[compiler-v2][lint] Needless mutable reference

third_party/move/move-compiler-v2/src/lint_common.rs

@@ -21,6 +21,7 @@ pub enum LintChecker {
     BlocksInConditions,
     NeedlessBool,
     NeedlessDerefRef,
+    NeedlessMutableReference,
     NeedlessRefDeref,
     NeedlessRefInFieldAccess,
     SimplerNumericExpression,

third_party/move/move-compiler-v2/src/pipeline/lint_processor.rs

@@ -6,10 +6,14 @@
 //! The lint checks also assume that all the correctness checks have already been performed.
 
 mod avoid_copy_on_identity_comparison;
+mod needless_mutable_reference;
 
 use crate::{
     lint_common::{lint_skips_from_attributes, LintChecker},
-    pipeline::lint_processor::avoid_copy_on_identity_comparison::AvoidCopyOnIdentityComparison,
+    pipeline::lint_processor::{
+        avoid_copy_on_identity_comparison::AvoidCopyOnIdentityComparison,
+        needless_mutable_reference::NeedlessMutableReference,
+    },
 };
 use move_compiler::shared::known_attributes::LintAttribute;
 use move_model::model::{FunctionEnv, GlobalEnv, Loc};
@@ -72,7 +76,10 @@ impl FunctionTargetProcessor for LintProcessor {
 impl LintProcessor {
     /// Returns the default pipeline of stackless bytecode linters to run.
     fn get_default_linter_pipeline() -> Vec<Box<dyn StacklessBytecodeLinter>> {
-        vec![Box::new(AvoidCopyOnIdentityComparison {})]
+        vec![
+            Box::new(AvoidCopyOnIdentityComparison {}),
+            Box::new(NeedlessMutableReference {}),
+        ]
     }
 
     /// Returns a filtered pipeline of stackless bytecode linters to run.

third_party/move/move-compiler-v2/src/pipeline/lint_processor/needless_mutable_reference.rs

@@ -0,0 +1,236 @@
+// Copyright (c) Aptos Foundation
+// SPDX-License-Identifier: Apache-2.0
+
+//! This module implements a stackless-bytecode linter that checks for mutable references
+//! that are never used mutably, and suggests to use immutable references instead.
+//! For example, if a mutable reference is never written to or passed as a mutable reference
+//! parameter to a function call, it can be replaced with an immutable reference.
+//!
+//! Currently, we only track mutable references that are:
+//! - function parameters,
+//! - obtained via `&mut` or `borrow_global_mut`.
+
+use crate::{lint_common::LintChecker, pipeline::lint_processor::StacklessBytecodeLinter};
+use move_model::{
+    ast::TempIndex,
+    model::{GlobalEnv, Loc, Parameter},
+};
+use move_stackless_bytecode::{
+    function_target::FunctionTarget,
+    stackless_bytecode::{Bytecode, Operation},
+};
+use std::collections::{BTreeMap, BTreeSet};
+
+/// Track "mutable" usages of certain mutable references in a function.
+/// Currently, the tracking is performed conservatively, in a flow-insensitive
+/// manner, to minimize perceived false positives.
+#[derive(Default)]
+struct MutableReferenceUsageTracker {
+    /// Keys are temps which are origins of certain mutable references.
+    /// Each key is mapped to a location where it acquires the mutable reference.
+    /// The origins tracked currently are:
+    /// - function parameters that are mutable references.
+    /// - mutable references acquired through `&mut` or `borrow_global_mut`.
+    /// The list above can be extended in the future.
+    origins: BTreeMap<TempIndex, Loc>,
+    /// Derived edges from mutable references.
+    /// A derived edge y -> x is created in the following cases:
+    /// - `x = y;`,  where y: &mut
+    /// - `x = &mut y.f;`
+    /// Each origin also has an entry in `derived_edges` (usually mapping to an
+    /// empty set, unless an origin is also derived).
+    derived_edges: BTreeMap<TempIndex, BTreeSet<TempIndex>>,
+    /// The set of mutable references that are known to be used mutably, either directly
+    /// or through a derived edge. To be used mutably, the mutable reference is:
+    /// - written to via `WriteRef`, or
+    /// - passed as an argument to a function call's mutable reference parameter.
+    mutably_used: BTreeSet<TempIndex>,
+}
+
+impl MutableReferenceUsageTracker {
+    /// For the `target` function, get locations we can warn about needless mutable references.
+    pub fn get_needless_mutable_refs(target: &FunctionTarget) -> Vec<Loc> {
+        let mut tracker = Self::get_tracker_from_params(target);
+        for instr in target.get_bytecode() {
+            tracker.update(target, instr);
+        }
+        tracker.get_mutably_unused_locations()
+    }
+
+    /// Get an initial tracker from function parameters.
+    fn get_tracker_from_params(target: &FunctionTarget) -> Self {
+        let mut tracker = Self::default();
+        for (origin, loc) in Self::get_mut_reference_params(target) {
+            tracker.add_origin(origin, loc);
+        }
+        tracker
+    }
+
+    /// Get locations where origins are not used mutably.
+    fn get_mutably_unused_locations(self) -> Vec<Loc> {
+        self.origins
+            .into_iter()
+            .filter_map(|(t, loc)| {
+                if !self.mutably_used.contains(&t) {
+                    Some(loc)
+                } else {
+                    None
+                }
+            })
+            .collect()
+    }
+
+    /// Get mutable reference function parameters, along with their location info.
+    fn get_mut_reference_params(target: &FunctionTarget) -> BTreeMap<TempIndex, Loc> {
+        target
+            .func_env
+            .get_parameters_ref()
+            .iter()
+            .enumerate()
+            .filter_map(|(i, Parameter(_, ty, loc))| {
+                if ty.is_mutable_reference() {
+                    // Note: we assume that parameters are laid out as the initial temps.
+                    Some((i, loc.clone()))
+                } else {
+                    None
+                }
+            })
+            .collect()
+    }
+
+    /// Update the tracker given `instr`.
+    fn update(&mut self, target: &FunctionTarget, instr: &Bytecode) {
+        self.update_origins(target, instr);
+        self.update_derived_edges(target, instr);
+        self.update_mutably_used(target.global_env(), instr);
+    }
+
+    /// Update origins given `instr`.
+    fn update_origins(&mut self, target: &FunctionTarget, instr: &Bytecode) {
+        use Bytecode::*;
+        use Operation::*;
+        // We currently track `&mut` and `borrow_global_mut` as origins.
+        if let Call(id, dsts, BorrowLoc | BorrowGlobal(..), _, _) = instr {
+            debug_assert!(dsts.len() == 1);
+            if target.get_local_type(dsts[0]).is_mutable_reference() {
+                // The location of whichever instruction appears first for the origin
+                // is used for reporting.
+                self.add_origin(dsts[0], target.get_bytecode_loc(*id));
+            }
+        }
+    }
+
+    /// Update derived edges given `instr`.
+    fn update_derived_edges(&mut self, target: &FunctionTarget, instr: &Bytecode) {
+        use Bytecode::*;
+        use Operation::*;
+        match instr {
+            Assign(_, dst, src, _) => {
+                if self.node_exists(*src) {
+                    self.add_derived_edge(*dst, *src);
+                }
+            },
+            Call(_, dsts, BorrowField(..) | BorrowVariantField(..), srcs, ..) => {
+                debug_assert!(srcs.len() == 1 && dsts.len() == 1);
+                if self.node_exists(srcs[0])
+                    && target.get_local_type(dsts[0]).is_mutable_reference()
+                {
+                    self.add_derived_edge(dsts[0], srcs[0]);
+                }
+            },
+            _ => {},
+        }
+    }
+
+    /// Update mutable usages given `instr`.
+    fn update_mutably_used(&mut self, env: &GlobalEnv, instr: &Bytecode) {
+        use Bytecode::*;
+        use Operation::*;
+        match instr {
+            Call(_, _, WriteRef, srcs, _) => {
+                self.set_and_propagate_mutably_used(srcs[0]);
+            },
+            Call(_, _, Function(mid, fid, _), srcs, _) => {
+                let callee_env = env.get_function_qid(mid.qualified(*fid));
+                callee_env
+                    .get_parameter_types()
+                    .iter()
+                    .enumerate()
+                    .filter(|(_, ty)| ty.is_mutable_reference())
+                    .map(|(i, _)| srcs[i])
+                    .for_each(|src| self.set_and_propagate_mutably_used(src));
+            },
+            _ => {},
+        }
+    }
+
+    /// Add an origin to the tracker.
+    fn add_origin(&mut self, origin: TempIndex, loc: Loc) {
+        self.origins.entry(origin).or_insert(loc);
+        self.derived_edges.entry(origin).or_default();
+    }
+
+    /// Check if a node exists in the tracker.
+    fn node_exists(&self, node: TempIndex) -> bool {
+        self.derived_edges.contains_key(&node)
+    }
+
+    /// Add a derived edge to the tracker.
+    fn add_derived_edge(&mut self, from: TempIndex, to: TempIndex) {
+        self.derived_edges.entry(from).or_default().insert(to);
+        self.propagate_mutably_used(from, to);
+    }
+
+    /// Propagate mutably used information from `from` to `to`.
+    fn propagate_mutably_used(&mut self, from: TempIndex, to: TempIndex) {
+        if self.mutably_used.contains(&from) {
+            self.set_and_propagate_mutably_used(to);
+        }
+    }
+
+    /// Set a mutable reference as mutably used.
+    /// Propagate this information transitively through derived edges.
+    /// Propagation is stopped early if a node is already marked as mutably used.
+    fn set_and_propagate_mutably_used(&mut self, node: TempIndex) {
+        let mut mutably_used = std::mem::take(&mut self.mutably_used);
+        self._set_and_propagate_mutably_used(node, &mut mutably_used);
+        self.mutably_used = mutably_used;
+    }
+
+    /// Helper function for `set_and_propagate_mutably_used`.
+    fn _set_and_propagate_mutably_used(
+        &self,
+        node: TempIndex,
+        mutably_used: &mut BTreeSet<TempIndex>,
+    ) {
+        if !mutably_used.insert(node) {
+            // Stop early if a node is already marked as mutably used.
+            return;
+        }
+        if let Some(parents) = self.derived_edges.get(&node) {
+            for parent in parents {
+                self._set_and_propagate_mutably_used(*parent, mutably_used);
+            }
+        }
+    }
+}
+
+/// Linter for detecting needless mutable references.
+pub struct NeedlessMutableReference {}
+
+impl StacklessBytecodeLinter for NeedlessMutableReference {
+    fn get_lint_checker(&self) -> LintChecker {
+        LintChecker::NeedlessMutableReference
+    }
+
+    fn check(&self, target: &FunctionTarget) {
+        let needless_mutable_refs = MutableReferenceUsageTracker::get_needless_mutable_refs(target);
+        for loc in needless_mutable_refs {
+            self.warning(
+                target.global_env(),
+                &loc,
+                "Needless mutable reference or borrow: consider using immutable reference or borrow instead",
+            );
+        }
+    }
+}

third_party/move/move-compiler-v2/tests/lints/model_ast_lints/needless_deref_ref_warn.exp

@@ -129,4 +129,14 @@ warning: [lint] Needless pair of `*` and `&` operators: consider removing them
     = To suppress this warning, annotate the function/module with the attribute `#[lint::skip(needless_deref_ref)]`.
 
 
+Diagnostics:
+warning: [lint] Needless mutable reference or borrow: consider using immutable reference or borrow instead
+   ┌─ tests/lints/model_ast_lints/needless_deref_ref_warn.move:35:15
+   │
+35 │         *&mut borrow_global_mut<S>(addr).y
+   │               ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   │
+   = To suppress this warning, annotate the function/module with the attribute `#[lint::skip(needless_mutable_reference)]`.
+
+
 ============ bytecode verification succeeded ========

third_party/move/move-compiler-v2/tests/lints/model_ast_lints/needless_ref_in_field_access_warn.exp

@@ -177,4 +177,38 @@ warning: [lint] Needless `&mut` taken for field access: consider removing `&mut`
    = To suppress this warning, annotate the function/module with the attribute `#[lint::skip(needless_ref_in_field_access)]`.
 
 
+Diagnostics:
+warning: [lint] Needless mutable reference or borrow: consider using immutable reference or borrow instead
+   ┌─ tests/lints/model_ast_lints/needless_ref_in_field_access_warn.move:51:9
+   │
+51 │         (&mut make_S()).y.a + (&mut (&mut s).y).a
+   │         ^^^^^^^^^^^^^^^
+   │
+   = To suppress this warning, annotate the function/module with the attribute `#[lint::skip(needless_mutable_reference)]`.
+
+warning: [lint] Needless mutable reference or borrow: consider using immutable reference or borrow instead
+   ┌─ tests/lints/model_ast_lints/needless_ref_in_field_access_warn.move:51:37
+   │
+51 │         (&mut make_S()).y.a + (&mut (&mut s).y).a
+   │                                     ^^^^^^^^
+   │
+   = To suppress this warning, annotate the function/module with the attribute `#[lint::skip(needless_mutable_reference)]`.
+
+warning: [lint] Needless mutable reference or borrow: consider using immutable reference or borrow instead
+   ┌─ tests/lints/model_ast_lints/needless_ref_in_field_access_warn.move:90:18
+   │
+90 │         (&e).0 + (&mut e).0
+   │                  ^^^^^^^^
+   │
+   = To suppress this warning, annotate the function/module with the attribute `#[lint::skip(needless_mutable_reference)]`.
+
+warning: [lint] Needless mutable reference or borrow: consider using immutable reference or borrow instead
+    ┌─ tests/lints/model_ast_lints/needless_ref_in_field_access_warn.move:125:18
+    │
+125 │         (&s).x + (&mut s).x
+    │                  ^^^^^^^^
+    │
+    = To suppress this warning, annotate the function/module with the attribute `#[lint::skip(needless_mutable_reference)]`.
+
+
 ============ bytecode verification succeeded ========