src/librustc_lint/unused.rs

// Copyright 2015 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 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use rustc::hir::def_id::DefId;
use rustc::ty;
use rustc::ty::adjustment;
use lint::{LateContext, EarlyContext, LintContext, LintArray};
use lint::{LintPass, EarlyLintPass, LateLintPass};

use syntax::ast;
use syntax::attr;
use syntax::feature_gate::{BUILTIN_ATTRIBUTES, AttributeType};
use syntax::print::pprust;
use syntax::symbol::keywords;
use syntax::util::parser;
use syntax_pos::Span;

use rustc::hir;

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

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

#[derive(Copy, Clone)]
pub struct UnusedResults;

impl LintPass for UnusedResults {
    fn get_lints(&self) -> LintArray {
        lint_array!(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::StmtSemi(ref expr, _) => &**expr,
            _ => return,
        };

        if let hir::ExprRet(..) = expr.node {
            return;
        }

        let t = cx.tables.expr_ty(&expr);
        let ty_warned = match t.sty {
            ty::TyTuple(ref tys, _) if tys.is_empty() => return,
            ty::TyNever => return,
            ty::TyAdt(def, _) => {
                if def.variants.is_empty() {
                    return;
                } else {
                    check_must_use(cx, def.did, s.span, "")
                }
            },
            _ => false,
        };

        let mut fn_warned = false;
        let mut op_warned = false;
        if cx.tcx.sess.features.borrow().fn_must_use {
            let maybe_def = match expr.node {
                hir::ExprCall(ref callee, _) => {
                    match callee.node {
                        hir::ExprPath(ref qpath) => {
                            Some(cx.tables.qpath_def(qpath, callee.hir_id))
                        },
                        _ => None
                    }
                },
                hir::ExprMethodCall(..) => {
                    cx.tables.type_dependent_defs().get(expr.hir_id).cloned()
                },
                _ => None
            };
            if let Some(def) = maybe_def {
                let def_id = def.def_id();
                fn_warned = check_must_use(cx, def_id, s.span, "return value of ");
            }

            if let hir::ExprBinary(bin_op, ..) = expr.node {
                match bin_op.node {
                    // Hardcoding the comparison 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::BiEq | hir::BiLt | hir::BiLe | hir::BiNe | hir::BiGe | hir::BiGt => {
                        let msg = "unused comparison which must be used";
                        cx.span_lint(UNUSED_MUST_USE, expr.span, msg);
                        op_warned = true;
                    },
                    _ => {},
                }
            }
        }

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

        fn check_must_use(cx: &LateContext, def_id: DefId, sp: Span, describe_path: &str) -> bool {
            for attr in cx.tcx.get_attrs(def_id).iter() {
                if attr.check_name("must_use") {
                    let mut msg = format!("unused {}`{}` which must be used",
                                          describe_path, cx.tcx.item_path_str(def_id));
                    // check for #[must_use="..."]
                    if let Some(s) = attr.value_str() {
                        msg.push_str(": ");
                        msg.push_str(&s.as_str());
                    }
                    cx.span_lint(UNUSED_MUST_USE, sp, &msg);
                    return true;
                }
            }
            false
        }
    }
}

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

#[derive(Copy, Clone)]
pub struct PathStatements;

impl LintPass for PathStatements {
    fn get_lints(&self) -> LintArray {
        lint_array!(PATH_STATEMENTS)
    }
}

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for PathStatements {
    fn check_stmt(&mut self, cx: &LateContext, s: &hir::Stmt) {
        if let hir::StmtSemi(ref expr, _) = s.node {
            if let hir::ExprPath(_) = expr.node {
                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;

impl LintPass for UnusedAttributes {
    fn get_lints(&self) -> LintArray {
        lint_array!(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 name = unwrap_or!(attr.name(), return);

        // Note that check_name() marks the attribute as used if it matches.
        for &(ref name, ty, _) in BUILTIN_ATTRIBUTES {
            match ty {
                AttributeType::Whitelisted if attr.check_name(name) => {
                    debug!("{:?} is Whitelisted", name);
                    break;
                }
                _ => (),
            }
        }

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

        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 = BUILTIN_ATTRIBUTES.iter()
                .find(|&&(builtin, ty, _)| name == builtin && ty == AttributeType::CrateLevel)
                .is_some();

            // Has a plugin registered this attribute as one which must be used at
            // the crate level?
            let plugin_crate = plugin_attributes.iter()
                .find(|&&(ref 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"
}

#[derive(Copy, Clone)]
pub struct UnusedParens;

impl UnusedParens {
    fn check_unused_parens_core(&self,
                                cx: &EarlyContext,
                                value: &ast::Expr,
                                msg: &str,
                                struct_lit_needs_parens: bool) {
        if let ast::ExprKind::Paren(ref inner) = value.node {
            let necessary = struct_lit_needs_parens &&
                            parser::contains_exterior_struct_lit(&inner);
            if !necessary {
                let span_msg = format!("unnecessary parentheses around {}", msg);
                let mut err = cx.struct_span_lint(UNUSED_PARENS,
                                                  value.span,
                                                  &span_msg);
                // Remove exactly one pair of parentheses (rather than naïvely
                // stripping all paren characters)
                let mut ate_left_paren = false;
                let mut ate_right_paren = false;
                let parens_removed = pprust::expr_to_string(value)
                    .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,
                        }
                    }).to_owned();
                err.span_suggestion_short(value.span,
                                          "remove these parentheses",
                                          parens_removed);
                err.emit();
            }
        }
    }
}

impl LintPass for UnusedParens {
    fn get_lints(&self) -> LintArray {
        lint_array!(UNUSED_PARENS)
    }
}

impl EarlyLintPass for UnusedParens {
    fn check_expr(&mut self, cx: &EarlyContext, e: &ast::Expr) {
        use syntax::ast::ExprKind::*;
        let (value, msg, struct_lit_needs_parens) = match e.node {
            If(ref cond, ..) => (cond, "`if` condition", true),
            While(ref cond, ..) => (cond, "`while` condition", true),
            IfLet(_, ref cond, ..) => (cond, "`if let` head expression", true),
            WhileLet(_, ref cond, ..) => (cond, "`while let` head expression", true),
            ForLoop(_, ref cond, ..) => (cond, "`for` head expression", true),
            Match(ref head, _) => (head, "`match` head expression", true),
            Ret(Some(ref value)) => (value, "`return` value", false),
            Assign(_, ref value) => (value, "assigned value", false),
            AssignOp(.., ref value) => (value, "assigned value", false),
            InPlace(_, ref value) => (value, "emplacement value", false),
            _ => return,
        };
        self.check_unused_parens_core(cx, &value, msg, struct_lit_needs_parens);
    }

    fn check_stmt(&mut self, cx: &EarlyContext, s: &ast::Stmt) {
        let (value, msg) = match s.node {
            ast::StmtKind::Local(ref local) => {
                match local.init {
                    Some(ref value) => (value, "assigned value"),
                    None => return,
                }
            }
            _ => return,
        };
        self.check_unused_parens_core(cx, &value, msg, false);
    }
}

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

#[derive(Copy, Clone)]
pub struct UnusedImportBraces;

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_ident;
            match items[0].0.kind {
                ast::UseTreeKind::Simple(ident) => {
                    if ident.name == keywords::SelfValue.name() {
                        return;
                    } else {
                        node_ident = ident;
                    }
                }
                ast::UseTreeKind::Glob => {
                    node_ident = ast::Ident::from_str("*");
                }
                ast::UseTreeKind::Nested(_) => {
                    return;
                }
            }

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

impl LintPass for UnusedImportBraces {
    fn get_lints(&self) -> LintArray {
        lint_array!(UNUSED_IMPORT_BRACES)
    }
}

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"
}

#[derive(Copy, Clone)]
pub struct UnusedAllocation;

impl LintPass for UnusedAllocation {
    fn get_lints(&self) -> LintArray {
        lint_array!(UNUSED_ALLOCATION)
    }
}

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

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