rust/src/librustc_lint/unused.rs

use rustc::hir::def::{Res, DefKind};
use rustc::hir::def_id::DefId;
use rustc::lint;
use rustc::ty::{self, Ty};
use rustc::ty::adjustment;
use rustc_data_structures::fx::FxHashMap;
use lint::{LateContext, EarlyContext, LintContext, LintArray};
use lint::{LintPass, EarlyLintPass, LateLintPass};

use syntax::ast;
use syntax::attr;
use syntax::errors::{Applicability, pluralise};
use syntax::feature_gate::{AttributeType, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
use syntax::print::pprust;
use syntax::symbol::{kw, sym};
use syntax::symbol::Symbol;
use syntax::util::parser;
use syntax_pos::{Span, BytePos};

use rustc::hir;

use log::debug;

declare_lint! {
    pub UNUSED_MUST_USE,
    Warn,
    "unused result of a type flagged as `#[must_use]`",
    report_in_external_macro: true
}

declare_lint! {
    pub UNUSED_RESULTS,
    Allow,
    "unused result of an expression in a statement"
}

declare_lint_pass!(UnusedResults => [UNUSED_MUST_USE, UNUSED_RESULTS]);

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedResults {
    fn check_stmt(&mut self, cx: &LateContext<'_, '_>, s: &hir::Stmt) {
        let expr = match s.node {
            hir::StmtKind::Semi(ref expr) => &**expr,
            _ => return,
        };

        if let hir::ExprKind::Ret(..) = expr.kind {
            return;
        }

        let ty = cx.tables.expr_ty(&expr);
        let type_permits_lack_of_use = check_must_use_ty(cx, ty, &expr, s.span, "", "", 1);

        let mut fn_warned = false;
        let mut op_warned = false;
        let maybe_def_id = match expr.kind {
            hir::ExprKind::Call(ref callee, _) => {
                match callee.kind {
                    hir::ExprKind::Path(ref qpath) => {
                        match cx.tables.qpath_res(qpath, callee.hir_id) {
                            Res::Def(DefKind::Fn, def_id)
                            | Res::Def(DefKind::Method, def_id) => Some(def_id),
                            // `Res::Local` if it was a closure, for which we
                            // do not currently support must-use linting
                            _ => None
                        }
                    },
                    _ => None
                }
            },
            hir::ExprKind::MethodCall(..) => {
                cx.tables.type_dependent_def_id(expr.hir_id)
            },
            _ => None
        };
        if let Some(def_id) = maybe_def_id {
            fn_warned = check_must_use_def(cx, def_id, s.span, "return value of ", "");
        } else if type_permits_lack_of_use {
            // We don't warn about unused unit or uninhabited types.
            // (See https://github.com/rust-lang/rust/issues/43806 for details.)
            return;
        }

        let must_use_op = match expr.kind {
            // Hardcoding operators here seemed more expedient than the
            // refactoring that would be needed to look up the `#[must_use]`
            // attribute which does exist on the comparison trait methods
            hir::ExprKind::Binary(bin_op, ..)  => {
                match bin_op.node {
                    hir::BinOpKind::Eq |
                    hir::BinOpKind::Lt |
                    hir::BinOpKind::Le |
                    hir::BinOpKind::Ne |
                    hir::BinOpKind::Ge |
                    hir::BinOpKind::Gt => {
                        Some("comparison")
                    },
                    hir::BinOpKind::Add |
                    hir::BinOpKind::Sub |
                    hir::BinOpKind::Div |
                    hir::BinOpKind::Mul |
                    hir::BinOpKind::Rem => {
                        Some("arithmetic operation")
                    },
                    hir::BinOpKind::And | hir::BinOpKind::Or => {
                        Some("logical operation")
                    },
                    hir::BinOpKind::BitXor |
                    hir::BinOpKind::BitAnd |
                    hir::BinOpKind::BitOr |
                    hir::BinOpKind::Shl |
                    hir::BinOpKind::Shr => {
                        Some("bitwise operation")
                    },
                }
            },
            hir::ExprKind::Unary(..) => Some("unary operation"),
            _ => None
        };

        if let Some(must_use_op) = must_use_op {
            cx.span_lint(UNUSED_MUST_USE, expr.span,
                         &format!("unused {} that must be used", must_use_op));
            op_warned = true;
        }

        if !(type_permits_lack_of_use || fn_warned || op_warned) {
            cx.span_lint(UNUSED_RESULTS, s.span, "unused result");
        }

        // Returns whether an error has been emitted (and thus another does not need to be later).
        fn check_must_use_ty<'tcx>(
            cx: &LateContext<'_, 'tcx>,
            ty: Ty<'tcx>,
            expr: &hir::Expr,
            span: Span,
            descr_pre: &str,
            descr_post: &str,
            plural_len: usize,
        ) -> bool {
            if ty.is_unit() || cx.tcx.is_ty_uninhabited_from(
                cx.tcx.hir().get_module_parent(expr.hir_id), ty)
            {
                return true;
            }

            let plural_suffix = pluralise!(plural_len);

            match ty.kind {
                ty::Adt(..) if ty.is_box() => {
                    let boxed_ty = ty.boxed_ty();
                    let descr_pre = &format!("{}boxed ", descr_pre);
                    check_must_use_ty(cx, boxed_ty, expr, span, descr_pre, descr_post, plural_len)
                }
                ty::Adt(def, _) => {
                    check_must_use_def(cx, def.did, span, descr_pre, descr_post)
                }
                ty::Opaque(def, _) => {
                    let mut has_emitted = false;
                    for (predicate, _) in &cx.tcx.predicates_of(def).predicates {
                        if let ty::Predicate::Trait(ref poly_trait_predicate) = predicate {
                            let trait_ref = poly_trait_predicate.skip_binder().trait_ref;
                            let def_id = trait_ref.def_id;
                            let descr_pre = &format!(
                                "{}implementer{} of ",
                                descr_pre,
                                plural_suffix,
                            );
                            if check_must_use_def(cx, def_id, span, descr_pre, descr_post) {
                                has_emitted = true;
                                break;
                            }
                        }
                    }
                    has_emitted
                }
                ty::Dynamic(binder, _) => {
                    let mut has_emitted = false;
                    for predicate in binder.skip_binder().iter() {
                        if let ty::ExistentialPredicate::Trait(ref trait_ref) = predicate {
                            let def_id = trait_ref.def_id;
                            let descr_post = &format!(
                                " trait object{}{}",
                                plural_suffix,
                                descr_post,
                            );
                            if check_must_use_def(cx, def_id, span, descr_pre, descr_post) {
                                has_emitted = true;
                                break;
                            }
                        }
                    }
                    has_emitted
                }
                ty::Tuple(ref tys) => {
                    let mut has_emitted = false;
                    let spans = if let hir::ExprKind::Tup(comps) = &expr.kind {
                        debug_assert_eq!(comps.len(), tys.len());
                        comps.iter().map(|e| e.span).collect()
                    } else {
                        vec![]
                    };
                    for (i, ty) in tys.iter().map(|k| k.expect_ty()).enumerate() {
                        let descr_post = &format!(" in tuple element {}", i);
                        let span = *spans.get(i).unwrap_or(&span);
                        if check_must_use_ty(
                            cx,
                            ty,
                            expr,
                            span,
                            descr_pre,
                            descr_post,
                            plural_len
                        ) {
                            has_emitted = true;
                        }
                    }
                    has_emitted
                }
                ty::Array(ty, len) => match len.try_eval_usize(cx.tcx, cx.param_env) {
                    // If the array is definitely non-empty, we can do `#[must_use]` checking.
                    Some(n) if n != 0 => {
                        let descr_pre = &format!(
                            "{}array{} of ",
                            descr_pre,
                            plural_suffix,
                        );
                        check_must_use_ty(cx, ty, expr, span, descr_pre, descr_post, n as usize + 1)
                    }
                    // Otherwise, we don't lint, to avoid false positives.
                    _ => false,
                }
                _ => false,
            }
        }

        // Returns whether an error has been emitted (and thus another does not need to be later).
        fn check_must_use_def(
            cx: &LateContext<'_, '_>,
            def_id: DefId,
            span: Span,
            descr_pre_path: &str,
            descr_post_path: &str,
        ) -> bool {
            for attr in cx.tcx.get_attrs(def_id).iter() {
                if attr.check_name(sym::must_use) {
                    let msg = format!("unused {}`{}`{} that must be used",
                        descr_pre_path, cx.tcx.def_path_str(def_id), descr_post_path);
                    let mut err = cx.struct_span_lint(UNUSED_MUST_USE, span, &msg);
                    // check for #[must_use = "..."]
                    if let Some(note) = attr.value_str() {
                        err.note(&note.as_str());
                    }
                    err.emit();
                    return true;
                }
            }
            false
        }
    }
}

declare_lint! {
    pub PATH_STATEMENTS,
    Warn,
    "path statements with no effect"
}

declare_lint_pass!(PathStatements => [PATH_STATEMENTS]);

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for PathStatements {
    fn check_stmt(&mut self, cx: &LateContext<'_, '_>, s: &hir::Stmt) {
        if let hir::StmtKind::Semi(ref expr) = s.node {
            if let hir::ExprKind::Path(_) = expr.kind {
                cx.span_lint(PATH_STATEMENTS, s.span, "path statement with no effect");
            }
        }
    }
}

declare_lint! {
    pub UNUSED_ATTRIBUTES,
    Warn,
    "detects attributes that were not used by the compiler"
}

#[derive(Copy, Clone)]
pub struct UnusedAttributes {
    builtin_attributes: &'static FxHashMap<Symbol, &'static BuiltinAttribute>,
}

impl UnusedAttributes {
    pub fn new() -> Self {
        UnusedAttributes {
            builtin_attributes: &*BUILTIN_ATTRIBUTE_MAP,
        }
    }
}

impl_lint_pass!(UnusedAttributes => [UNUSED_ATTRIBUTES]);

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedAttributes {
    fn check_attribute(&mut self, cx: &LateContext<'_, '_>, attr: &ast::Attribute) {
        debug!("checking attribute: {:?}", attr);

        let attr_info = attr.ident().and_then(|ident| self.builtin_attributes.get(&ident.name));

        if let Some(&&(name, ty, ..)) = attr_info {
            match ty {
                AttributeType::Whitelisted => {
                    debug!("{:?} is Whitelisted", name);
                    return;
                }
                _ => (),
            }
        }

        let plugin_attributes = cx.sess().plugin_attributes.borrow_mut();
        for &(name, ty) in plugin_attributes.iter() {
            if ty == AttributeType::Whitelisted && attr.check_name(name) {
                debug!("{:?} (plugin attr) is whitelisted with ty {:?}", name, ty);
                break;
            }
        }

        let name = attr.name_or_empty();
        if !attr::is_used(attr) {
            debug!("emitting warning for: {:?}", attr);
            cx.span_lint(UNUSED_ATTRIBUTES, attr.span, "unused attribute");
            // Is it a builtin attribute that must be used at the crate level?
            let known_crate = attr_info.map(|&&(_, ty, ..)| {
                    ty == AttributeType::CrateLevel
            }).unwrap_or(false);

            // Has a plugin registered this attribute as one that must be used at
            // the crate level?
            let plugin_crate = plugin_attributes.iter()
                .find(|&&(x, t)| name == x && AttributeType::CrateLevel == t)
                .is_some();
            if known_crate || plugin_crate {
                let msg = match attr.style {
                    ast::AttrStyle::Outer => {
                        "crate-level attribute should be an inner attribute: add an exclamation \
                         mark: `#![foo]`"
                    }
                    ast::AttrStyle::Inner => "crate-level attribute should be in the root module",
                };
                cx.span_lint(UNUSED_ATTRIBUTES, attr.span, msg);
            }
        } else {
            debug!("Attr was used: {:?}", attr);
        }
    }
}

declare_lint! {
    pub(super) UNUSED_PARENS,
    Warn,
    "`if`, `match`, `while` and `return` do not need parentheses"
}

declare_lint_pass!(UnusedParens => [UNUSED_PARENS]);

impl UnusedParens {

    fn is_expr_parens_necessary(inner: &ast::Expr, followed_by_block: bool) -> bool {
        followed_by_block && match inner.kind {
            ast::ExprKind::Ret(_) | ast::ExprKind::Break(..) => true,
            _ => parser::contains_exterior_struct_lit(&inner),
        }
    }

    fn check_unused_parens_expr(&self,
                                     cx: &EarlyContext<'_>,
                                     value: &ast::Expr,
                                     msg: &str,
                                     followed_by_block: bool,
                                     left_pos: Option<BytePos>,
                                     right_pos: Option<BytePos>) {
        match value.kind {
            ast::ExprKind::Paren(ref inner) => {
                if !Self::is_expr_parens_necessary(inner, followed_by_block) &&
                    value.attrs.is_empty() {
                    let expr_text = if let Ok(snippet) = cx.sess().source_map()
                        .span_to_snippet(value.span) {
                            snippet
                        } else {
                            pprust::expr_to_string(value)
                        };
                    let keep_space = (
                        left_pos.map(|s| s >= value.span.lo()).unwrap_or(false),
                        right_pos.map(|s| s <= value.span.hi()).unwrap_or(false),
                    );
                    Self::remove_outer_parens(cx, value.span, &expr_text, msg, keep_space);
                }
            }
            ast::ExprKind::Let(_, ref expr) => {
                // FIXME(#60336): Properly handle `let true = (false && true)`
                // actually needing the parenthesis.
                self.check_unused_parens_expr(
                    cx, expr,
                    "`let` head expression",
                    followed_by_block,
                    None, None
                );
            }
            _ => {}
        }
    }

    fn check_unused_parens_pat(
        &self,
        cx: &EarlyContext<'_>,
        value: &ast::Pat,
        avoid_or: bool,
        avoid_mut: bool,
    ) {
        use ast::{PatKind, BindingMode::ByValue, Mutability::Mutable};

        if let PatKind::Paren(inner) = &value.kind {
            match inner.kind {
                // The lint visitor will visit each subpattern of `p`. We do not want to lint
                // any range pattern no matter where it occurs in the pattern. For something like
                // `&(a..=b)`, there is a recursive `check_pat` on `a` and `b`, but we will assume
                // that if there are unnecessary parens they serve a purpose of readability.
                PatKind::Range(..) => return,
                // Avoid `p0 | .. | pn` if we should.
                PatKind::Or(..) if avoid_or => return,
                // Avoid `mut x` and `mut x @ p` if we should:
                PatKind::Ident(ByValue(Mutable), ..) if avoid_mut => return,
                // Otherwise proceed with linting.
                _ => {}
            }

            let pattern_text = if let Ok(snippet) = cx.sess().source_map()
                .span_to_snippet(value.span) {
                    snippet
                } else {
                    pprust::pat_to_string(value)
                };
            Self::remove_outer_parens(cx, value.span, &pattern_text, "pattern", (false, false));
        }
    }

    fn remove_outer_parens(cx: &EarlyContext<'_>,
                           span: Span,
                           pattern: &str,
                           msg: &str,
                           keep_space: (bool, bool)) {
        let span_msg = format!("unnecessary parentheses around {}", msg);
        let mut err = cx.struct_span_lint(UNUSED_PARENS, span, &span_msg);
        let mut ate_left_paren = false;
        let mut ate_right_paren = false;
        let parens_removed = pattern
            .trim_matches(|c| {
                match c {
                    '(' => {
                        if ate_left_paren {
                            false
                        } else {
                            ate_left_paren = true;
                            true
                        }
                    },
                    ')' => {
                        if ate_right_paren {
                            false
                        } else {
                            ate_right_paren = true;
                            true
                        }
                    },
                    _ => false,
                }
            });

        let replace = {
            let mut replace = if keep_space.0 {
                let mut s = String::from(" ");
                s.push_str(parens_removed);
                s
            } else {
                String::from(parens_removed)
            };

            if keep_space.1 {
                replace.push(' ');
            }
            replace
        };

        err.span_suggestion_short(
            span,
            "remove these parentheses",
            replace,
            Applicability::MachineApplicable,
        );
        err.emit();
    }
}

impl EarlyLintPass for UnusedParens {
    fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
        use syntax::ast::ExprKind::*;
        let (value, msg, followed_by_block, left_pos, right_pos) = match e.kind {
            Let(ref pat, ..) => {
                self.check_unused_parens_pat(cx, pat, false, false);
                return;
            }

            If(ref cond, ref block, ..) => {
                let left = e.span.lo() + syntax_pos::BytePos(2);
                let right = block.span.lo();
                (cond, "`if` condition", true, Some(left), Some(right))
            }

            While(ref cond, ref block, ..) => {
                let left = e.span.lo() + syntax_pos::BytePos(5);
                let right = block.span.lo();
                (cond, "`while` condition", true, Some(left), Some(right))
            },

            ForLoop(ref pat, ref cond, ref block, ..) => {
                self.check_unused_parens_pat(cx, pat, false, false);
                (cond, "`for` head expression", true, None, Some(block.span.lo()))
            }

            Match(ref head, _) => {
                let left = e.span.lo() + syntax_pos::BytePos(5);
                (head, "`match` head expression", true, Some(left), None)
            }

            Ret(Some(ref value)) => {
                let left = e.span.lo() + syntax_pos::BytePos(3);
                (value, "`return` value", false, Some(left), None)
            }

            Assign(_, ref value) => (value, "assigned value", false, None, None),
            AssignOp(.., ref value) => (value, "assigned value", false, None, None),
            // either function/method call, or something this lint doesn't care about
            ref call_or_other => {
                let (args_to_check, call_kind) = match *call_or_other {
                    Call(_, ref args) => (&args[..], "function"),
                    // first "argument" is self (which sometimes needs parens)
                    MethodCall(_, ref args) => (&args[1..], "method"),
                    // actual catch-all arm
                    _ => {
                        return;
                    }
                };
                // Don't lint if this is a nested macro expansion: otherwise, the lint could
                // trigger in situations that macro authors shouldn't have to care about, e.g.,
                // when a parenthesized token tree matched in one macro expansion is matched as
                // an expression in another and used as a fn/method argument (Issue #47775)
                if e.span.ctxt().outer_expn_data().call_site.from_expansion() {
                    return;
                }
                let msg = format!("{} argument", call_kind);
                for arg in args_to_check {
                    self.check_unused_parens_expr(cx, arg, &msg, false, None, None);
                }
                return;
            }
        };
        self.check_unused_parens_expr(cx, &value, msg, followed_by_block, left_pos, right_pos);
    }

    fn check_pat(&mut self, cx: &EarlyContext<'_>, p: &ast::Pat) {
        use ast::{PatKind::*, Mutability};
        match &p.kind {
            // Do not lint on `(..)` as that will result in the other arms being useless.
            Paren(_)
            // The other cases do not contain sub-patterns.
            | Wild | Rest | Lit(..) | Mac(..) | Range(..) | Ident(.., None) | Path(..) => return,
            // These are list-like patterns; parens can always be removed.
            TupleStruct(_, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
                self.check_unused_parens_pat(cx, p, false, false);
            },
            Struct(_, fps, _) => for f in fps {
                self.check_unused_parens_pat(cx, &f.pat, false, false);
            },
            // Avoid linting on `i @ (p0 | .. | pn)` and `box (p0 | .. | pn)`, #64106.
            Ident(.., Some(p)) | Box(p) => self.check_unused_parens_pat(cx, p, true, false),
            // Avoid linting on `&(mut x)` as `&mut x` has a different meaning, #55342.
            // Also avoid linting on `& mut? (p0 | .. | pn)`, #64106.
            Ref(p, m) => self.check_unused_parens_pat(cx, p, true, *m == Mutability::Immutable),
        }
    }

    fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
        if let ast::StmtKind::Local(ref local) = s.node {
            self.check_unused_parens_pat(cx, &local.pat, false, false);

            if let Some(ref value) = local.init {
                self.check_unused_parens_expr(cx, &value, "assigned value", false, None, None);
            }
        }
    }

    fn check_param(&mut self, cx: &EarlyContext<'_>, param: &ast::Param) {
        self.check_unused_parens_pat(cx, &param.pat, true, false);
    }

    fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
        self.check_unused_parens_pat(cx, &arm.pat, false, false);
    }
}

declare_lint! {
    UNUSED_IMPORT_BRACES,
    Allow,
    "unnecessary braces around an imported item"
}

declare_lint_pass!(UnusedImportBraces => [UNUSED_IMPORT_BRACES]);

impl UnusedImportBraces {
    fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &ast::Item) {
        if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
            // Recursively check nested UseTrees
            for &(ref tree, _) in items {
                self.check_use_tree(cx, tree, item);
            }

            // Trigger the lint only if there is one nested item
            if items.len() != 1 {
                return;
            }

            // Trigger the lint if the nested item is a non-self single item
            let node_name = match items[0].0.kind {
                ast::UseTreeKind::Simple(rename, ..) => {
                    let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
                    if orig_ident.name == kw::SelfLower {
                        return;
                    }
                    rename.unwrap_or(orig_ident).name
                }
                ast::UseTreeKind::Glob => Symbol::intern("*"),
                ast::UseTreeKind::Nested(_) => return,
            };

            let msg = format!("braces around {} is unnecessary", node_name);
            cx.span_lint(UNUSED_IMPORT_BRACES, item.span, &msg);
        }
    }
}

impl EarlyLintPass for UnusedImportBraces {
    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &ast::Item) {
        if let ast::ItemKind::Use(ref use_tree) = item.node {
            self.check_use_tree(cx, use_tree, item);
        }
    }
}

declare_lint! {
    pub(super) UNUSED_ALLOCATION,
    Warn,
    "detects unnecessary allocations that can be eliminated"
}

declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedAllocation {
    fn check_expr(&mut self, cx: &LateContext<'_, '_>, e: &hir::Expr) {
        match e.kind {
            hir::ExprKind::Box(_) => {}
            _ => return,
        }

        for adj in cx.tables.expr_adjustments(e) {
            if let adjustment::Adjust::Borrow(adjustment::AutoBorrow::Ref(_, m)) = adj.kind {
                let msg = match m {
                    adjustment::AutoBorrowMutability::Immutable =>
                        "unnecessary allocation, use `&` instead",
                    adjustment::AutoBorrowMutability::Mutable { .. }=>
                        "unnecessary allocation, use `&mut` instead"
                };
                cx.span_lint(UNUSED_ALLOCATION, e.span, msg);
            }
        }
    }
}