// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 or the MIT license // , at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! 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 //! id. //! //! # 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 usize) and the id for a variable //! is called a `variable` (another newtype'd usize). //! //! 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 `Users` struct //! //! At each live node `N`, we track three pieces of information for each //! variable `V` (these are encapsulated in the `Users` 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 //! //! - `no_ret_var`: a synthetic variable that is only 'read' from, the //! fallthrough node. This allows us to detect functions where we fail //! to return explicitly. //! - `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::LoopKind::*; use self::LiveNodeKind::*; use self::VarKind::*; use dep_graph::DepNode; use middle::def::*; use middle::pat_util; use middle::ty::{self, TyCtxt, ParameterEnvironment}; use middle::traits::{self, ProjectionMode}; use middle::infer; use lint; use util::nodemap::NodeMap; use std::{fmt, usize}; use std::io::prelude::*; use std::io; use std::rc::Rc; use syntax::ast::{self, NodeId}; use syntax::codemap::{BytePos, original_sp, Span}; use syntax::parse::token::special_idents; use syntax::ptr::P; use rustc_front::hir::Expr; use rustc_front::hir; use rustc_front::print::pprust::{expr_to_string, block_to_string}; use rustc_front::intravisit::{self, Visitor, FnKind}; /// For use with `propagate_through_loop`. enum LoopKind<'a> { /// An endless `loop` loop. LoopLoop, /// A `while` loop, with the given expression as condition. WhileLoop(&'a Expr), } #[derive(Copy, Clone, PartialEq)] struct Variable(usize); #[derive(Copy, PartialEq)] struct LiveNode(usize); impl Variable { fn get(&self) -> usize { let Variable(v) = *self; v } } impl LiveNode { fn get(&self) -> usize { let LiveNode(v) = *self; v } } impl Clone for LiveNode { fn clone(&self) -> LiveNode { LiveNode(self.get()) } } #[derive(Copy, Clone, PartialEq, Debug)] enum LiveNodeKind { FreeVarNode(Span), ExprNode(Span), VarDefNode(Span), ExitNode } fn live_node_kind_to_string(lnk: LiveNodeKind, cx: &TyCtxt) -> String { let cm = cx.sess.codemap(); match lnk { FreeVarNode(s) => { format!("Free var node [{}]", cm.span_to_string(s)) } ExprNode(s) => { format!("Expr node [{}]", cm.span_to_string(s)) } VarDefNode(s) => { format!("Var def node [{}]", cm.span_to_string(s)) } ExitNode => "Exit node".to_string(), } } impl<'a, 'tcx, 'v> Visitor<'v> for IrMaps<'a, 'tcx> { fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v hir::FnDecl, b: &'v hir::Block, s: Span, id: NodeId) { visit_fn(self, fk, fd, b, s, id); } fn visit_local(&mut self, l: &hir::Local) { visit_local(self, l); } fn visit_expr(&mut self, ex: &Expr) { visit_expr(self, ex); } fn visit_arm(&mut self, a: &hir::Arm) { visit_arm(self, a); } } pub fn check_crate(tcx: &TyCtxt) { let _task = tcx.dep_graph.in_task(DepNode::Liveness); tcx.map.krate().visit_all_items(&mut IrMaps::new(tcx)); tcx.sess.abort_if_errors(); } 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.get() != usize::MAX } } fn invalid_node() -> LiveNode { LiveNode(usize::MAX) } struct CaptureInfo { ln: LiveNode, var_nid: NodeId } #[derive(Copy, Clone, Debug)] struct LocalInfo { id: NodeId, name: ast::Name } #[derive(Copy, Clone, Debug)] enum VarKind { Arg(NodeId, ast::Name), Local(LocalInfo), ImplicitRet, CleanExit } struct IrMaps<'a, 'tcx: 'a> { tcx: &'a TyCtxt<'tcx>, num_live_nodes: usize, num_vars: usize, live_node_map: NodeMap, variable_map: NodeMap, capture_info_map: NodeMap>>, var_kinds: Vec, lnks: Vec, } impl<'a, 'tcx> IrMaps<'a, 'tcx> { fn new(tcx: &'a TyCtxt<'tcx>) -> IrMaps<'a, 'tcx> { IrMaps { tcx: tcx, num_live_nodes: 0, num_vars: 0, live_node_map: NodeMap(), variable_map: NodeMap(), capture_info_map: NodeMap(), var_kinds: Vec::new(), lnks: Vec::new(), } } fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode { let ln = LiveNode(self.num_live_nodes); 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, node_id: NodeId, lnk: LiveNodeKind) { let ln = self.add_live_node(lnk); self.live_node_map.insert(node_id, ln); debug!("{:?} is node {}", ln, node_id); } fn add_variable(&mut self, vk: VarKind) -> Variable { let v = Variable(self.num_vars); self.var_kinds.push(vk); self.num_vars += 1; match vk { Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => { self.variable_map.insert(node_id, v); }, ImplicitRet | CleanExit => {} } debug!("{:?} is {:?}", v, vk); v } fn variable(&self, node_id: NodeId, span: Span) -> Variable { match self.variable_map.get(&node_id) { Some(&var) => var, None => { self.tcx .sess .span_bug(span, &format!("no variable registered for id {}", node_id)); } } } fn variable_name(&self, var: Variable) -> String { match self.var_kinds[var.get()] { Local(LocalInfo { name, .. }) | Arg(_, name) => { name.to_string() }, ImplicitRet => "".to_string(), CleanExit => "".to_string() } } fn set_captures(&mut self, node_id: NodeId, cs: Vec) { self.capture_info_map.insert(node_id, Rc::new(cs)); } fn lnk(&self, ln: LiveNode) -> LiveNodeKind { self.lnks[ln.get()] } } impl<'a, 'tcx, 'v> Visitor<'v> for Liveness<'a, 'tcx> { fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v hir::FnDecl, b: &'v hir::Block, s: Span, n: NodeId) { check_fn(self, fk, fd, b, s, n); } fn visit_local(&mut self, l: &hir::Local) { check_local(self, l); } fn visit_expr(&mut self, ex: &Expr) { check_expr(self, ex); } fn visit_arm(&mut self, a: &hir::Arm) { check_arm(self, a); } } fn visit_fn(ir: &mut IrMaps, fk: FnKind, decl: &hir::FnDecl, body: &hir::Block, sp: Span, id: ast::NodeId) { debug!("visit_fn"); // swap in a new set of IR maps for this function body: let mut fn_maps = IrMaps::new(ir.tcx); debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps); for arg in &decl.inputs { pat_util::pat_bindings(&ir.tcx.def_map, &arg.pat, |_bm, arg_id, _x, path1| { debug!("adding argument {}", arg_id); let name = path1.node; fn_maps.add_variable(Arg(arg_id, name)); }) }; // gather up the various local variables, significant expressions, // and so forth: intravisit::walk_fn(&mut fn_maps, fk, decl, body, sp); // 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: fn_maps.add_live_node(ExitNode), fallthrough_ln: fn_maps.add_live_node(ExitNode), no_ret_var: fn_maps.add_variable(ImplicitRet), clean_exit_var: fn_maps.add_variable(CleanExit) }; // compute liveness let mut lsets = Liveness::new(&mut fn_maps, specials); let entry_ln = lsets.compute(decl, body); // check for various error conditions lsets.visit_block(body); lsets.check_ret(id, sp, fk, entry_ln, body); lsets.warn_about_unused_args(decl, entry_ln); } fn visit_local(ir: &mut IrMaps, local: &hir::Local) { pat_util::pat_bindings(&ir.tcx.def_map, &local.pat, |_, p_id, sp, path1| { debug!("adding local variable {}", p_id); let name = path1.node; ir.add_live_node_for_node(p_id, VarDefNode(sp)); ir.add_variable(Local(LocalInfo { id: p_id, name: name })); }); intravisit::walk_local(ir, local); } fn visit_arm(ir: &mut IrMaps, arm: &hir::Arm) { for pat in &arm.pats { pat_util::pat_bindings(&ir.tcx.def_map, &pat, |bm, p_id, sp, path1| { debug!("adding local variable {} from match with bm {:?}", p_id, bm); let name = path1.node; ir.add_live_node_for_node(p_id, VarDefNode(sp)); ir.add_variable(Local(LocalInfo { id: p_id, name: name })); }) } intravisit::walk_arm(ir, arm); } fn visit_expr(ir: &mut IrMaps, expr: &Expr) { match expr.node { // live nodes required for uses or definitions of variables: hir::ExprPath(..) => { let def = ir.tcx.def_map.borrow().get(&expr.id).unwrap().full_def(); debug!("expr {}: path that leads to {:?}", expr.id, def); if let Def::Local(..) = def { ir.add_live_node_for_node(expr.id, ExprNode(expr.span)); } intravisit::walk_expr(ir, expr); } hir::ExprClosure(..) => { // Interesting control flow (for loops can contain labeled // breaks or continues) ir.add_live_node_for_node(expr.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(); ir.tcx.with_freevars(expr.id, |freevars| { for fv in freevars { if let Def::Local(_, rv) = fv.def { let fv_ln = ir.add_live_node(FreeVarNode(fv.span)); call_caps.push(CaptureInfo {ln: fv_ln, var_nid: rv}); } } }); ir.set_captures(expr.id, call_caps); intravisit::walk_expr(ir, expr); } // live nodes required for interesting control flow: hir::ExprIf(..) | hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) => { ir.add_live_node_for_node(expr.id, ExprNode(expr.span)); intravisit::walk_expr(ir, expr); } hir::ExprBinary(op, _, _) if ::rustc_front::util::lazy_binop(op.node) => { ir.add_live_node_for_node(expr.id, ExprNode(expr.span)); intravisit::walk_expr(ir, expr); } // otherwise, live nodes are not required: hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) | hir::ExprVec(..) | hir::ExprCall(..) | hir::ExprMethodCall(..) | hir::ExprTup(..) | hir::ExprBinary(..) | hir::ExprAddrOf(..) | hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprBreak(_) | hir::ExprAgain(_) | hir::ExprLit(_) | hir::ExprRet(..) | hir::ExprBlock(..) | hir::ExprAssign(..) | hir::ExprAssignOp(..) | hir::ExprStruct(..) | hir::ExprRepeat(..) | hir::ExprInlineAsm(..) | hir::ExprBox(..) | hir::ExprType(..) => { 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 Users { reader: LiveNode, writer: LiveNode, used: bool } fn invalid_users() -> Users { Users { reader: invalid_node(), writer: invalid_node(), used: false } } #[derive(Copy, Clone)] struct Specials { exit_ln: LiveNode, fallthrough_ln: LiveNode, no_ret_var: Variable, clean_exit_var: Variable } const ACC_READ: u32 = 1; const ACC_WRITE: u32 = 2; const ACC_USE: u32 = 4; struct Liveness<'a, 'tcx: 'a> { ir: &'a mut IrMaps<'a, 'tcx>, s: Specials, successors: Vec, users: Vec, // The list of node IDs for the nested loop scopes // we're in. loop_scope: Vec, // mappings from loop node ID to LiveNode // ("break" label should map to loop node ID, // it probably doesn't now) break_ln: NodeMap, cont_ln: NodeMap } impl<'a, 'tcx> Liveness<'a, 'tcx> { fn new(ir: &'a mut IrMaps<'a, 'tcx>, specials: Specials) -> Liveness<'a, 'tcx> { let num_live_nodes = ir.num_live_nodes; let num_vars = ir.num_vars; Liveness { ir: ir, s: specials, successors: vec![invalid_node(); num_live_nodes], users: vec![invalid_users(); num_live_nodes * num_vars], loop_scope: Vec::new(), break_ln: NodeMap(), cont_ln: NodeMap(), } } fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode { match self.ir.live_node_map.get(&node_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. self.ir.tcx.sess.span_bug( span, &format!("no live node registered for node {}", node_id)); } } } fn variable(&self, node_id: NodeId, span: Span) -> Variable { self.ir.variable(node_id, span) } fn pat_bindings(&mut self, pat: &hir::Pat, mut f: F) where F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId), { pat_util::pat_bindings(&self.ir.tcx.def_map, pat, |_bm, p_id, sp, _n| { let ln = self.live_node(p_id, sp); let var = self.variable(p_id, sp); f(self, ln, var, sp, p_id); }) } fn arm_pats_bindings(&mut self, pat: Option<&hir::Pat>, f: F) where F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId), { match pat { Some(pat) => { self.pat_bindings(pat, f); } None => {} } } fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode) -> LiveNode { self.define_bindings_in_arm_pats(Some(pat), succ) } fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode) -> LiveNode { let mut succ = succ; self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| { this.init_from_succ(ln, succ); this.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.users[self.idx(ln, var)].reader; 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.users[self.idx(ln, var)].used } fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> Option { assert!(ln.is_valid()); let writer = self.users[self.idx(ln, var)].writer; 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 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() { try!(write!(wr, " {:?}", Variable(var_idx))); } } Ok(()) } fn find_loop_scope(&self, opt_label: Option, id: NodeId, sp: Span) -> NodeId { match opt_label { Some(_) => { // Refers to a labeled loop. Use the results of resolve // to find with one match self.ir.tcx.def_map.borrow().get(&id).map(|d| d.full_def()) { Some(Def::Label(loop_id)) => loop_id, _ => self.ir.tcx.sess.span_bug(sp, "label on break/loop \ doesn't refer to a loop") } } None => { // Vanilla 'break' or 'loop', so use the enclosing // loop scope if self.loop_scope.is_empty() { self.ir.tcx.sess.span_bug(sp, "break outside loop"); } else { *self.loop_scope.last().unwrap() } } } } #[allow(unused_must_use)] fn ln_str(&self, ln: LiveNode) -> String { let mut wr = Vec::new(); { let wr = &mut wr as &mut Write; write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln)); self.write_vars(wr, ln, |idx| self.users[idx].reader); write!(wr, " writes"); self.write_vars(wr, ln, |idx| self.users[idx].writer); 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 // values to empty because this is the value // they have when they are created, and the sets // only grow during iterations. // // self.indices(ln) { |idx| // self.users[idx] = invalid_users(); // } } 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.users[idx] = this.users[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 changed = false; self.indices2(ln, succ_ln, |this, idx, succ_idx| { changed |= copy_if_invalid(this.users[succ_idx].reader, &mut this.users[idx].reader); changed |= copy_if_invalid(this.users[succ_idx].writer, &mut this.users[idx].writer); if this.users[succ_idx].used && !this.users[idx].used { this.users[idx].used = true; changed = true; } }); debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})", ln, self.ln_str(succ_ln), first_merge, changed); return changed; fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool { if src.is_valid() && !dst.is_valid() { *dst = src; true } else { false } } } // 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.users[idx].reader = invalid_node(); self.users[idx].writer = invalid_node(); 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 user = &mut self.users[idx]; if (acc & ACC_WRITE) != 0 { user.reader = invalid_node(); user.writer = ln; } // Important: if we both read/write, must do read second // or else the write will override. if (acc & ACC_READ) != 0 { user.reader = ln; } if (acc & ACC_USE) != 0 { user.used = true; } } // _______________________________________________________________________ fn compute(&mut self, decl: &hir::FnDecl, body: &hir::Block) -> LiveNode { // if there is a `break` or `again` at the top level, then it's // effectively a return---this only occurs in `for` loops, // where the body is really a closure. debug!("compute: using id for block, {}", block_to_string(body)); let exit_ln = self.s.exit_ln; let entry_ln: LiveNode = self.with_loop_nodes(body.id, exit_ln, exit_ln, |this| this.propagate_through_fn_block(decl, body)); // 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))); } body.id }, entry_ln); entry_ln } fn propagate_through_fn_block(&mut self, _: &hir::FnDecl, blk: &hir::Block) -> LiveNode { // 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); if blk.expr.is_none() { self.acc(s.fallthrough_ln, s.no_ret_var, ACC_READ) } self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ); self.propagate_through_block(blk, s.fallthrough_ln) } fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode) -> LiveNode { 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.node { hir::StmtDecl(ref decl, _) => { self.propagate_through_decl(&decl, succ) } hir::StmtExpr(ref expr, _) | hir::StmtSemi(ref expr, _) => { self.propagate_through_expr(&expr, succ) } } } fn propagate_through_decl(&mut self, decl: &hir::Decl, succ: LiveNode) -> LiveNode { match decl.node { hir::DeclLocal(ref local) => { self.propagate_through_local(&local, succ) } hir::DeclItem(_) => succ, } } fn propagate_through_local(&mut self, local: &hir::Local, succ: LiveNode) -> LiveNode { // 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) } fn propagate_through_exprs(&mut self, exprs: &[P], 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_to_string(expr)); match expr.node { // Interesting cases with control flow or which gen/kill hir::ExprPath(..) => { self.access_path(expr, succ, ACC_READ | ACC_USE) } hir::ExprField(ref e, _) => { self.propagate_through_expr(&e, succ) } hir::ExprTupField(ref e, _) => { self.propagate_through_expr(&e, succ) } hir::ExprClosure(_, _, ref blk) => { debug!("{} is an ExprClosure", expr_to_string(expr)); /* The next-node for a break is the successor of the entire loop. The next-node for a continue is the top of this loop. */ let node = self.live_node(expr.id, expr.span); self.with_loop_nodes(blk.id, succ, node, |this| { // the construction of a closure itself is not important, // but we have to consider the closed over variables. let caps = match this.ir.capture_info_map.get(&expr.id) { Some(caps) => caps.clone(), None => { this.ir.tcx.sess.span_bug(expr.span, "no registered caps"); } }; caps.iter().rev().fold(succ, |succ, cap| { this.init_from_succ(cap.ln, succ); let var = this.variable(cap.var_nid, expr.span); this.acc(cap.ln, var, ACC_READ | ACC_USE); cap.ln }) }) } hir::ExprIf(ref cond, ref then, ref els) => { // // (cond) // | // v // (expr) // / \ // | | // v v // (then)(els) // | | // v v // ( succ ) // let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ); let then_ln = self.propagate_through_block(&then, succ); let ln = self.live_node(expr.id, expr.span); self.init_from_succ(ln, else_ln); self.merge_from_succ(ln, then_ln, false); self.propagate_through_expr(&cond, ln) } hir::ExprWhile(ref cond, ref blk, _) => { self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ) } // Note that labels have been resolved, so we don't need to look // at the label ident hir::ExprLoop(ref blk, _) => { self.propagate_through_loop(expr, LoopLoop, &blk, succ) } hir::ExprMatch(ref e, ref arms, _) => { // // (e) // | // v // (expr) // / | \ // | | | // v v v // (..arms..) // | | | // v v v // ( succ ) // // let ln = self.live_node(expr.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(|e| &**e), body_succ); // 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 let arm_succ = self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p), guard_succ); self.merge_from_succ(ln, arm_succ, first_merge); first_merge = false; }; self.propagate_through_expr(&e, ln) } hir::ExprRet(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::ExprBreak(opt_label) => { // Find which label this break jumps to let sc = self.find_loop_scope(opt_label.map(|l| l.node.name), expr.id, expr.span); // Now that we know the label we're going to, // look it up in the break loop nodes table match self.break_ln.get(&sc) { Some(&b) => b, None => self.ir.tcx.sess.span_bug(expr.span, "break to unknown label") } } hir::ExprAgain(opt_label) => { // Find which label this expr continues to let sc = self.find_loop_scope(opt_label.map(|l| l.node.name), expr.id, expr.span); // Now that we know the label we're going to, // look it up in the continue loop nodes table match self.cont_ln.get(&sc) { Some(&b) => b, None => self.ir.tcx.sess.span_bug(expr.span, "loop to unknown label") } } hir::ExprAssign(ref l, ref r) => { // see comment on lvalues in // propagate_through_lvalue_components() let succ = self.write_lvalue(&l, succ, ACC_WRITE); let succ = self.propagate_through_lvalue_components(&l, succ); self.propagate_through_expr(&r, succ) } hir::ExprAssignOp(_, ref l, ref r) => { // an overloaded assign op is like a method call if self.ir.tcx.is_method_call(expr.id) { let succ = self.propagate_through_expr(&l, succ); self.propagate_through_expr(&r, succ) } else { // see comment on lvalues in // propagate_through_lvalue_components() let succ = self.write_lvalue(&l, succ, ACC_WRITE|ACC_READ); let succ = self.propagate_through_expr(&r, succ); self.propagate_through_lvalue_components(&l, succ) } } // Uninteresting cases: just propagate in rev exec order hir::ExprVec(ref exprs) => { self.propagate_through_exprs(&exprs[..], succ) } hir::ExprRepeat(ref element, ref count) => { let succ = self.propagate_through_expr(&count, succ); self.propagate_through_expr(&element, succ) } hir::ExprStruct(_, 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::ExprCall(ref f, ref args) => { let diverges = !self.ir.tcx.is_method_call(expr.id) && self.ir.tcx.expr_ty_adjusted(&f).fn_ret().diverges(); let succ = if diverges { self.s.exit_ln } else { succ }; let succ = self.propagate_through_exprs(&args[..], succ); self.propagate_through_expr(&f, succ) } hir::ExprMethodCall(_, _, ref args) => { let method_call = ty::MethodCall::expr(expr.id); let method_ty = self.ir.tcx.tables.borrow().method_map[&method_call].ty; let succ = if method_ty.fn_ret().diverges() { self.s.exit_ln } else { succ }; self.propagate_through_exprs(&args[..], succ) } hir::ExprTup(ref exprs) => { self.propagate_through_exprs(&exprs[..], succ) } hir::ExprBinary(op, ref l, ref r) if ::rustc_front::util::lazy_binop(op.node) => { let r_succ = self.propagate_through_expr(&r, succ); let ln = self.live_node(expr.id, expr.span); self.init_from_succ(ln, succ); self.merge_from_succ(ln, r_succ, false); self.propagate_through_expr(&l, ln) } hir::ExprIndex(ref l, ref r) | hir::ExprBinary(_, ref l, ref r) => { let r_succ = self.propagate_through_expr(&r, succ); self.propagate_through_expr(&l, r_succ) } hir::ExprBox(ref e) | hir::ExprAddrOf(_, ref e) | hir::ExprCast(ref e, _) | hir::ExprType(ref e, _) | hir::ExprUnary(_, ref e) => { self.propagate_through_expr(&e, succ) } hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => { let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| { // see comment on lvalues // in propagate_through_lvalue_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_lvalue(output, succ, acc); self.propagate_through_lvalue_components(output, succ) } }); // Inputs are executed first. Propagate last because of rev order self.propagate_through_exprs(inputs, succ) } hir::ExprLit(..) => { succ } hir::ExprBlock(ref blk) => { self.propagate_through_block(&blk, succ) } } } fn propagate_through_lvalue_components(&mut self, expr: &Expr, succ: LiveNode) -> LiveNode { // # Lvalues // // 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 lvalue Untracked lvalue // ----------------------++----------------------- // || // | || | // v || v // (rvalue) || (rvalue) // | || | // v || v // (write of lvalue) || (lvalue components) // | || | // v || v // (succ) || (succ) // || // ----------------------++----------------------- // // I will cover the two cases in turn: // // # Tracked lvalues // // A tracked lvalue 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_lvalue()` routine generates // the contents of this node. There are no subcomponents to // consider. // // # Non-tracked lvalues // // These are lvalues 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_lvalue_components()` (this fn). // // # Illegal lvalues // // It is still possible to observe assignments to non-lvalues; // these errors are detected in the later pass borrowck. We // just ignore such cases and treat them as reads. match expr.node { hir::ExprPath(..) => succ, hir::ExprField(ref e, _) => self.propagate_through_expr(&e, succ), hir::ExprTupField(ref e, _) => self.propagate_through_expr(&e, succ), _ => self.propagate_through_expr(expr, succ) } } // see comment on propagate_through_lvalue() fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: u32) -> LiveNode { match expr.node { hir::ExprPath(..) => { self.access_path(expr, succ, acc) } // We do not track other lvalues, so just propagate through // to their subcomponents. Also, it may happen that // non-lvalues occur here, because those are detected in the // later pass borrowck. _ => succ } } fn access_path(&mut self, expr: &Expr, succ: LiveNode, acc: u32) -> LiveNode { match self.ir.tcx.def_map.borrow().get(&expr.id).unwrap().full_def() { Def::Local(_, nid) => { let ln = self.live_node(expr.id, expr.span); if acc != 0 { self.init_from_succ(ln, succ); let var = self.variable(nid, expr.span); self.acc(ln, var, acc); } ln } _ => succ } } fn propagate_through_loop(&mut self, expr: &Expr, kind: LoopKind, body: &hir::Block, succ: LiveNode) -> LiveNode { /* We model control flow like this: (cond) <--+ | | v | +-- (expr) | | | | | v | | (body) ---+ | | v (succ) */ // first iteration: let mut first_merge = true; let ln = self.live_node(expr.id, expr.span); self.init_empty(ln, succ); match kind { LoopLoop => {} _ => { // If this is not a `loop` loop, then it's possible we bypass // the body altogether. Otherwise, the only way is via a `break` // in the loop body. self.merge_from_succ(ln, succ, first_merge); first_merge = false; } } debug!("propagate_through_loop: using id for loop body {} {}", expr.id, block_to_string(body)); let cond_ln = match kind { LoopLoop => ln, WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln), }; let body_ln = self.with_loop_nodes(expr.id, succ, ln, |this| { this.propagate_through_block(body, cond_ln) }); // repeat until fixed point is reached: while self.merge_from_succ(ln, body_ln, first_merge) { first_merge = false; let new_cond_ln = match kind { LoopLoop => ln, WhileLoop(ref cond) => { self.propagate_through_expr(&cond, ln) } }; assert!(cond_ln == new_cond_ln); assert!(body_ln == self.with_loop_nodes(expr.id, succ, ln, |this| this.propagate_through_block(body, cond_ln))); } cond_ln } fn with_loop_nodes(&mut self, loop_node_id: NodeId, break_ln: LiveNode, cont_ln: LiveNode, f: F) -> R where F: FnOnce(&mut Liveness<'a, 'tcx>) -> R, { debug!("with_loop_nodes: {} {}", loop_node_id, break_ln.get()); self.loop_scope.push(loop_node_id); self.break_ln.insert(loop_node_id, break_ln); self.cont_ln.insert(loop_node_id, cont_ln); let r = f(self); self.loop_scope.pop(); r } } // _______________________________________________________________________ // Checking for error conditions fn check_local(this: &mut Liveness, local: &hir::Local) { match local.init { Some(_) => { this.warn_about_unused_or_dead_vars_in_pat(&local.pat); }, None => { this.pat_bindings(&local.pat, |this, ln, var, sp, id| { this.warn_about_unused(sp, id, ln, var); }) } } intravisit::walk_local(this, local); } fn check_arm(this: &mut Liveness, arm: &hir::Arm) { // 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 this.arm_pats_bindings(arm.pats.first().map(|p| &**p), |this, ln, var, sp, id| { this.warn_about_unused(sp, id, ln, var); }); intravisit::walk_arm(this, arm); } fn check_expr(this: &mut Liveness, expr: &Expr) { match expr.node { hir::ExprAssign(ref l, _) => { this.check_lvalue(&l); intravisit::walk_expr(this, expr); } hir::ExprAssignOp(_, ref l, _) => { if !this.ir.tcx.is_method_call(expr.id) { this.check_lvalue(&l); } intravisit::walk_expr(this, expr); } hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => { for input in inputs { this.visit_expr(input); } // Output operands must be lvalues for (o, output) in ia.outputs.iter().zip(outputs) { if !o.is_indirect { this.check_lvalue(output); } this.visit_expr(output); } intravisit::walk_expr(this, expr); } // no correctness conditions related to liveness hir::ExprCall(..) | hir::ExprMethodCall(..) | hir::ExprIf(..) | hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) | hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) | hir::ExprVec(..) | hir::ExprTup(..) | hir::ExprBinary(..) | hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprRet(..) | hir::ExprBreak(..) | hir::ExprAgain(..) | hir::ExprLit(_) | hir::ExprBlock(..) | hir::ExprAddrOf(..) | hir::ExprStruct(..) | hir::ExprRepeat(..) | hir::ExprClosure(..) | hir::ExprPath(..) | hir::ExprBox(..) | hir::ExprType(..) => { intravisit::walk_expr(this, expr); } } } fn check_fn(_v: &Liveness, _fk: FnKind, _decl: &hir::FnDecl, _body: &hir::Block, _sp: Span, _id: NodeId) { // do not check contents of nested fns } impl<'a, 'tcx> Liveness<'a, 'tcx> { fn fn_ret(&self, id: NodeId) -> ty::PolyFnOutput<'tcx> { let fn_ty = self.ir.tcx.node_id_to_type(id); match fn_ty.sty { ty::TyClosure(closure_def_id, ref substs) => self.ir.tcx.closure_type(closure_def_id, substs).sig.output(), _ => fn_ty.fn_ret() } } fn check_ret(&self, id: NodeId, sp: Span, _fk: FnKind, entry_ln: LiveNode, body: &hir::Block) { // within the fn body, late-bound regions are liberated // and must outlive the *call-site* of the function. let fn_ret = self.ir.tcx.liberate_late_bound_regions( self.ir.tcx.region_maps.call_site_extent(id, body.id), &self.fn_ret(id)); match fn_ret { ty::FnConverging(t_ret) if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() => { let param_env = ParameterEnvironment::for_item(&self.ir.tcx, id); let infcx = infer::new_infer_ctxt(&self.ir.tcx, &self.ir.tcx.tables, Some(param_env), ProjectionMode::Any); let cause = traits::ObligationCause::dummy(); let norm = traits::fully_normalize(&infcx, cause, &t_ret); if norm.unwrap().is_nil() { // for nil return types, it is ok to not return a value expl. } else { let ends_with_stmt = match body.expr { None if !body.stmts.is_empty() => match body.stmts.first().unwrap().node { hir::StmtSemi(ref e, _) => { self.ir.tcx.expr_ty(&e) == t_ret }, _ => false }, _ => false }; let mut err = struct_span_err!(self.ir.tcx.sess, sp, E0269, "not all control paths return a value"); if ends_with_stmt { let last_stmt = body.stmts.first().unwrap(); let original_span = original_sp(self.ir.tcx.sess.codemap(), last_stmt.span, sp); let span_semicolon = Span { lo: original_span.hi - BytePos(1), hi: original_span.hi, expn_id: original_span.expn_id }; err.span_help(span_semicolon, "consider removing this semicolon:"); } err.emit(); } } ty::FnDiverging if self.live_on_entry(entry_ln, self.s.clean_exit_var).is_some() => { span_err!(self.ir.tcx.sess, sp, E0270, "computation may converge in a function marked as diverging"); } _ => {} } } fn check_lvalue(&mut self, expr: &Expr) { match expr.node { hir::ExprPath(..) => { if let Def::Local(_, nid) = self.ir.tcx.def_map.borrow().get(&expr.id) .unwrap() .full_def() { // 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.id, expr.span); let var = self.variable(nid, expr.span); self.warn_about_dead_assign(expr.span, expr.id, ln, var); } } _ => { // For other kinds of lvalues, 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] == ('_' as u8) { None } else { Some(name) } } fn warn_about_unused_args(&self, decl: &hir::FnDecl, entry_ln: LiveNode) { for arg in &decl.inputs { pat_util::pat_bindings(&self.ir.tcx.def_map, &arg.pat, |_bm, p_id, sp, path1| { let var = self.variable(p_id, sp); // Ignore unused self. let name = path1.node; if name != special_idents::self_.name { if !self.warn_about_unused(sp, p_id, entry_ln, var) { if self.live_on_entry(entry_ln, var).is_none() { self.report_dead_assign(p_id, sp, var, true); } } } }) } } fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) { self.pat_bindings(pat, |this, ln, var, sp, id| { if !this.warn_about_unused(sp, id, ln, var) { this.warn_about_dead_assign(sp, id, ln, var); } }) } fn warn_about_unused(&self, sp: Span, id: NodeId, ln: LiveNode, var: Variable) -> bool { if !self.used_on_entry(ln, var) { let r = self.should_warn(var); if let Some(name) = r { // 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.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp, format!("variable `{}` is assigned to, but never used", name)); } else if name != "self" { self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp, format!("unused variable: `{}`", name)); } } true } else { false } } fn warn_about_dead_assign(&self, sp: Span, id: NodeId, ln: LiveNode, var: Variable) { if self.live_on_exit(ln, var).is_none() { self.report_dead_assign(id, sp, var, false); } } fn report_dead_assign(&self, id: NodeId, sp: Span, var: Variable, is_argument: bool) { if let Some(name) = self.should_warn(var) { if is_argument { self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp, format!("value passed to `{}` is never read", name)); } else { self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp, format!("value assigned to `{}` is never read", name)); } } } }