rust/src/librustc/ty/error.rs

use crate::hir::def_id::DefId;
use crate::ty::{self, BoundRegion, Region, Ty, TyCtxt};
use std::borrow::Cow;
use std::fmt;
use rustc_target::spec::abi;
use syntax::ast;
use syntax::errors::pluralise;
use errors::{Applicability, DiagnosticBuilder};
use syntax_pos::Span;

use crate::hir;

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct ExpectedFound<T> {
    pub expected: T,
    pub found: T,
}

// Data structures used in type unification
#[derive(Clone, Debug)]
pub enum TypeError<'tcx> {
    Mismatch,
    UnsafetyMismatch(ExpectedFound<hir::Unsafety>),
    AbiMismatch(ExpectedFound<abi::Abi>),
    Mutability,
    TupleSize(ExpectedFound<usize>),
    FixedArraySize(ExpectedFound<u64>),
    ArgCount,

    RegionsDoesNotOutlive(Region<'tcx>, Region<'tcx>),
    RegionsInsufficientlyPolymorphic(BoundRegion, Region<'tcx>),
    RegionsOverlyPolymorphic(BoundRegion, Region<'tcx>),
    RegionsPlaceholderMismatch,

    Sorts(ExpectedFound<Ty<'tcx>>),
    IntMismatch(ExpectedFound<ty::IntVarValue>),
    FloatMismatch(ExpectedFound<ast::FloatTy>),
    Traits(ExpectedFound<DefId>),
    VariadicMismatch(ExpectedFound<bool>),

    /// Instantiating a type variable with the given type would have
    /// created a cycle (because it appears somewhere within that
    /// type).
    CyclicTy(Ty<'tcx>),
    ProjectionMismatched(ExpectedFound<DefId>),
    ProjectionBoundsLength(ExpectedFound<usize>),
    ExistentialMismatch(ExpectedFound<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>),

    ConstMismatch(ExpectedFound<&'tcx ty::Const<'tcx>>),

    IntrinsicCast,
}

#[derive(Clone, RustcEncodable, RustcDecodable, PartialEq, Eq, Hash, Debug, Copy)]
pub enum UnconstrainedNumeric {
    UnconstrainedFloat,
    UnconstrainedInt,
    Neither,
}

/// Explains the source of a type err in a short, human readable way. This is meant to be placed
/// in parentheses after some larger message. You should also invoke `note_and_explain_type_err()`
/// afterwards to present additional details, particularly when it comes to lifetime-related
/// errors.
impl<'tcx> fmt::Display for TypeError<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        use self::TypeError::*;
        fn report_maybe_different(f: &mut fmt::Formatter<'_>,
                                  expected: &str, found: &str) -> fmt::Result {
            // A naive approach to making sure that we're not reporting silly errors such as:
            // (expected closure, found closure).
            if expected == found {
                write!(f, "expected {}, found a different {}", expected, found)
            } else {
                write!(f, "expected {}, found {}", expected, found)
            }
        }

        let br_string = |br: ty::BoundRegion| {
            match br {
                ty::BrNamed(_, name) => format!(" {}", name),
                _ => String::new(),
            }
        };

        match *self {
            CyclicTy(_) => write!(f, "cyclic type of infinite size"),
            Mismatch => write!(f, "types differ"),
            UnsafetyMismatch(values) => {
                write!(f, "expected {} fn, found {} fn",
                       values.expected,
                       values.found)
            }
            AbiMismatch(values) => {
                write!(f, "expected {} fn, found {} fn",
                       values.expected,
                       values.found)
            }
            Mutability => write!(f, "types differ in mutability"),
            TupleSize(values) => {
                write!(f, "expected a tuple with {} element{}, \
                           found one with {} element{}",
                       values.expected,
                       pluralise!(values.expected),
                       values.found,
                       pluralise!(values.found))
            }
            FixedArraySize(values) => {
                write!(f, "expected an array with a fixed size of {} element{}, \
                           found one with {} element{}",
                       values.expected,
                       pluralise!(values.expected),
                       values.found,
                       pluralise!(values.found))
            }
            ArgCount => {
                write!(f, "incorrect number of function parameters")
            }
            RegionsDoesNotOutlive(..) => {
                write!(f, "lifetime mismatch")
            }
            RegionsInsufficientlyPolymorphic(br, _) => {
                write!(f,
                       "expected bound lifetime parameter{}, found concrete lifetime",
                       br_string(br))
            }
            RegionsOverlyPolymorphic(br, _) => {
                write!(f,
                       "expected concrete lifetime, found bound lifetime parameter{}",
                       br_string(br))
            }
            RegionsPlaceholderMismatch => {
                write!(f, "one type is more general than the other")
            }
            Sorts(values) => ty::tls::with(|tcx| {
                report_maybe_different(f, &values.expected.sort_string(tcx),
                                       &values.found.sort_string(tcx))
            }),
            Traits(values) => ty::tls::with(|tcx| {
                report_maybe_different(f,
                                       &format!("trait `{}`",
                                                tcx.def_path_str(values.expected)),
                                       &format!("trait `{}`",
                                                tcx.def_path_str(values.found)))
            }),
            IntMismatch(ref values) => {
                write!(f, "expected `{:?}`, found `{:?}`",
                       values.expected,
                       values.found)
            }
            FloatMismatch(ref values) => {
                write!(f, "expected `{:?}`, found `{:?}`",
                       values.expected,
                       values.found)
            }
            VariadicMismatch(ref values) => {
                write!(f, "expected {} fn, found {} function",
                       if values.expected { "variadic" } else { "non-variadic" },
                       if values.found { "variadic" } else { "non-variadic" })
            }
            ProjectionMismatched(ref values) => ty::tls::with(|tcx| {
                write!(f, "expected {}, found {}",
                       tcx.def_path_str(values.expected),
                       tcx.def_path_str(values.found))
            }),
            ProjectionBoundsLength(ref values) => {
                write!(f, "expected {} associated type binding{}, found {}",
                       values.expected,
                       pluralise!(values.expected),
                       values.found)
            },
            ExistentialMismatch(ref values) => {
                report_maybe_different(f, &format!("trait `{}`", values.expected),
                                       &format!("trait `{}`", values.found))
            }
            ConstMismatch(ref values) => {
                write!(f, "expected `{}`, found `{}`", values.expected, values.found)
            }
            IntrinsicCast => {
                write!(f, "cannot coerce intrinsics to function pointers")
            }
        }
    }
}

impl<'tcx> ty::TyS<'tcx> {
    pub fn sort_string(&self, tcx: TyCtxt<'_>) -> Cow<'static, str> {
        match self.kind {
            ty::Bool | ty::Char | ty::Int(_) |
            ty::Uint(_) | ty::Float(_) | ty::Str | ty::Never => self.to_string().into(),
            ty::Tuple(ref tys) if tys.is_empty() => self.to_string().into(),

            ty::Adt(def, _) => format!("{} `{}`", def.descr(), tcx.def_path_str(def.did)).into(),
            ty::Foreign(def_id) => format!("extern type `{}`", tcx.def_path_str(def_id)).into(),
            ty::Array(_, n) => {
                let n = tcx.lift(&n).unwrap();
                match n.try_eval_usize(tcx, ty::ParamEnv::empty()) {
                    Some(n) => {
                        format!("array of {} element{}", n, pluralise!(n)).into()
                    }
                    None => "array".into(),
                }
            }
            ty::Slice(_) => "slice".into(),
            ty::RawPtr(_) => "*-ptr".into(),
            ty::Ref(region, ty, mutbl) => {
                let tymut = ty::TypeAndMut { ty, mutbl };
                let tymut_string = tymut.to_string();
                if tymut_string == "_" ||         //unknown type name,
                   tymut_string.len() > 10 ||     //name longer than saying "reference",
                   region.to_string() != "'_"     //... or a complex type
                {
                    format!("{}reference", match mutbl {
                        hir::Mutability::MutMutable => "mutable ",
                        _ => ""
                    }).into()
                } else {
                    format!("&{}", tymut_string).into()
                }
            }
            ty::FnDef(..) => "fn item".into(),
            ty::FnPtr(_) => "fn pointer".into(),
            ty::Dynamic(ref inner, ..) => {
                if let Some(principal) = inner.principal() {
                    format!("trait {}", tcx.def_path_str(principal.def_id())).into()
                } else {
                    "trait".into()
                }
            }
            ty::Closure(..) => "closure".into(),
            ty::Generator(..) => "generator".into(),
            ty::GeneratorWitness(..) => "generator witness".into(),
            ty::Tuple(..) => "tuple".into(),
            ty::Infer(ty::TyVar(_)) => "inferred type".into(),
            ty::Infer(ty::IntVar(_)) => "integer".into(),
            ty::Infer(ty::FloatVar(_)) => "floating-point number".into(),
            ty::Placeholder(..) => "placeholder type".into(),
            ty::Bound(..) => "bound type".into(),
            ty::Infer(ty::FreshTy(_)) => "fresh type".into(),
            ty::Infer(ty::FreshIntTy(_)) => "fresh integral type".into(),
            ty::Infer(ty::FreshFloatTy(_)) => "fresh floating-point type".into(),
            ty::Projection(_) => "associated type".into(),
            ty::UnnormalizedProjection(_) => "non-normalized associated type".into(),
            ty::Param(_) => "type parameter".into(),
            ty::Opaque(..) => "opaque type".into(),
            ty::Error => "type error".into(),
        }
    }
}

impl<'tcx> TyCtxt<'tcx> {
    pub fn note_and_explain_type_err(
        self,
        db: &mut DiagnosticBuilder<'_>,
        err: &TypeError<'tcx>,
        sp: Span,
    ) {
        use self::TypeError::*;

        match err {
            Sorts(values) => {
                let expected_str = values.expected.sort_string(self);
                let found_str = values.found.sort_string(self);
                if expected_str == found_str && expected_str == "closure" {
                    db.note("no two closures, even if identical, have the same type");
                    db.help("consider boxing your closure and/or using it as a trait object");
                }
                if expected_str == found_str && expected_str == "opaque type" { // Issue #63167
                    db.note("distinct uses of `impl Trait` result in different opaque types");
                    let e_str = values.expected.to_string();
                    let f_str = values.found.to_string();
                    if &e_str == &f_str && &e_str == "impl std::future::Future" {
                        // FIXME: use non-string based check.
                        db.help("if both `Future`s have the same `Output` type, consider \
                                 `.await`ing on both of them");
                    }
                }
                match (&values.expected.kind, &values.found.kind) {
                    (ty::Float(_), ty::Infer(ty::IntVar(_))) => if let Ok( // Issue #53280
                        snippet,
                    ) = self.sess.source_map().span_to_snippet(sp) {
                        if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
                            db.span_suggestion(
                                sp,
                                "use a float literal",
                                format!("{}.0", snippet),
                                Applicability::MachineApplicable
                            );
                        }
                    },
                    (ty::Param(_), ty::Param(_)) => {
                        db.note("a type parameter was expected, but a different one was found; \
                                 you might be missing a type parameter or trait bound");
                        db.note("for more information, visit \
                                 https://doc.rust-lang.org/book/ch10-02-traits.html\
                                 #traits-as-parameters");
                    }
                    (ty::Projection(_), ty::Projection(_)) => {
                        db.note("an associated type was expected, but a different one was found");
                    }
                    (ty::Param(_), ty::Projection(_)) | (ty::Projection(_), ty::Param(_)) => {
                        db.note("you might be missing a type parameter or trait bound");
                    }
                    (ty::Param(_), _) | (_, ty::Param(_)) => {
                        db.help("type parameters must be constrained to match other types");
                        if self.sess.teach(&db.get_code().unwrap()) {
                            db.help("given a type parameter `T` and a method `foo`:
```
trait Trait<T> { fn foo(&self) -> T; }
```
the only ways to implement method `foo` are:
- constrain `T` with an explicit type:
```
impl Trait<String> for X {
    fn foo(&self) -> String { String::new() }
}
```
- add a trait bound to `T` and call a method on that trait that returns `Self`:
```
impl<T: std::default::Default> Trait<T> for X {
    fn foo(&self) -> T { <T as std::default::Default>::default() }
}
```
- change `foo` to return an argument of type `T`:
```
impl<T> Trait<T> for X {
    fn foo(&self, x: T) -> T { x }
}
```");
                        }
                        db.note("for more information, visit \
                                 https://doc.rust-lang.org/book/ch10-02-traits.html\
                                 #traits-as-parameters");
                    }
                    (ty::Projection(_), _) => {
                        db.note(&format!(
                            "consider constraining the associated type `{}` to `{}` or calling a \
                             method that returns `{}`",
                            values.expected,
                            values.found,
                            values.expected,
                        ));
                        if self.sess.teach(&db.get_code().unwrap()) {
                            db.help("given an associated type `T` and a method `foo`:
```
trait Trait {
    type T;
    fn foo(&self) -> Self::T;
}
```
the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
```
impl Trait for X {
    type T = String;
    fn foo(&self) -> Self::T { String::new() }
}
```");
                        }
                        db.note("for more information, visit \
                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html");
                    }
                    (_, ty::Projection(_)) => {
                        db.note(&format!(
                            "consider constraining the associated type `{}` to `{}`",
                            values.found,
                            values.expected,
                        ));
                        db.note("for more information, visit \
                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html");
                    }
                    _ => {}
                }
                debug!(
                    "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
                    values.expected,
                    values.expected.kind,
                    values.found,
                    values.found.kind,
                );
            },
            CyclicTy(ty) => {
                // Watch out for various cases of cyclic types and try to explain.
                if ty.is_closure() || ty.is_generator() {
                    db.note("closures cannot capture themselves or take themselves as argument;\n\
                             this error may be the result of a recent compiler bug-fix,\n\
                             see https://github.com/rust-lang/rust/issues/46062 for more details");
                }
            }
            _ => {}
        }
    }
}