ExprKind::Ret(..) | hir::ExprKind::Break(..) | hir::ExprKind::Continue(..) | hir::ExprKind::Lit(_) | hir::ExprKind::ConstBlock(..) | 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 { // 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 == kw::Empty { return None; } let name: &str = &name.as_str(); if name.as_bytes()[0] == b'_' { return None; } Some(name.to_owned()) } fn warn_about_unused_upvars(&self, entry_ln: LiveNode) { let closure_captures = match self.closure_captures { None => return, Some(closure_captures) => closure_captures, }; // If closure_captures is Some(), upvars must be Some() too. let upvars = self.upvars.unwrap(); for &var_hir_id in closure_captures.keys() { let upvar = upvars[&var_hir_id]; let var = self.variable(var_hir_id, upvar.span); let upvar_id = ty::UpvarId { var_path: ty::UpvarPath { hir_id: var_hir_id }, closure_expr_id: self.body_owner, }; match self.typeck_results.upvar_capture(upvar_id) { ty::UpvarCapture::ByValue(_) => {} ty::UpvarCapture::ByRef(..) => continue, }; if self.used_on_entry(entry_ln, var) { if !self.live_on_entry(entry_ln, var) { if let Some(name) = self.should_warn(var) { self.ir.tcx.struct_span_lint_hir( lint::builtin::UNUSED_ASSIGNMENTS, var_hir_id, vec![upvar.span], |lint| { lint.build(&format!("value captured by `{}` is never read", name)) .help("did you mean to capture by reference instead?") .emit(); }, ); } } } else { if let Some(name) = self.should_warn(var) { self.ir.tcx.struct_span_lint_hir( lint::builtin::UNUSED_VARIABLES, var_hir_id, vec![upvar.span], |lint| { lint.build(&format!("unused variable: `{}`", name)) .help("did you mean to capture by reference instead?") .emit(); }, ); } } } } 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) { self.report_unsed_assign(hir_id, spans, var, |name| { format!("value passed to `{}` is never read", name) }); } }); } } 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 ids and 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); let id_and_sp = (hir_id, pat_sp); vars.entry(self.ir.variable_name(var)) .and_modify(|(.., hir_ids_and_spans)| hir_ids_and_spans.push(id_and_sp)) .or_insert_with(|| (ln, var, vec![id_and_sp])); }); for (_, (ln, var, hir_ids_and_spans)) in vars { if self.used_on_entry(ln, var) { let id = hir_ids_and_spans[0].0; let spans = hir_ids_and_spans.into_iter().map(|(_, sp)| sp).collect(); on_used_on_entry(spans, id, ln, var); } else { self.report_unused(hir_ids_and_spans, ln, var); } } } fn report_unused(&self, hir_ids_and_spans: Vec<(HirId, Span)>, ln: LiveNode, var: Variable) { let first_hir_id = hir_ids_and_spans[0].0; 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.exit_ln { false } else { self.assigned_on_exit(ln, var) }; if is_assigned { self.ir.tcx.struct_span_lint_hir( lint::builtin::UNUSED_VARIABLES, first_hir_id, hir_ids_and_spans.into_iter().map(|(_, sp)| sp).collect::>(), |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, first_hir_id, hir_ids_and_spans.iter().map(|(_, sp)| *sp).collect::>(), |lint| { let mut err = lint.build(&format!("unused variable: `{}`", name)); let (shorthands, non_shorthands): (Vec<_>, Vec<_>) = hir_ids_and_spans.into_iter().partition(|(hir_id, span)| { let var = self.variable(*hir_id, *span); self.ir.variable_is_shorthand(var) }); let mut shorthands = shorthands .into_iter() .map(|(_, span)| (span, format!("{}: _", name))) .collect::>(); // If we have both shorthand and non-shorthand, prefer the "try ignoring // the field" message, and suggest `_` for the non-shorthands. If we only // have non-shorthand, then prefix with an underscore instead. if !shorthands.is_empty() { shorthands.extend( non_shorthands .into_iter() .map(|(_, span)| (span, "_".to_string())) .collect::>(), ); err.multipart_suggestion( "try ignoring the field", shorthands, Applicability::MachineApplicable, ); } else { err.multipart_suggestion( "if this is intentional, prefix it with an underscore", non_shorthands .into_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) { self.report_unsed_assign(hir_id, spans, var, |name| { format!("value assigned to `{}` is never read", name) }); } } fn report_unsed_assign( &self, hir_id: HirId, spans: Vec, var: Variable, message: impl Fn(&str) -> String, ) { if let Some(name) = self.should_warn(var) { self.ir.tcx.struct_span_lint_hir( lint::builtin::UNUSED_ASSIGNMENTS, hir_id, spans, |lint| { lint.build(&message(&name)) .help("maybe it is overwritten before being read?") .emit(); }, ) } } }