check_attr.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.

//! This module implements some validity checks for attributes.
//! In particular it verifies that `#[inline]` and `#[repr]` attributes are
//! attached to items that actually support them and if there are
//! conflicts between multiple such attributes attached to the same
//! item.

use syntax_pos::{BytePos, Span};
use ty::TyCtxt;

use hir;
use hir::intravisit::{self, Visitor, NestedVisitorMap};

#[derive(Copy, Clone, PartialEq)]
enum Target {
    Fn,
    Struct,
    Union,
    Enum,
    Const,
    ForeignMod,
    Expression,
    Statement,
    Closure,
    Static,
    Other,
}

impl Target {
    fn from_item(item: &hir::Item) -> Target {
        match item.node {
            hir::ItemFn(..) => Target::Fn,
            hir::ItemStruct(..) => Target::Struct,
            hir::ItemUnion(..) => Target::Union,
            hir::ItemEnum(..) => Target::Enum,
            hir::ItemConst(..) => Target::Const,
            hir::ItemForeignMod(..) => Target::ForeignMod,
            hir::ItemStatic(..) => Target::Static,
            _ => Target::Other,
        }
    }
}

struct CheckAttrVisitor<'a, 'tcx: 'a> {
    tcx: TyCtxt<'a, 'tcx, 'tcx>,
}

impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
    /// Check any attribute.
    fn check_attributes(&self, item: &hir::Item, target: Target) {
        if target == Target::Fn {
            self.tcx.codegen_fn_attrs(self.tcx.hir.local_def_id(item.id));
        } else if let Some(a) = item.attrs.iter().find(|a| a.check_name("target_feature")) {
            self.tcx.sess.struct_span_err(a.span, "attribute should be applied to a function")
                .span_label(item.span, "not a function")
                .emit();
        }

        let mut has_wasm_import_module = false;
        for attr in &item.attrs {
            if attr.check_name("inline") {
                self.check_inline(attr, &item.span, target)
            } else if attr.check_name("non_exhaustive") {
                self.check_non_exhaustive(attr, item, target)
            } else if attr.check_name("wasm_import_module") {
                has_wasm_import_module = true;
                if attr.value_str().is_none() {
                    self.tcx.sess.span_err(attr.span, "\
                        must be of the form #[wasm_import_module = \"...\"]");
                }
                if target != Target::ForeignMod {
                    self.tcx.sess.span_err(attr.span, "\
                        must only be attached to foreign modules");
                }
            } else if attr.check_name("wasm_custom_section") {
                if target != Target::Const {
                    self.tcx.sess.span_err(attr.span, "only allowed on consts");
                }

                if attr.value_str().is_none() {
                    self.tcx.sess.span_err(attr.span, "must be of the form \
                        #[wasm_custom_section = \"foo\"]");
                }
            }
        }

        if target == Target::ForeignMod &&
            !has_wasm_import_module &&
            self.tcx.sess.target.target.arch == "wasm32" &&
            false // FIXME: eventually enable this warning when stable
        {
            self.tcx.sess.span_warn(item.span, "\
                must have a #[wasm_import_module = \"...\"] attribute, this \
                will become a hard error before too long");
        }

        self.check_repr(item, target);
        self.check_used(item, target);
    }

    /// Check if an `#[inline]` is applied to a function or a closure.
    fn check_inline(&self, attr: &hir::Attribute, span: &Span, target: Target) {
        if target != Target::Fn && target != Target::Closure {
            struct_span_err!(self.tcx.sess,
                             attr.span,
                             E0518,
                             "attribute should be applied to function or closure")
                .span_label(*span, "not a function or closure")
                .emit();
        }
    }

    /// Check if the `#[non_exhaustive]` attribute on an `item` is valid.
    fn check_non_exhaustive(&self, attr: &hir::Attribute, item: &hir::Item, target: Target) {
        match target {
            Target::Struct | Target::Enum => { /* Valid */ },
            _ => {
                struct_span_err!(self.tcx.sess,
                                 attr.span,
                                 E0910,
                                 "attribute can only be applied to a struct or enum")
                    .span_label(item.span, "not a struct or enum")
                    .emit();
                return;
            }
        }

        if attr.meta_item_list().is_some() || attr.value_str().is_some() {
            struct_span_err!(self.tcx.sess,
                             attr.span,
                             E0911,
                             "attribute should be empty")
                .span_label(item.span, "not empty")
                .emit();
        }
    }

    /// Check if the `#[repr]` attributes on `item` are valid.
    fn check_repr(&self, item: &hir::Item, target: Target) {
        // Extract the names of all repr hints, e.g., [foo, bar, align] for:
        // ```
        // #[repr(foo)]
        // #[repr(bar, align(8))]
        // ```
        let hints: Vec<_> = item.attrs
            .iter()
            .filter(|attr| attr.name() == "repr")
            .filter_map(|attr| {
                let list = attr.meta_item_list();

                // Emit warnings with `repr` either has a literal assignment (`#[repr = "C"]`) or
                // no hints (``#[repr]`)
                let has_hints = list.as_ref().map(|ref list| !list.is_empty()).unwrap_or(false);
                if !has_hints {
                    let mut suggested = false;
                    let mut warn = if let Some(ref lit) = attr.value_str() {
                        // avoid warning about empty `repr` on `#[repr = "foo"]`
                        let sp = match format!("{}", lit).as_ref() {
                            "C" | "packed" | "rust" | "u*" | "i*" => {
                                let lo = attr.span.lo() + BytePos(2);
                                let hi = attr.span.hi() - BytePos(1);
                                suggested = true;
                                attr.span.with_lo(lo).with_hi(hi)
                            }
                            _ => attr.span,  // the literal wasn't a valid `repr` arg
                        };
                        let mut warn = self.tcx.sess.struct_span_warn(
                            sp,
                            "`repr` attribute isn't configurable with a literal",
                        );
                        if suggested {
                            // if the literal could have been a valid `repr` arg,
                            // suggest the correct syntax
                            warn.span_suggestion(
                                sp,
                                "give `repr` a hint",
                                format!("repr({})", lit),
                            );
                        } else {
                            warn.span_label(attr.span, "needs a hint");
                        }
                        warn
                    } else {
                        let mut warn = self.tcx.sess.struct_span_warn(
                            attr.span,
                            "`repr` attribute must have a hint",
                        );
                        warn.span_label(attr.span, "needs a hint");
                        warn
                    };
                    if !suggested {
                        warn.help("valid hints include `#[repr(C)]`, `#[repr(packed)]` and \
                                   `#[repr(rust)]`");
                        warn.note("for more information, visit \
                                   <https://doc.rust-lang.org/nomicon/other-reprs.html>");
                    }
                    warn.emit();
                }
                list
            })
            .flat_map(|hints| hints)
            .collect();

        let mut int_reprs = 0;
        let mut is_c = false;
        let mut is_simd = false;
        let mut is_transparent = false;

        for hint in &hints {
            let name = if let Some(name) = hint.name() {
                name
            } else {
                // Invalid repr hint like repr(42). We don't check for unrecognized hints here
                // (libsyntax does that), so just ignore it.
                continue;
            };

            let (article, allowed_targets) = match &*name.as_str() {
                "C" => {
                    is_c = true;
                    if target != Target::Struct &&
                            target != Target::Union &&
                            target != Target::Enum {
                                ("a", "struct, enum or union")
                    } else {
                        continue
                    }
                }
                "packed" => {
                    if target != Target::Struct &&
                            target != Target::Union {
                                ("a", "struct or union")
                    } else {
                        continue
                    }
                }
                "simd" => {
                    is_simd = true;
                    if target != Target::Struct {
                        ("a", "struct")
                    } else {
                        continue
                    }
                }
                "align" => {
                    if target != Target::Struct &&
                            target != Target::Union {
                        ("a", "struct or union")
                    } else {
                        continue
                    }
                }
                "transparent" => {
                    is_transparent = true;
                    if target != Target::Struct {
                        ("a", "struct")
                    } else {
                        continue
                    }
                }
                "i8" | "u8" | "i16" | "u16" |
                "i32" | "u32" | "i64" | "u64" |
                "isize" | "usize" => {
                    int_reprs += 1;
                    if target != Target::Enum {
                        ("an", "enum")
                    } else {
                        continue
                    }
                }
                _ => continue,
            };
            self.emit_repr_error(
                hint.span,
                item.span,
                &format!("attribute should be applied to {}", allowed_targets),
                &format!("not {} {}", article, allowed_targets),
            )
        }

        // Just point at all repr hints if there are any incompatibilities.
        // This is not ideal, but tracking precisely which ones are at fault is a huge hassle.
        let hint_spans = hints.iter().map(|hint| hint.span);

        // Error on repr(transparent, <anything else>).
        if is_transparent && hints.len() > 1 {
            let hint_spans: Vec<_> = hint_spans.clone().collect();
            span_err!(self.tcx.sess, hint_spans, E0692,
                      "transparent struct cannot have other repr hints");
        }
        // Warn on repr(u8, u16), repr(C, simd), and c-like-enum-repr(C, u8)
        if (int_reprs > 1)
           || (is_simd && is_c)
           || (int_reprs == 1 && is_c && is_c_like_enum(item)) {
            let hint_spans: Vec<_> = hint_spans.collect();
            span_warn!(self.tcx.sess, hint_spans, E0566,
                       "conflicting representation hints");
        }
    }

    fn emit_repr_error(
        &self,
        hint_span: Span,
        label_span: Span,
        hint_message: &str,
        label_message: &str,
    ) {
        struct_span_err!(self.tcx.sess, hint_span, E0517, "{}", hint_message)
            .span_label(label_span, label_message)
            .emit();
    }

    fn check_stmt_attributes(&self, stmt: &hir::Stmt) {
        // When checking statements ignore expressions, they will be checked later
        if let hir::Stmt_::StmtDecl(_, _) = stmt.node {
            for attr in stmt.node.attrs() {
                if attr.check_name("inline") {
                    self.check_inline(attr, &stmt.span, Target::Statement);
                }
                if attr.check_name("repr") {
                    self.emit_repr_error(
                        attr.span,
                        stmt.span,
                        &format!("attribute should not be applied to a statement"),
                        &format!("not a struct, enum or union"),
                    );
                }
            }
        }
    }

    fn check_expr_attributes(&self, expr: &hir::Expr) {
        let target = match expr.node {
            hir::ExprClosure(..) => Target::Closure,
            _ => Target::Expression,
        };
        for attr in expr.attrs.iter() {
            if attr.check_name("inline") {
                self.check_inline(attr, &expr.span, target);
            }
            if attr.check_name("repr") {
                self.emit_repr_error(
                    attr.span,
                    expr.span,
                    &format!("attribute should not be applied to an expression"),
                    &format!("not defining a struct, enum or union"),
                );
            }
        }
    }

    fn check_used(&self, item: &hir::Item, target: Target) {
        for attr in &item.attrs {
            if attr.name() == "used" && target != Target::Static {
                self.tcx.sess
                    .span_err(attr.span, "attribute must be applied to a `static` variable");
            }
        }
    }
}

impl<'a, 'tcx> Visitor<'tcx> for CheckAttrVisitor<'a, 'tcx> {
    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
        NestedVisitorMap::OnlyBodies(&self.tcx.hir)
    }

    fn visit_item(&mut self, item: &'tcx hir::Item) {
        let target = Target::from_item(item);
        self.check_attributes(item, target);
        intravisit::walk_item(self, item)
    }


    fn visit_stmt(&mut self, stmt: &'tcx hir::Stmt) {
        self.check_stmt_attributes(stmt);
        intravisit::walk_stmt(self, stmt)
    }

    fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
        self.check_expr_attributes(expr);
        intravisit::walk_expr(self, expr)
    }
}

pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
    let mut checker = CheckAttrVisitor { tcx };
    tcx.hir.krate().visit_all_item_likes(&mut checker.as_deep_visitor());
}

fn is_c_like_enum(item: &hir::Item) -> bool {
    if let hir::ItemEnum(ref def, _) = item.node {
        for variant in &def.variants {
            match variant.node.data {
                hir::VariantData::Unit(_) => { /* continue */ }
                _ => { return false; }
            }
        }
        true
    } else {
        false
    }
}