Rudimentary escape analysis for Box<T>

README.md

@@ -6,7 +6,7 @@ A collection of lints to catch common mistakes and improve your Rust code.
 [Jump to usage instructions](#usage)
 
 ##Lints
-There are 80 lints included in this crate:
+There are 81 lints included in this crate:
 
 name                                                                                                     | default | meaning
 ---------------------------------------------------------------------------------------------------------|---------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
@@ -15,6 +15,7 @@ name
 [block_in_if_condition_expr](https://github.com/Manishearth/rust-clippy/wiki#block_in_if_condition_expr) | warn    | braces can be eliminated in conditions that are expressions, e.g `if { true } ...`
 [block_in_if_condition_stmt](https://github.com/Manishearth/rust-clippy/wiki#block_in_if_condition_stmt) | warn    | avoid complex blocks in conditions, instead move the block higher and bind it with 'let'; e.g: `if { let x = true; x } ...`
 [box_vec](https://github.com/Manishearth/rust-clippy/wiki#box_vec)                                       | warn    | usage of `Box<Vec<T>>`, vector elements are already on the heap
+[boxed_local](https://github.com/Manishearth/rust-clippy/wiki#boxed_local)                               | warn    | using Box<T> where unnecessary
 [cast_possible_truncation](https://github.com/Manishearth/rust-clippy/wiki#cast_possible_truncation)     | allow   | casts that may cause truncation of the value, e.g `x as u8` where `x: u32`, or `x as i32` where `x: f32`
 [cast_possible_wrap](https://github.com/Manishearth/rust-clippy/wiki#cast_possible_wrap)                 | allow   | casts that may cause wrapping around the value, e.g `x as i32` where `x: u32` and `x > i32::MAX`
 [cast_precision_loss](https://github.com/Manishearth/rust-clippy/wiki#cast_precision_loss)               | allow   | casts that cause loss of precision, e.g `x as f32` where `x: u64`

src/escape.rs

@@ -0,0 +1,156 @@
+use rustc::lint::*;
+use rustc_front::hir::*;
+use rustc_front::intravisit as visit;
+use rustc::front::map::Node;
+use rustc::middle::ty;
+use rustc::middle::ty::adjustment::AutoAdjustment;
+use rustc::middle::expr_use_visitor::*;
+use rustc::middle::infer;
+use rustc::middle::mem_categorization::{cmt, Categorization};
+use rustc::util::nodemap::NodeSet;
+use syntax::ast::NodeId;
+use syntax::codemap::Span;
+use utils::span_lint;
+
+pub struct EscapePass;
+
+declare_lint!(pub BOXED_LOCAL, Warn, "using Box<T> where unnecessary");
+
+struct EscapeDelegate<'a, 'tcx: 'a> {
+    cx: &'a LateContext<'a, 'tcx>,
+    set: NodeSet,
+}
+
+impl LintPass for EscapePass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(BOXED_LOCAL)
+    }
+}
+
+impl LateLintPass for EscapePass {
+    fn check_fn(&mut self,
+                cx: &LateContext,
+                _: visit::FnKind,
+                decl: &FnDecl,
+                body: &Block,
+                _: Span,
+                id: NodeId) {
+        let param_env = ty::ParameterEnvironment::for_item(cx.tcx, id);
+        let infcx = infer::new_infer_ctxt(cx.tcx, &cx.tcx.tables, Some(param_env), false);
+        let mut v = EscapeDelegate {
+            cx: cx,
+            set: NodeSet(),
+        };
+        {
+            let mut vis = ExprUseVisitor::new(&mut v, &infcx);
+            vis.walk_fn(decl, body);
+        }
+        for node in v.set {
+            span_lint(cx,
+                      BOXED_LOCAL,
+                      cx.tcx.map.span(node),
+                      "local variable doesn't need to be boxed here");
+        }
+    }
+}
+
+impl<'a, 'tcx: 'a> Delegate<'tcx> for EscapeDelegate<'a, 'tcx> {
+    fn consume(&mut self,
+               _: NodeId,
+               _: Span,
+               cmt: cmt<'tcx>,
+               mode: ConsumeMode) {
+
+        if let Categorization::Local(lid) = cmt.cat {
+            if self.set.contains(&lid) {
+                if let Move(DirectRefMove) = mode {
+                    // moved out or in. clearly can't be localized
+                    self.set.remove(&lid);
+                }
+            }
+        }
+    }
+    fn matched_pat(&mut self, _: &Pat, _: cmt<'tcx>, _: MatchMode) {}
+    fn consume_pat(&mut self, consume_pat: &Pat, cmt: cmt<'tcx>, _: ConsumeMode) {
+        if let Categorization::Rvalue(..) = cmt.cat {
+            if let Some(Node::NodeStmt(st)) = self.cx
+                                                  .tcx
+                                                  .map
+                                                  .find(self.cx.tcx.map.get_parent_node(cmt.id)) {
+                if let StmtDecl(ref decl, _) = st.node {
+                    if let DeclLocal(ref loc) = decl.node {
+                        if let Some(ref ex) = loc.init {
+                            if let ExprBox(..) = ex.node {
+                                if let ty::TyBox(..) = cmt.ty.sty {
+                                    // let x = box (...)
+                                    self.set.insert(consume_pat.id);
+                                }
+                                // TODO Box::new
+                                // TODO vec![]
+                                // TODO "foo".to_owned() and friends
+                            }
+                        }
+                    }
+                }
+            }
+        }
+        if let Categorization::Local(lid) = cmt.cat {
+            if self.set.contains(&lid) {
+                // let y = x where x is known
+                // remove x, insert y
+                self.set.insert(consume_pat.id);
+                self.set.remove(&lid);
+            }
+        }
+
+    }
+    fn borrow(&mut self,
+              borrow_id: NodeId,
+              _: Span,
+              cmt: cmt<'tcx>,
+              _: ty::Region,
+              _: ty::BorrowKind,
+              loan_cause: LoanCause) {
+
+        if let Categorization::Local(lid) = cmt.cat {
+            if self.set.contains(&lid) {
+                if let Some(&AutoAdjustment::AdjustDerefRef(adj)) = self.cx
+                                                                        .tcx
+                                                                        .tables
+                                                                        .borrow()
+                                                                        .adjustments
+                                                                        .get(&borrow_id) {
+                    if LoanCause::AutoRef == loan_cause {
+                        // x.foo()
+                        if adj.autoderefs <= 0 {
+                            self.set.remove(&lid); // Used without autodereffing (i.e. x.clone())
+                        }
+                    } else {
+                        self.cx.sess().span_bug(cmt.span, "Unknown adjusted AutoRef");
+                    }
+                } else if LoanCause::AddrOf == loan_cause {
+                    // &x
+                    if let Some(&AutoAdjustment::AdjustDerefRef(adj)) =
+                           self.cx.tcx.tables.borrow().adjustments
+                               .get(&self.cx.tcx.map.get_parent_node(borrow_id)) {
+                        if adj.autoderefs <= 1 {
+                            // foo(&x) where no extra autoreffing is happening
+                            self.set.remove(&lid);
+                        }
+                    }
+
+                } else if LoanCause::MatchDiscriminant == loan_cause {
+                    self.set.remove(&lid); // `match x` can move
+                }
+                // do nothing for matches, etc. These can't escape
+            }
+        }
+    }
+    fn decl_without_init(&mut self, _: NodeId, _: Span) {}
+    fn mutate(&mut self,
+              _: NodeId,
+              _: Span,
+              _: cmt<'tcx>,
+              _: MutateMode) {
+    }
+}

src/lib.rs

@@ -64,6 +64,7 @@ pub mod no_effect;
 pub mod temporary_assignment;
 pub mod transmute;
 pub mod cyclomatic_complexity;
+pub mod escape;
 
 mod reexport {
     pub use syntax::ast::{Name, Ident, NodeId};
@@ -116,6 +117,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_late_lint_pass(box temporary_assignment::TemporaryAssignmentPass);
     reg.register_late_lint_pass(box transmute::UselessTransmute);
     reg.register_late_lint_pass(box cyclomatic_complexity::CyclomaticComplexity::new(25));
+    reg.register_late_lint_pass(box escape::EscapePass);
 
     reg.register_lint_group("clippy_pedantic", vec![
         methods::OPTION_UNWRAP_USED,
@@ -146,6 +148,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         collapsible_if::COLLAPSIBLE_IF,
         cyclomatic_complexity::CYCLOMATIC_COMPLEXITY,
         eq_op::EQ_OP,
+        escape::BOXED_LOCAL,
         eta_reduction::REDUNDANT_CLOSURE,
         identity_op::IDENTITY_OP,
         len_zero::LEN_WITHOUT_IS_EMPTY,

tests/compile-fail/escape_analysis.rs

@@ -0,0 +1,81 @@
+#![feature(plugin, box_syntax)]
+#![plugin(clippy)]
+#![allow(warnings, clippy)]
+
+#![deny(boxed_local)]
+
+#[derive(Clone)]
+struct A;
+
+impl A {
+    fn foo(&self){}
+}
+
+fn main() {
+}
+
+fn warn_call() {
+    let x = box A; //~ ERROR local variable
+    x.foo(); 
+}
+
+fn warn_rename_call() {
+    let x = box A;
+
+    let y = x; //~ ERROR local variable
+    y.foo(); // via autoderef
+}
+
+fn warn_notuse() {
+    let bz = box A; //~ ERROR local variable
+}
+
+fn warn_pass() {
+    let bz = box A; //~ ERROR local variable
+    take_ref(&bz); // via deref coercion
+}
+
+fn nowarn_return() -> Box<A> {
+    let fx = box A;
+    fx // moved out, "escapes"
+}
+
+fn nowarn_move() {
+    let bx = box A;
+    drop(bx) // moved in, "escapes"
+}
+fn nowarn_call() {
+    let bx = box A;
+    bx.clone(); // method only available to Box, not via autoderef
+}
+
+fn nowarn_pass() {
+    let bx = box A;
+    take_box(&bx); // fn needs &Box
+}
+
+
+fn take_box(x: &Box<A>) {}
+fn take_ref(x: &A) {}
+
+
+fn nowarn_ref_take() {
+    // false positive, should actually warn
+    let x = box A; //~ ERROR local variable
+    let y = &x;
+    take_box(y);
+}
+
+fn nowarn_match() {
+    let x = box A; // moved into a match
+    match x {
+        y => drop(y)
+    }
+}
+
+fn warn_match() {
+    let x = box A; //~ ERROR local variable
+    match &x { // not moved
+        ref y => ()
+    }
+}