const_val.rs

// Copyright 2012-2016 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.

pub use rustc_const_math::ConstInt;

use hir::def_id::DefId;
use ty::{self, TyCtxt, layout};
use ty::subst::Substs;
use rustc_const_math::*;

use graphviz::IntoCow;
use errors::DiagnosticBuilder;
use serialize::{self, Encodable, Encoder, Decodable, Decoder};
use syntax::symbol::InternedString;
use syntax::ast;
use syntax_pos::Span;

use std::borrow::Cow;

pub type EvalResult<'tcx> = Result<&'tcx ty::Const<'tcx>, ConstEvalErr<'tcx>>;

#[derive(Copy, Clone, Debug, Hash, RustcEncodable, RustcDecodable, Eq, PartialEq)]
pub enum ConstVal<'tcx> {
    Integral(ConstInt),
    Float(ConstFloat),
    Str(InternedString),
    ByteStr(ByteArray<'tcx>),
    Bool(bool),
    Char(char),
    Variant(DefId),
    Function(DefId, &'tcx Substs<'tcx>),
    Aggregate(ConstAggregate<'tcx>),
    Unevaluated(DefId, &'tcx Substs<'tcx>),
}

#[derive(Copy, Clone, Debug, Hash, RustcEncodable, Eq, PartialEq)]
pub struct ByteArray<'tcx> {
    pub data: &'tcx [u8],
}

impl<'tcx> serialize::UseSpecializedDecodable for ByteArray<'tcx> {}

#[derive(Copy, Clone, Debug, Hash, Eq, PartialEq)]
pub enum ConstAggregate<'tcx> {
    Struct(&'tcx [(ast::Name, &'tcx ty::Const<'tcx>)]),
    Tuple(&'tcx [&'tcx ty::Const<'tcx>]),
    Array(&'tcx [&'tcx ty::Const<'tcx>]),
    Repeat(&'tcx ty::Const<'tcx>, u64),
}

impl<'tcx> Encodable for ConstAggregate<'tcx> {
    fn encode<S: Encoder>(&self, _: &mut S) -> Result<(), S::Error> {
        bug!("should never encode ConstAggregate::{:?}", self)
    }
}

impl<'tcx> Decodable for ConstAggregate<'tcx> {
    fn decode<D: Decoder>(_: &mut D) -> Result<Self, D::Error> {
        bug!("should never decode ConstAggregate")
    }
}

impl<'tcx> ConstVal<'tcx> {
    pub fn to_const_int(&self) -> Option<ConstInt> {
        match *self {
            ConstVal::Integral(i) => Some(i),
            ConstVal::Bool(b) => Some(ConstInt::U8(b as u8)),
            ConstVal::Char(ch) => Some(ConstInt::U32(ch as u32)),
            _ => None
        }
    }
}

#[derive(Clone, Debug)]
pub struct ConstEvalErr<'tcx> {
    pub span: Span,
    pub kind: ErrKind<'tcx>,
}

#[derive(Clone, Debug)]
pub enum ErrKind<'tcx> {
    CannotCast,
    MissingStructField,

    NonConstPath,
    UnimplementedConstVal(&'static str),
    ExpectedConstTuple,
    ExpectedConstStruct,
    IndexedNonVec,
    IndexNotUsize,
    IndexOutOfBounds { len: u64, index: u64 },

    MiscBinaryOp,
    MiscCatchAll,

    IndexOpFeatureGated,
    Math(ConstMathErr),
    LayoutError(layout::LayoutError<'tcx>),

    ErroneousReferencedConstant(Box<ConstEvalErr<'tcx>>),

    TypeckError
}

impl<'tcx> From<ConstMathErr> for ErrKind<'tcx> {
    fn from(err: ConstMathErr) -> ErrKind<'tcx> {
        match err {
            ConstMathErr::UnsignedNegation => ErrKind::TypeckError,
            _ => ErrKind::Math(err)
        }
    }
}

#[derive(Clone, Debug)]
pub enum ConstEvalErrDescription<'a> {
    Simple(Cow<'a, str>),
}

impl<'a> ConstEvalErrDescription<'a> {
    /// Return a one-line description of the error, for lints and such
    pub fn into_oneline(self) -> Cow<'a, str> {
        match self {
            ConstEvalErrDescription::Simple(simple) => simple,
        }
    }
}

impl<'a, 'gcx, 'tcx> ConstEvalErr<'tcx> {
    pub fn description(&self) -> ConstEvalErrDescription {
        use self::ErrKind::*;
        use self::ConstEvalErrDescription::*;

        macro_rules! simple {
            ($msg:expr) => ({ Simple($msg.into_cow()) });
            ($fmt:expr, $($arg:tt)+) => ({
                Simple(format!($fmt, $($arg)+).into_cow())
            })
        }

        match self.kind {
            CannotCast => simple!("can't cast this type"),
            MissingStructField  => simple!("nonexistent struct field"),
            NonConstPath        => simple!("non-constant path in constant expression"),
            UnimplementedConstVal(what) =>
                simple!("unimplemented constant expression: {}", what),
            ExpectedConstTuple => simple!("expected constant tuple"),
            ExpectedConstStruct => simple!("expected constant struct"),
            IndexedNonVec => simple!("indexing is only supported for arrays"),
            IndexNotUsize => simple!("indices must be of type `usize`"),
            IndexOutOfBounds { len, index } => {
                simple!("index out of bounds: the len is {} but the index is {}",
                        len, index)
            }

            MiscBinaryOp => simple!("bad operands for binary"),
            MiscCatchAll => simple!("unsupported constant expr"),
            IndexOpFeatureGated => simple!("the index operation on const values is unstable"),
            Math(ref err) => Simple(err.description().into_cow()),
            LayoutError(ref err) => Simple(err.to_string().into_cow()),

            ErroneousReferencedConstant(_) => simple!("could not evaluate referenced constant"),

            TypeckError => simple!("type-checking failed"),
        }
    }

    pub fn struct_error(&self,
        tcx: TyCtxt<'a, 'gcx, 'tcx>,
        primary_span: Span,
        primary_kind: &str)
        -> DiagnosticBuilder<'gcx>
    {
        let mut err = self;
        while let &ConstEvalErr {
            kind: ErrKind::ErroneousReferencedConstant(box ref i_err), ..
        } = err {
            err = i_err;
        }

        let mut diag = struct_span_err!(tcx.sess, err.span, E0080, "constant evaluation error");
        err.note(tcx, primary_span, primary_kind, &mut diag);
        diag
    }

    pub fn note(&self,
        _tcx: TyCtxt<'a, 'gcx, 'tcx>,
        primary_span: Span,
        primary_kind: &str,
        diag: &mut DiagnosticBuilder)
    {
        match self.description() {
            ConstEvalErrDescription::Simple(message) => {
                diag.span_label(self.span, message);
            }
        }

        if !primary_span.contains(self.span) {
            diag.span_note(primary_span,
                        &format!("for {} here", primary_kind));
        }
    }

    pub fn report(&self,
        tcx: TyCtxt<'a, 'gcx, 'tcx>,
        primary_span: Span,
        primary_kind: &str)
    {
        if let ErrKind::TypeckError = self.kind {
            return;
        }
        self.struct_error(tcx, primary_span, primary_kind).emit();
    }
}