context.rs

//! Implementation of lint checking.
//!
//! The lint checking is mostly consolidated into one pass which runs
//! after all other analyses. Throughout compilation, lint warnings
//! can be added via the `add_lint` method on the Session structure. This
//! requires a span and an ID of the node that the lint is being added to. The
//! lint isn't actually emitted at that time because it is unknown what the
//! actual lint level at that location is.
//!
//! To actually emit lint warnings/errors, a separate pass is used.
//! A context keeps track of the current state of all lint levels.
//! Upon entering a node of the ast which can modify the lint settings, the
//! previous lint state is pushed onto a stack and the ast is then recursed
//! upon. As the ast is traversed, this keeps track of the current lint level
//! for all lint attributes.

use self::TargetLint::*;

use crate::levels::LintLevelsBuilder;
use crate::passes::{EarlyLintPassObject, LateLintPassObject};
use rustc_ast as ast;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::sync;
use rustc_errors::{
    add_elided_lifetime_in_path_suggestion, struct_span_err, Applicability, SuggestionStyle,
};
use rustc_hir as hir;
use rustc_hir::def::Res;
use rustc_hir::def_id::{CrateNum, DefId};
use rustc_hir::definitions::{DefPathData, DisambiguatedDefPathData};
use rustc_middle::lint::LintDiagnosticBuilder;
use rustc_middle::middle::privacy::AccessLevels;
use rustc_middle::middle::stability;
use rustc_middle::ty::layout::{LayoutError, TyAndLayout};
use rustc_middle::ty::print::with_no_trimmed_paths;
use rustc_middle::ty::{self, print::Printer, subst::GenericArg, Ty, TyCtxt};
use rustc_serialize::json::Json;
use rustc_session::lint::{BuiltinLintDiagnostics, ExternDepSpec};
use rustc_session::lint::{FutureIncompatibleInfo, Level, Lint, LintBuffer, LintId};
use rustc_session::Session;
use rustc_session::SessionLintStore;
use rustc_span::lev_distance::find_best_match_for_name;
use rustc_span::{symbol::Symbol, MultiSpan, Span, DUMMY_SP};
use rustc_target::abi::LayoutOf;
use tracing::debug;

use std::cell::Cell;
use std::slice;

/// Information about the registered lints.
///
/// This is basically the subset of `Context` that we can
/// build early in the compile pipeline.
pub struct LintStore {
    /// Registered lints.
    lints: Vec<&'static Lint>,

    /// Constructor functions for each variety of lint pass.
    ///
    /// These should only be called once, but since we want to avoid locks or
    /// interior mutability, we don't enforce this (and lints should, in theory,
    /// be compatible with being constructed more than once, though not
    /// necessarily in a sane manner. This is safe though.)
    pub pre_expansion_passes: Vec<Box<dyn Fn() -> EarlyLintPassObject + sync::Send + sync::Sync>>,
    pub early_passes: Vec<Box<dyn Fn() -> EarlyLintPassObject + sync::Send + sync::Sync>>,
    pub late_passes: Vec<Box<dyn Fn() -> LateLintPassObject + sync::Send + sync::Sync>>,
    /// This is unique in that we construct them per-module, so not once.
    pub late_module_passes: Vec<Box<dyn Fn() -> LateLintPassObject + sync::Send + sync::Sync>>,

    /// Lints indexed by name.
    by_name: FxHashMap<String, TargetLint>,

    /// Map of registered lint groups to what lints they expand to.
    lint_groups: FxHashMap<&'static str, LintGroup>,
}

impl SessionLintStore for LintStore {
    fn name_to_lint(&self, lint_name: &str) -> LintId {
        let lints = self
            .find_lints(lint_name)
            .unwrap_or_else(|_| panic!("Failed to find lint with name `{}`", lint_name));

        if let &[lint] = lints.as_slice() {
            return lint;
        } else {
            panic!("Found mutliple lints with name `{}`: {:?}", lint_name, lints);
        }
    }
}

/// The target of the `by_name` map, which accounts for renaming/deprecation.
#[derive(Debug)]
enum TargetLint {
    /// A direct lint target
    Id(LintId),

    /// Temporary renaming, used for easing migration pain; see #16545
    Renamed(String, LintId),

    /// Lint with this name existed previously, but has been removed/deprecated.
    /// The string argument is the reason for removal.
    Removed(String),

    /// A lint name that should give no warnings and have no effect.
    ///
    /// This is used by rustc to avoid warning about old rustdoc lints before rustdoc registers them as tool lints.
    Ignored,
}

pub enum FindLintError {
    NotFound,
    Removed,
}

struct LintAlias {
    name: &'static str,
    /// Whether deprecation warnings should be suppressed for this alias.
    silent: bool,
}

struct LintGroup {
    lint_ids: Vec<LintId>,
    from_plugin: bool,
    depr: Option<LintAlias>,
}

pub enum CheckLintNameResult<'a> {
    Ok(&'a [LintId]),
    /// Lint doesn't exist. Potentially contains a suggestion for a correct lint name.
    NoLint(Option<Symbol>),
    /// The lint is either renamed or removed. This is the warning
    /// message, and an optional new name (`None` if removed).
    Warning(String, Option<String>),
    /// The lint is from a tool. If the Option is None, then either
    /// the lint does not exist in the tool or the code was not
    /// compiled with the tool and therefore the lint was never
    /// added to the `LintStore`. Otherwise the `LintId` will be
    /// returned as if it where a rustc lint.
    Tool(Result<&'a [LintId], (Option<&'a [LintId]>, String)>),
}

impl LintStore {
    pub fn new() -> LintStore {
        LintStore {
            lints: vec![],
            pre_expansion_passes: vec![],
            early_passes: vec![],
            late_passes: vec![],
            late_module_passes: vec![],
            by_name: Default::default(),
            lint_groups: Default::default(),
        }
    }

    pub fn get_lints<'t>(&'t self) -> &'t [&'static Lint] {
        &self.lints
    }

    pub fn get_lint_groups<'t>(&'t self) -> Vec<(&'static str, Vec<LintId>, bool)> {
        self.lint_groups
            .iter()
            .filter(|(_, LintGroup { depr, .. })| {
                // Don't display deprecated lint groups.
                depr.is_none()
            })
            .map(|(k, LintGroup { lint_ids, from_plugin, .. })| {
                (*k, lint_ids.clone(), *from_plugin)
            })
            .collect()
    }

    pub fn register_early_pass(
        &mut self,
        pass: impl Fn() -> EarlyLintPassObject + 'static + sync::Send + sync::Sync,
    ) {
        self.early_passes.push(Box::new(pass));
    }

    pub fn register_pre_expansion_pass(
        &mut self,
        pass: impl Fn() -> EarlyLintPassObject + 'static + sync::Send + sync::Sync,
    ) {
        self.pre_expansion_passes.push(Box::new(pass));
    }

    pub fn register_late_pass(
        &mut self,
        pass: impl Fn() -> LateLintPassObject + 'static + sync::Send + sync::Sync,
    ) {
        self.late_passes.push(Box::new(pass));
    }

    pub fn register_late_mod_pass(
        &mut self,
        pass: impl Fn() -> LateLintPassObject + 'static + sync::Send + sync::Sync,
    ) {
        self.late_module_passes.push(Box::new(pass));
    }

    // Helper method for register_early/late_pass
    pub fn register_lints(&mut self, lints: &[&'static Lint]) {
        for lint in lints {
            self.lints.push(lint);

            let id = LintId::of(lint);
            if self.by_name.insert(lint.name_lower(), Id(id)).is_some() {
                bug!("duplicate specification of lint {}", lint.name_lower())
            }

            if let Some(FutureIncompatibleInfo { edition, .. }) = lint.future_incompatible {
                if let Some(edition) = edition {
                    self.lint_groups
                        .entry(edition.lint_name())
                        .or_insert(LintGroup {
                            lint_ids: vec![],
                            from_plugin: lint.is_plugin,
                            depr: None,
                        })
                        .lint_ids
                        .push(id);
                }

                self.lint_groups
                    .entry("future_incompatible")
                    .or_insert(LintGroup {
                        lint_ids: vec![],
                        from_plugin: lint.is_plugin,
                        depr: None,
                    })
                    .lint_ids
                    .push(id);
            }
        }
    }

    pub fn register_group_alias(&mut self, lint_name: &'static str, alias: &'static str) {
        self.lint_groups.insert(
            alias,
            LintGroup {
                lint_ids: vec![],
                from_plugin: false,
                depr: Some(LintAlias { name: lint_name, silent: true }),
            },
        );
    }

    pub fn register_group(
        &mut self,
        from_plugin: bool,
        name: &'static str,
        deprecated_name: Option<&'static str>,
        to: Vec<LintId>,
    ) {
        let new = self
            .lint_groups
            .insert(name, LintGroup { lint_ids: to, from_plugin, depr: None })
            .is_none();
        if let Some(deprecated) = deprecated_name {
            self.lint_groups.insert(
                deprecated,
                LintGroup {
                    lint_ids: vec![],
                    from_plugin,
                    depr: Some(LintAlias { name, silent: false }),
                },
            );
        }

        if !new {
            bug!("duplicate specification of lint group {}", name);
        }
    }

    /// This lint should be available with either the old or the new name.
    ///
    /// Using the old name will not give a warning.
    /// You must register a lint with the new name before calling this function.
    #[track_caller]
    pub fn register_alias(&mut self, old_name: &str, new_name: &str) {
        let target = match self.by_name.get(new_name) {
            Some(&Id(lint_id)) => lint_id,
            _ => bug!("cannot add alias {} for lint {} that does not exist", old_name, new_name),
        };
        match self.by_name.insert(old_name.to_string(), Id(target)) {
            None | Some(Ignored) => {}
            Some(x) => bug!("duplicate specification of lint {} (was {:?})", old_name, x),
        }
    }

    /// This lint should give no warning and have no effect.
    ///
    /// This is used by rustc to avoid warning about old rustdoc lints before rustdoc registers them as tool lints.
    #[track_caller]
    pub fn register_ignored(&mut self, name: &str) {
        if self.by_name.insert(name.to_string(), Ignored).is_some() {
            bug!("duplicate specification of lint {}", name);
        }
    }

    /// This lint has been renamed; warn about using the new name and apply the lint.
    #[track_caller]
    pub fn register_renamed(&mut self, old_name: &str, new_name: &str) {
        let target = match self.by_name.get(new_name) {
            Some(&Id(lint_id)) => lint_id,
            _ => bug!("invalid lint renaming of {} to {}", old_name, new_name),
        };
        self.by_name.insert(old_name.to_string(), Renamed(new_name.to_string(), target));
    }

    pub fn register_removed(&mut self, name: &str, reason: &str) {
        self.by_name.insert(name.into(), Removed(reason.into()));
    }

    pub fn find_lints(&self, mut lint_name: &str) -> Result<Vec<LintId>, FindLintError> {
        match self.by_name.get(lint_name) {
            Some(&Id(lint_id)) => Ok(vec![lint_id]),
            Some(&Renamed(_, lint_id)) => Ok(vec![lint_id]),
            Some(&Removed(_)) => Err(FindLintError::Removed),
            Some(&Ignored) => Ok(vec![]),
            None => loop {
                return match self.lint_groups.get(lint_name) {
                    Some(LintGroup { lint_ids, depr, .. }) => {
                        if let Some(LintAlias { name, .. }) = depr {
                            lint_name = name;
                            continue;
                        }
                        Ok(lint_ids.clone())
                    }
                    None => Err(FindLintError::Removed),
                };
            },
        }
    }

    /// Checks the validity of lint names derived from the command line
    pub fn check_lint_name_cmdline(&self, sess: &Session, lint_name: &str, level: Level) {
        let db = match self.check_lint_name(lint_name, None) {
            CheckLintNameResult::Ok(_) => None,
            CheckLintNameResult::Warning(ref msg, _) => Some(sess.struct_warn(msg)),
            CheckLintNameResult::NoLint(suggestion) => {
                let mut err =
                    struct_span_err!(sess, DUMMY_SP, E0602, "unknown lint: `{}`", lint_name);

                if let Some(suggestion) = suggestion {
                    err.help(&format!("did you mean: `{}`", suggestion));
                }

                Some(err)
            }
            CheckLintNameResult::Tool(result) => match result {
                Err((Some(_), new_name)) => Some(sess.struct_warn(&format!(
                    "lint name `{}` is deprecated \
                     and does not have an effect anymore. \
                     Use: {}",
                    lint_name, new_name
                ))),
                _ => None,
            },
        };

        if let Some(mut db) = db {
            let msg = format!(
                "requested on the command line with `{} {}`",
                match level {
                    Level::Allow => "-A",
                    Level::Warn => "-W",
                    Level::Deny => "-D",
                    Level::Forbid => "-F",
                },
                lint_name
            );
            db.note(&msg);
            db.emit();
        }
    }

    /// True if this symbol represents a lint group name.
    pub fn is_lint_group(&self, lint_name: Symbol) -> bool {
        debug!(
            "is_lint_group(lint_name={:?}, lint_groups={:?})",
            lint_name,
            self.lint_groups.keys().collect::<Vec<_>>()
        );
        let lint_name_str = &*lint_name.as_str();
        self.lint_groups.contains_key(&lint_name_str) || {
            let warnings_name_str = crate::WARNINGS.name_lower();
            lint_name_str == &*warnings_name_str
        }
    }

    /// Checks the name of a lint for its existence, and whether it was
    /// renamed or removed. Generates a DiagnosticBuilder containing a
    /// warning for renamed and removed lints. This is over both lint
    /// names from attributes and those passed on the command line. Since
    /// it emits non-fatal warnings and there are *two* lint passes that
    /// inspect attributes, this is only run from the late pass to avoid
    /// printing duplicate warnings.
    pub fn check_lint_name(
        &self,
        lint_name: &str,
        tool_name: Option<Symbol>,
    ) -> CheckLintNameResult<'_> {
        let complete_name = if let Some(tool_name) = tool_name {
            format!("{}::{}", tool_name, lint_name)
        } else {
            lint_name.to_string()
        };
        // If the lint was scoped with `tool::` check if the tool lint exists
        if let Some(tool_name) = tool_name {
            match self.by_name.get(&complete_name) {
                None => match self.lint_groups.get(&*complete_name) {
                    // If the lint isn't registered, there are two possibilities:
                    None => {
                        // 1. The tool is currently running, so this lint really doesn't exist.
                        // FIXME: should this handle tools that never register a lint, like rustfmt?
                        tracing::debug!("lints={:?}", self.by_name.keys().collect::<Vec<_>>());
                        let tool_prefix = format!("{}::", tool_name);
                        return if self.by_name.keys().any(|lint| lint.starts_with(&tool_prefix)) {
                            self.no_lint_suggestion(&complete_name)
                        } else {
                            // 2. The tool isn't currently running, so no lints will be registered.
                            // To avoid giving a false positive, ignore all unknown lints.
                            CheckLintNameResult::Tool(Err((None, String::new())))
                        };
                    }
                    Some(LintGroup { lint_ids, .. }) => {
                        return CheckLintNameResult::Tool(Ok(&lint_ids));
                    }
                },
                Some(&Id(ref id)) => return CheckLintNameResult::Tool(Ok(slice::from_ref(id))),
                // If the lint was registered as removed or renamed by the lint tool, we don't need
                // to treat tool_lints and rustc lints different and can use the code below.
                _ => {}
            }
        }
        match self.by_name.get(&complete_name) {
            Some(&Renamed(ref new_name, _)) => CheckLintNameResult::Warning(
                format!("lint `{}` has been renamed to `{}`", complete_name, new_name),
                Some(new_name.to_owned()),
            ),
            Some(&Removed(ref reason)) => CheckLintNameResult::Warning(
                format!("lint `{}` has been removed: {}", complete_name, reason),
                None,
            ),
            None => match self.lint_groups.get(&*complete_name) {
                // If neither the lint, nor the lint group exists check if there is a `clippy::`
                // variant of this lint
                None => self.check_tool_name_for_backwards_compat(&complete_name, "clippy"),
                Some(LintGroup { lint_ids, depr, .. }) => {
                    // Check if the lint group name is deprecated
                    if let Some(LintAlias { name, silent }) = depr {
                        let LintGroup { lint_ids, .. } = self.lint_groups.get(name).unwrap();
                        return if *silent {
                            CheckLintNameResult::Ok(&lint_ids)
                        } else {
                            CheckLintNameResult::Tool(Err((Some(&lint_ids), (*name).to_string())))
                        };
                    }
                    CheckLintNameResult::Ok(&lint_ids)
                }
            },
            Some(&Id(ref id)) => CheckLintNameResult::Ok(slice::from_ref(id)),
            Some(&Ignored) => CheckLintNameResult::Ok(&[]),
        }
    }

    fn no_lint_suggestion(&self, lint_name: &str) -> CheckLintNameResult<'_> {
        let name_lower = lint_name.to_lowercase();
        let symbols =
            self.get_lints().iter().map(|l| Symbol::intern(&l.name_lower())).collect::<Vec<_>>();

        if lint_name.chars().any(char::is_uppercase) && self.find_lints(&name_lower).is_ok() {
            // First check if the lint name is (partly) in upper case instead of lower case...
            CheckLintNameResult::NoLint(Some(Symbol::intern(&name_lower)))
        } else {
            // ...if not, search for lints with a similar name
            let suggestion = find_best_match_for_name(&symbols, Symbol::intern(&name_lower), None);
            CheckLintNameResult::NoLint(suggestion)
        }
    }

    fn check_tool_name_for_backwards_compat(
        &self,
        lint_name: &str,
        tool_name: &str,
    ) -> CheckLintNameResult<'_> {
        let complete_name = format!("{}::{}", tool_name, lint_name);
        match self.by_name.get(&complete_name) {
            None => match self.lint_groups.get(&*complete_name) {
                // Now we are sure, that this lint exists nowhere
                None => self.no_lint_suggestion(lint_name),
                Some(LintGroup { lint_ids, depr, .. }) => {
                    // Reaching this would be weird, but let's cover this case anyway
                    if let Some(LintAlias { name, silent }) = depr {
                        let LintGroup { lint_ids, .. } = self.lint_groups.get(name).unwrap();
                        return if *silent {
                            CheckLintNameResult::Tool(Err((Some(&lint_ids), complete_name)))
                        } else {
                            CheckLintNameResult::Tool(Err((Some(&lint_ids), (*name).to_string())))
                        };
                    }
                    CheckLintNameResult::Tool(Err((Some(&lint_ids), complete_name)))
                }
            },
            Some(&Id(ref id)) => {
                CheckLintNameResult::Tool(Err((Some(slice::from_ref(id)), complete_name)))
            }
            Some(other) => {
                tracing::debug!("got renamed lint {:?}", other);
                CheckLintNameResult::NoLint(None)
            }
        }
    }
}

/// Context for lint checking after type checking.
pub struct LateContext<'tcx> {
    /// Type context we're checking in.
    pub tcx: TyCtxt<'tcx>,

    /// Current body, or `None` if outside a body.
    pub enclosing_body: Option<hir::BodyId>,

    /// Type-checking results for the current body. Access using the `typeck_results`
    /// and `maybe_typeck_results` methods, which handle querying the typeck results on demand.
    // FIXME(eddyb) move all the code accessing internal fields like this,
    // to this module, to avoid exposing it to lint logic.
    pub(super) cached_typeck_results: Cell<Option<&'tcx ty::TypeckResults<'tcx>>>,

    /// Parameter environment for the item we are in.
    pub param_env: ty::ParamEnv<'tcx>,

    /// Items accessible from the crate being checked.
    pub access_levels: &'tcx AccessLevels,

    /// The store of registered lints and the lint levels.
    pub lint_store: &'tcx LintStore,

    pub last_node_with_lint_attrs: hir::HirId,

    /// Generic type parameters in scope for the item we are in.
    pub generics: Option<&'tcx hir::Generics<'tcx>>,

    /// We are only looking at one module
    pub only_module: bool,
}

/// Context for lint checking of the AST, after expansion, before lowering to
/// HIR.
pub struct EarlyContext<'a> {
    /// Type context we're checking in.
    pub sess: &'a Session,

    /// The crate being checked.
    pub krate: &'a ast::Crate,

    pub builder: LintLevelsBuilder<'a>,

    /// The store of registered lints and the lint levels.
    pub lint_store: &'a LintStore,

    pub buffered: LintBuffer,
}

pub trait LintPassObject: Sized {}

impl LintPassObject for EarlyLintPassObject {}

impl LintPassObject for LateLintPassObject {}

pub trait LintContext: Sized {
    type PassObject: LintPassObject;

    fn sess(&self) -> &Session;
    fn lints(&self) -> &LintStore;

    fn lookup_with_diagnostics(
        &self,
        lint: &'static Lint,
        span: Option<impl Into<MultiSpan>>,
        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a>),
        diagnostic: BuiltinLintDiagnostics,
    ) {
        self.lookup(lint, span, |lint| {
            // We first generate a blank diagnostic.
            let mut db = lint.build("");

            // Now, set up surrounding context.
            let sess = self.sess();
            match diagnostic {
                BuiltinLintDiagnostics::Normal => (),
                BuiltinLintDiagnostics::BareTraitObject(span, is_global) => {
                    let (sugg, app) = match sess.source_map().span_to_snippet(span) {
                        Ok(s) if is_global => {
                            (format!("dyn ({})", s), Applicability::MachineApplicable)
                        }
                        Ok(s) => (format!("dyn {}", s), Applicability::MachineApplicable),
                        Err(_) => ("dyn <type>".to_string(), Applicability::HasPlaceholders),
                    };
                    db.span_suggestion(span, "use `dyn`", sugg, app);
                }
                BuiltinLintDiagnostics::AbsPathWithModule(span) => {
                    let (sugg, app) = match sess.source_map().span_to_snippet(span) {
                        Ok(ref s) => {
                            // FIXME(Manishearth) ideally the emitting code
                            // can tell us whether or not this is global
                            let opt_colon =
                                if s.trim_start().starts_with("::") { "" } else { "::" };

                            (format!("crate{}{}", opt_colon, s), Applicability::MachineApplicable)
                        }
                        Err(_) => ("crate::<path>".to_string(), Applicability::HasPlaceholders),
                    };
                    db.span_suggestion(span, "use `crate`", sugg, app);
                }
                BuiltinLintDiagnostics::ProcMacroDeriveResolutionFallback(span) => {
                    db.span_label(
                        span,
                        "names from parent modules are not accessible without an explicit import",
                    );
                }
                BuiltinLintDiagnostics::MacroExpandedMacroExportsAccessedByAbsolutePaths(
                    span_def,
                ) => {
                    db.span_note(span_def, "the macro is defined here");
                }
                BuiltinLintDiagnostics::ElidedLifetimesInPaths(
                    n,
                    path_span,
                    incl_angl_brckt,
                    insertion_span,
                    anon_lts,
                ) => {
                    add_elided_lifetime_in_path_suggestion(
                        sess.source_map(),
                        &mut db,
                        n,
                        path_span,
                        incl_angl_brckt,
                        insertion_span,
                        anon_lts,
                    );
                }
                BuiltinLintDiagnostics::UnknownCrateTypes(span, note, sugg) => {
                    db.span_suggestion(span, &note, sugg, Applicability::MaybeIncorrect);
                }
                BuiltinLintDiagnostics::UnusedImports(message, replaces) => {
                    if !replaces.is_empty() {
                        db.tool_only_multipart_suggestion(
                            &message,
                            replaces,
                            Applicability::MachineApplicable,
                        );
                    }
                }
                BuiltinLintDiagnostics::RedundantImport(spans, ident) => {
                    for (span, is_imported) in spans {
                        let introduced = if is_imported { "imported" } else { "defined" };
                        db.span_label(
                            span,
                            format!("the item `{}` is already {} here", ident, introduced),
                        );
                    }
                }
                BuiltinLintDiagnostics::DeprecatedMacro(suggestion, span) => {
                    stability::deprecation_suggestion(&mut db, "macro", suggestion, span)
                }
                BuiltinLintDiagnostics::UnusedDocComment(span) => {
                    db.span_label(span, "rustdoc does not generate documentation for macro invocations");
                    db.help("to document an item produced by a macro, \
                                  the macro must produce the documentation as part of its expansion");
                }
                BuiltinLintDiagnostics::PatternsInFnsWithoutBody(span, ident) => {
                    db.span_suggestion(span, "remove `mut` from the parameter", ident.to_string(), Applicability::MachineApplicable);
                }
                BuiltinLintDiagnostics::MissingAbi(span, default_abi) => {
                    db.span_label(span, "ABI should be specified here");
                    db.help(&format!("the default ABI is {}", default_abi.name()));
                }
                BuiltinLintDiagnostics::LegacyDeriveHelpers(span) => {
                    db.span_label(span, "the attribute is introduced here");
                }
                BuiltinLintDiagnostics::ExternDepSpec(krate, loc) => {
                    let json = match loc {
                        ExternDepSpec::Json(json) => {
                            db.help(&format!("remove unnecessary dependency `{}`", krate));
                            json
                        }
                        ExternDepSpec::Raw(raw) => {
                            db.help(&format!("remove unnecessary dependency `{}` at `{}`", krate, raw));
                            db.span_suggestion_with_style(
                                DUMMY_SP,
                                "raw extern location",
                                raw.clone(),
                                Applicability::Unspecified,
                                SuggestionStyle::CompletelyHidden,
                            );
                            Json::String(raw)
                        }
                    };
                    db.tool_only_suggestion_with_metadata(
                        "json extern location",
                        Applicability::Unspecified,
                        json
                    );
                }
                BuiltinLintDiagnostics::ProcMacroBackCompat(note) => {
                    db.note(&note);
                }
            }
            // Rewrap `db`, and pass control to the user.
            decorate(LintDiagnosticBuilder::new(db));
        });
    }

    // FIXME: These methods should not take an Into<MultiSpan> -- instead, callers should need to
    // set the span in their `decorate` function (preferably using set_span).
    fn lookup<S: Into<MultiSpan>>(
        &self,
        lint: &'static Lint,
        span: Option<S>,
        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a>),
    );

    fn struct_span_lint<S: Into<MultiSpan>>(
        &self,
        lint: &'static Lint,
        span: S,
        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a>),
    ) {
        self.lookup(lint, Some(span), decorate);
    }
    /// Emit a lint at the appropriate level, with no associated span.
    fn lint(&self, lint: &'static Lint, decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a>)) {
        self.lookup(lint, None as Option<Span>, decorate);
    }
}

impl<'a> EarlyContext<'a> {
    pub fn new(
        sess: &'a Session,
        lint_store: &'a LintStore,
        krate: &'a ast::Crate,
        buffered: LintBuffer,
        warn_about_weird_lints: bool,
    ) -> EarlyContext<'a> {
        EarlyContext {
            sess,
            krate,
            lint_store,
            builder: LintLevelsBuilder::new(sess, warn_about_weird_lints, lint_store, &krate.attrs),
            buffered,
        }
    }
}

impl LintContext for LateContext<'_> {
    type PassObject = LateLintPassObject;

    /// Gets the overall compiler `Session` object.
    fn sess(&self) -> &Session {
        &self.tcx.sess
    }

    fn lints(&self) -> &LintStore {
        &*self.lint_store
    }

    fn lookup<S: Into<MultiSpan>>(
        &self,
        lint: &'static Lint,
        span: Option<S>,
        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a>),
    ) {
        let hir_id = self.last_node_with_lint_attrs;

        match span {
            Some(s) => self.tcx.struct_span_lint_hir(lint, hir_id, s, decorate),
            None => self.tcx.struct_lint_node(lint, hir_id, decorate),
        }
    }
}

impl LintContext for EarlyContext<'_> {
    type PassObject = EarlyLintPassObject;

    /// Gets the overall compiler `Session` object.
    fn sess(&self) -> &Session {
        &self.sess
    }

    fn lints(&self) -> &LintStore {
        &*self.lint_store
    }

    fn lookup<S: Into<MultiSpan>>(
        &self,
        lint: &'static Lint,
        span: Option<S>,
        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a>),
    ) {
        self.builder.struct_lint(lint, span.map(|s| s.into()), decorate)
    }
}

impl<'tcx> LateContext<'tcx> {
    /// Gets the type-checking results for the current body,
    /// or `None` if outside a body.
    pub fn maybe_typeck_results(&self) -> Option<&'tcx ty::TypeckResults<'tcx>> {
        self.cached_typeck_results.get().or_else(|| {
            self.enclosing_body.map(|body| {
                let typeck_results = self.tcx.typeck_body(body);
                self.cached_typeck_results.set(Some(typeck_results));
                typeck_results
            })
        })
    }

    /// Gets the type-checking results for the current body.
    /// As this will ICE if called outside bodies, only call when working with
    /// `Expr` or `Pat` nodes (they are guaranteed to be found only in bodies).
    #[track_caller]
    pub fn typeck_results(&self) -> &'tcx ty::TypeckResults<'tcx> {
        self.maybe_typeck_results().expect("`LateContext::typeck_results` called outside of body")
    }

    /// Returns the final resolution of a `QPath`, or `Res::Err` if unavailable.
    /// Unlike `.typeck_results().qpath_res(qpath, id)`, this can be used even outside
    /// bodies (e.g. for paths in `hir::Ty`), without any risk of ICE-ing.
    pub fn qpath_res(&self, qpath: &hir::QPath<'_>, id: hir::HirId) -> Res {
        match *qpath {
            hir::QPath::Resolved(_, ref path) => path.res,
            hir::QPath::TypeRelative(..) | hir::QPath::LangItem(..) => self
                .maybe_typeck_results()
                .filter(|typeck_results| typeck_results.hir_owner == id.owner)
                .or_else(|| {
                    if self.tcx.has_typeck_results(id.owner.to_def_id()) {
                        Some(self.tcx.typeck(id.owner))
                    } else {
                        None
                    }
                })
                .and_then(|typeck_results| typeck_results.type_dependent_def(id))
                .map_or(Res::Err, |(kind, def_id)| Res::Def(kind, def_id)),
        }
    }

    /// Check if a `DefId`'s path matches the given absolute type path usage.
    ///
    /// Anonymous scopes such as `extern` imports are matched with `kw::Empty`;
    /// inherent `impl` blocks are matched with the name of the type.
    ///
    /// Instead of using this method, it is often preferable to instead use
    /// `rustc_diagnostic_item` or a `lang_item`. This is less prone to errors
    /// as paths get invalidated if the target definition moves.
    ///
    /// # Examples
    ///
    /// ```rust,ignore (no context or def id available)
    /// if cx.match_def_path(def_id, &[sym::core, sym::option, sym::Option]) {
    ///     // The given `def_id` is that of an `Option` type
    /// }
    /// ```
    pub fn match_def_path(&self, def_id: DefId, path: &[Symbol]) -> bool {
        let names = self.get_def_path(def_id);

        names.len() == path.len() && names.into_iter().zip(path.iter()).all(|(a, &b)| a == b)
    }

    /// Gets the absolute path of `def_id` as a vector of `Symbol`.
    ///
    /// # Examples
    ///
    /// ```rust,ignore (no context or def id available)
    /// let def_path = cx.get_def_path(def_id);
    /// if let &[sym::core, sym::option, sym::Option] = &def_path[..] {
    ///     // The given `def_id` is that of an `Option` type
    /// }
    /// ```
    pub fn get_def_path(&self, def_id: DefId) -> Vec<Symbol> {
        pub struct AbsolutePathPrinter<'tcx> {
            pub tcx: TyCtxt<'tcx>,
        }

        impl<'tcx> Printer<'tcx> for AbsolutePathPrinter<'tcx> {
            type Error = !;

            type Path = Vec<Symbol>;
            type Region = ();
            type Type = ();
            type DynExistential = ();
            type Const = ();

            fn tcx(&self) -> TyCtxt<'tcx> {
                self.tcx
            }

            fn print_region(self, _region: ty::Region<'_>) -> Result<Self::Region, Self::Error> {
                Ok(())
            }

            fn print_type(self, _ty: Ty<'tcx>) -> Result<Self::Type, Self::Error> {
                Ok(())
            }

            fn print_dyn_existential(
                self,
                _predicates: &'tcx ty::List<ty::Binder<ty::ExistentialPredicate<'tcx>>>,
            ) -> Result<Self::DynExistential, Self::Error> {
                Ok(())
            }

            fn print_const(self, _ct: &'tcx ty::Const<'tcx>) -> Result<Self::Const, Self::Error> {
                Ok(())
            }

            fn path_crate(self, cnum: CrateNum) -> Result<Self::Path, Self::Error> {
                Ok(vec![self.tcx.original_crate_name(cnum)])
            }

            fn path_qualified(
                self,
                self_ty: Ty<'tcx>,
                trait_ref: Option<ty::TraitRef<'tcx>>,
            ) -> Result<Self::Path, Self::Error> {
                if trait_ref.is_none() {
                    if let ty::Adt(def, substs) = self_ty.kind() {
                        return self.print_def_path(def.did, substs);
                    }
                }

                // This shouldn't ever be needed, but just in case:
                with_no_trimmed_paths(|| {
                    Ok(vec![match trait_ref {
                        Some(trait_ref) => Symbol::intern(&format!("{:?}", trait_ref)),
                        None => Symbol::intern(&format!("<{}>", self_ty)),
                    }])
                })
            }

            fn path_append_impl(
                self,
                print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
                _disambiguated_data: &DisambiguatedDefPathData,
                self_ty: Ty<'tcx>,
                trait_ref: Option<ty::TraitRef<'tcx>>,
            ) -> Result<Self::Path, Self::Error> {
                let mut path = print_prefix(self)?;

                // This shouldn't ever be needed, but just in case:
                path.push(match trait_ref {
                    Some(trait_ref) => with_no_trimmed_paths(|| {
                        Symbol::intern(&format!(
                            "<impl {} for {}>",
                            trait_ref.print_only_trait_path(),
                            self_ty
                        ))
                    }),
                    None => {
                        with_no_trimmed_paths(|| Symbol::intern(&format!("<impl {}>", self_ty)))
                    }
                });

                Ok(path)
            }

            fn path_append(
                self,
                print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
                disambiguated_data: &DisambiguatedDefPathData,
            ) -> Result<Self::Path, Self::Error> {
                let mut path = print_prefix(self)?;

                // Skip `::{{constructor}}` on tuple/unit structs.
                if let DefPathData::Ctor = disambiguated_data.data {
                    return Ok(path);
                }

                path.push(Symbol::intern(&disambiguated_data.data.to_string()));
                Ok(path)
            }

            fn path_generic_args(
                self,
                print_prefix: impl FnOnce(Self) -> Result<Self::Path, Self::Error>,
                _args: &[GenericArg<'tcx>],
            ) -> Result<Self::Path, Self::Error> {
                print_prefix(self)
            }
        }

        AbsolutePathPrinter { tcx: self.tcx }.print_def_path(def_id, &[]).unwrap()
    }
}

impl<'tcx> LayoutOf for LateContext<'tcx> {
    type Ty = Ty<'tcx>;
    type TyAndLayout = Result<TyAndLayout<'tcx>, LayoutError<'tcx>>;

    fn layout_of(&self, ty: Ty<'tcx>) -> Self::TyAndLayout {
        self.tcx.layout_of(self.param_env.and(ty))
    }
}