fn_queries.rs

use rustc_attr as attr;
use rustc_hir as hir;
use rustc_hir::def_id::{DefId, LocalDefId};
use rustc_middle::hir::map::blocks::FnLikeNode;
use rustc_middle::ty::query::Providers;
use rustc_middle::ty::TyCtxt;
use rustc_span::symbol::Symbol;
use rustc_target::spec::abi::Abi;

/// Whether the `def_id` counts as const fn in your current crate, considering all active
/// feature gates
pub fn is_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
    tcx.is_const_fn_raw(def_id)
        && match is_unstable_const_fn(tcx, def_id) {
            Some(feature_name) => {
                // has a `rustc_const_unstable` attribute, check whether the user enabled the
                // corresponding feature gate.
                tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == feature_name)
            }
            // functions without const stability are either stable user written
            // const fn or the user is using feature gates and we thus don't
            // care what they do
            None => true,
        }
}

/// Whether the `def_id` is an unstable const fn and what feature gate is necessary to enable it
pub fn is_unstable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> Option<Symbol> {
    if tcx.is_const_fn_raw(def_id) {
        let const_stab = tcx.lookup_const_stability(def_id)?;
        if const_stab.level.is_unstable() { Some(const_stab.feature) } else { None }
    } else {
        None
    }
}

/// Returns `true` if this function must conform to `min_const_fn`
pub fn is_min_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
    // Bail out if the signature doesn't contain `const`
    if !tcx.is_const_fn_raw(def_id) {
        return false;
    }

    if tcx.features().staged_api {
        // In order for a libstd function to be considered min_const_fn
        // it needs to be stable and have no `rustc_const_unstable` attribute.
        match tcx.lookup_const_stability(def_id) {
            // `rustc_const_unstable` functions don't need to conform.
            Some(&attr::ConstStability { ref level, .. }) if level.is_unstable() => false,
            None => {
                if let Some(stab) = tcx.lookup_stability(def_id) {
                    if stab.level.is_stable() {
                        tcx.sess.span_err(
                            tcx.def_span(def_id),
                            "stable const functions must have either `rustc_const_stable` or \
                             `rustc_const_unstable` attribute",
                        );
                        // While we errored above, because we don't know if we need to conform, we
                        // err on the "safe" side and require min_const_fn.
                        true
                    } else {
                        // Unstable functions need not conform to min_const_fn.
                        false
                    }
                } else {
                    // Internal functions are forced to conform to min_const_fn.
                    // Annotate the internal function with a const stability attribute if
                    // you need to use unstable features.
                    // Note: this is an arbitrary choice that does not affect stability or const
                    // safety or anything, it just changes whether we need to annotate some
                    // internal functions with `rustc_const_stable` or with `rustc_const_unstable`
                    true
                }
            }
            // Everything else needs to conform, because it would be callable from
            // other `min_const_fn` functions.
            _ => true,
        }
    } else {
        // users enabling the `const_fn` feature gate can do what they want
        !tcx.features().const_fn
    }
}

pub fn is_parent_const_impl_raw(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
    let parent_id = tcx.hir().get_parent_did(hir_id);
    if !parent_id.is_top_level_module() {
        is_const_impl_raw(tcx, parent_id.expect_local())
    } else {
        false
    }
}

/// Checks whether the function has a `const` modifier or, in case it is an intrinsic, whether
/// said intrinsic is on the whitelist for being const callable.
fn is_const_fn_raw(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
    let hir_id =
        tcx.hir().as_local_hir_id(def_id).expect("Non-local call to local provider is_const_fn");

    let node = tcx.hir().get(hir_id);

    if let Some(whitelisted) = is_const_intrinsic(tcx, def_id) {
        whitelisted
    } else if let Some(fn_like) = FnLikeNode::from_node(node) {
        if fn_like.constness() == hir::Constness::Const {
            return true;
        }

        // If the function itself is not annotated with `const`, it may still be a `const fn`
        // if it resides in a const trait impl.
        is_parent_const_impl_raw(tcx, hir_id)
    } else if let hir::Node::Ctor(_) = node {
        true
    } else {
        false
    }
}

/// Const evaluability whitelist is here to check evaluability at the
/// top level beforehand.
fn is_const_intrinsic(tcx: TyCtxt<'_>, def_id: DefId) -> Option<bool> {
    if tcx.is_closure(def_id) {
        return None;
    }

    match tcx.fn_sig(def_id).abi() {
        Abi::RustIntrinsic | Abi::PlatformIntrinsic => {
            Some(tcx.lookup_const_stability(def_id).is_some())
        }
        _ => None,
    }
}

/// Checks whether the given item is an `impl` that has a `const` modifier.
fn is_const_impl_raw(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
    let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
    let node = tcx.hir().get(hir_id);
    matches!(
        node,
        hir::Node::Item(hir::Item {
            kind: hir::ItemKind::Impl { constness: hir::Constness::Const, .. },
            ..
        })
    )
}

fn is_promotable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
    is_const_fn(tcx, def_id)
        && match tcx.lookup_const_stability(def_id) {
            Some(stab) => {
                if cfg!(debug_assertions) && stab.promotable {
                    let sig = tcx.fn_sig(def_id);
                    assert_eq!(
                        sig.unsafety(),
                        hir::Unsafety::Normal,
                        "don't mark const unsafe fns as promotable",
                        // https://github.com/rust-lang/rust/pull/53851#issuecomment-418760682
                    );
                }
                stab.promotable
            }
            None => false,
        }
}

fn const_fn_is_allowed_fn_ptr(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
    is_const_fn(tcx, def_id)
        && tcx.lookup_const_stability(def_id).map(|stab| stab.allow_const_fn_ptr).unwrap_or(false)
}

pub fn provide(providers: &mut Providers<'_>) {
    *providers = Providers {
        is_const_fn_raw,
        is_const_impl_raw: |tcx, def_id| is_const_impl_raw(tcx, def_id.expect_local()),
        is_promotable_const_fn,
        const_fn_is_allowed_fn_ptr,
        ..*providers
    };
}