rust/compiler/rustc_lint/src/lifetime_syntax.rs

use rustc_data_structures::fx::FxIndexMap;
use rustc_hir::intravisit::{self, Visitor};
use rustc_hir::{self as hir, LifetimeSource};
use rustc_session::{declare_lint, declare_lint_pass};
use rustc_span::Span;
use tracing::instrument;

use crate::{LateContext, LateLintPass, LintContext, lints};

declare_lint! {
    /// The `mismatched_lifetime_syntaxes` lint detects when the same
    /// lifetime is referred to by different syntaxes between function
    /// arguments and return values.
    ///
    /// The three kinds of syntaxes are:
    ///
    /// 1. Named lifetimes. These are references (`&'a str`) or paths
    ///    (`Person<'a>`) that use a lifetime with a name, such as
    ///    `'static` or `'a`.
    ///
    /// 2. Elided lifetimes. These are references with no explicit
    ///    lifetime (`&str`), references using the anonymous lifetime
    ///    (`&'_ str`), and paths using the anonymous lifetime
    ///    (`Person<'_>`).
    ///
    /// 3. Hidden lifetimes. These are paths that do not contain any
    ///    visual indication that it contains a lifetime (`Person`).
    ///
    /// ### Example
    ///
    /// ```rust,compile_fail
    /// #![deny(mismatched_lifetime_syntaxes)]
    ///
    /// pub fn mixing_named_with_elided(v: &'static u8) -> &u8 {
    ///     v
    /// }
    ///
    /// struct Person<'a> {
    ///     name: &'a str,
    /// }
    ///
    /// pub fn mixing_hidden_with_elided(v: Person) -> Person<'_> {
    ///     v
    /// }
    ///
    /// struct Foo;
    ///
    /// impl Foo {
    ///     // Lifetime elision results in the output lifetime becoming
    ///     // `'static`, which is not what was intended.
    ///     pub fn get_mut(&'static self, x: &mut u8) -> &mut u8 {
    ///         unsafe { &mut *(x as *mut _) }
    ///     }
    /// }
    /// ```
    ///
    /// {{produces}}
    ///
    /// ### Explanation
    ///
    /// Lifetime elision is useful because it frees you from having to
    /// give each lifetime its own name and show the relation of input
    /// and output lifetimes for common cases. However, a lifetime
    /// that uses inconsistent syntax between related arguments and
    /// return values is more confusing.
    ///
    /// In certain `unsafe` code, lifetime elision combined with
    /// inconsistent lifetime syntax may result in unsound code.
    pub MISMATCHED_LIFETIME_SYNTAXES,
    Warn,
    "detects when a lifetime uses different syntax between arguments and return values"
}

declare_lint_pass!(LifetimeSyntax => [MISMATCHED_LIFETIME_SYNTAXES]);

impl<'tcx> LateLintPass<'tcx> for LifetimeSyntax {
    #[instrument(skip_all)]
    fn check_fn(
        &mut self,
        cx: &LateContext<'tcx>,
        _: hir::intravisit::FnKind<'tcx>,
        fd: &'tcx hir::FnDecl<'tcx>,
        _: &'tcx hir::Body<'tcx>,
        _: rustc_span::Span,
        _: rustc_span::def_id::LocalDefId,
    ) {
        let mut input_map = Default::default();
        let mut output_map = Default::default();

        for input in fd.inputs {
            LifetimeInfoCollector::collect(input, &mut input_map);
        }

        if let hir::FnRetTy::Return(output) = fd.output {
            LifetimeInfoCollector::collect(output, &mut output_map);
        }

        report_mismatches(cx, &input_map, &output_map);
    }
}

#[instrument(skip_all)]
fn report_mismatches<'tcx>(
    cx: &LateContext<'tcx>,
    inputs: &LifetimeInfoMap<'tcx>,
    outputs: &LifetimeInfoMap<'tcx>,
) {
    for (resolved_lifetime, output_info) in outputs {
        if let Some(input_info) = inputs.get(resolved_lifetime) {
            if !lifetimes_use_matched_syntax(input_info, output_info) {
                emit_mismatch_diagnostic(cx, input_info, output_info);
            }
        }
    }
}

fn lifetimes_use_matched_syntax(input_info: &[Info<'_>], output_info: &[Info<'_>]) -> bool {
    // Categorize lifetimes into source/syntax buckets.
    let mut n_hidden = 0;
    let mut n_elided = 0;
    let mut n_named = 0;

    for info in input_info.iter().chain(output_info) {
        use LifetimeSource::*;
        use hir::LifetimeSyntax::*;

        let syntax_source = (info.lifetime.syntax, info.lifetime.source);

        match syntax_source {
            // Ignore any other kind of lifetime.
            (_, Other) => continue,

            // E.g. `&T`.
            (Implicit, Reference | OutlivesBound | PreciseCapturing) |
            // E.g. `&'_ T`.
            (ExplicitAnonymous, Reference | OutlivesBound | PreciseCapturing) |
            // E.g. `ContainsLifetime<'_>`.
            (ExplicitAnonymous, Path { .. }) => n_elided += 1,

            // E.g. `ContainsLifetime`.
            (Implicit, Path { .. }) => n_hidden += 1,

            // E.g. `&'a T`.
            (ExplicitBound, Reference | OutlivesBound | PreciseCapturing) |
            // E.g. `ContainsLifetime<'a>`.
            (ExplicitBound, Path { .. }) => n_named += 1,
        };
    }

    let syntax_counts = (n_hidden, n_elided, n_named);
    tracing::debug!(?syntax_counts);

    matches!(syntax_counts, (_, 0, 0) | (0, _, 0) | (0, 0, _))
}

fn emit_mismatch_diagnostic<'tcx>(
    cx: &LateContext<'tcx>,
    input_info: &[Info<'_>],
    output_info: &[Info<'_>],
) {
    // There can only ever be zero or one bound lifetime
    // for a given lifetime resolution.
    let mut bound_lifetime = None;

    // We offer the following kinds of suggestions (when appropriate
    // such that the suggestion wouldn't violate the lint):
    //
    // 1. Every lifetime becomes named, when there is already a
    //    user-provided name.
    //
    // 2. A "mixed" signature, where references become implicit
    //    and paths become explicitly anonymous.
    //
    // 3. Every lifetime becomes implicit.
    //
    // 4. Every lifetime becomes explicitly anonymous.
    //
    // Number 2 is arguably the most common pattern and the one we
    // should push strongest. Number 3 is likely the next most common,
    // followed by number 1. Coming in at a distant last would be
    // number 4.
    //
    // Beyond these, there are variants of acceptable signatures that
    // we won't suggest because they are very low-value. For example,
    // we will never suggest `fn(&T1, &'_ T2) -> &T3` even though that
    // would pass the lint.
    //
    // The following collections are the lifetime instances that we
    // suggest changing to a given alternate style.

    // 1. Convert all to named.
    let mut suggest_change_to_explicit_bound = Vec::new();

    // 2. Convert to mixed. We track each kind of change separately.
    let mut suggest_change_to_mixed_implicit = Vec::new();
    let mut suggest_change_to_mixed_explicit_anonymous = Vec::new();

    // 3. Convert all to implicit.
    let mut suggest_change_to_implicit = Vec::new();

    // 4. Convert all to explicit anonymous.
    let mut suggest_change_to_explicit_anonymous = Vec::new();

    // Some styles prevent using implicit syntax at all.
    let mut allow_suggesting_implicit = true;

    // It only makes sense to suggest mixed if we have both sources.
    let mut saw_a_reference = false;
    let mut saw_a_path = false;

    for info in input_info.iter().chain(output_info) {
        use LifetimeSource::*;
        use hir::LifetimeSyntax::*;

        let syntax_source = (info.lifetime.syntax, info.lifetime.source);

        if let (_, Other) = syntax_source {
            // Ignore any other kind of lifetime.
            continue;
        }

        if let (ExplicitBound, _) = syntax_source {
            bound_lifetime = Some(info);
        }

        match syntax_source {
            // E.g. `&T`.
            (Implicit, Reference) => {
                suggest_change_to_explicit_anonymous.push(info);
                suggest_change_to_explicit_bound.push(info);
            }

            // E.g. `&'_ T`.
            (ExplicitAnonymous, Reference) => {
                suggest_change_to_implicit.push(info);
                suggest_change_to_mixed_implicit.push(info);
                suggest_change_to_explicit_bound.push(info);
            }

            // E.g. `ContainsLifetime`.
            (Implicit, Path { .. }) => {
                suggest_change_to_mixed_explicit_anonymous.push(info);
                suggest_change_to_explicit_anonymous.push(info);
                suggest_change_to_explicit_bound.push(info);
            }

            // E.g. `ContainsLifetime<'_>`.
            (ExplicitAnonymous, Path { .. }) => {
                suggest_change_to_explicit_bound.push(info);
            }

            // E.g. `&'a T`.
            (ExplicitBound, Reference) => {
                suggest_change_to_implicit.push(info);
                suggest_change_to_mixed_implicit.push(info);
                suggest_change_to_explicit_anonymous.push(info);
            }

            // E.g. `ContainsLifetime<'a>`.
            (ExplicitBound, Path { .. }) => {
                suggest_change_to_mixed_explicit_anonymous.push(info);
                suggest_change_to_explicit_anonymous.push(info);
            }

            (Implicit, OutlivesBound | PreciseCapturing) => {
                panic!("This syntax / source combination is not possible");
            }

            // E.g. `+ '_`, `+ use<'_>`.
            (ExplicitAnonymous, OutlivesBound | PreciseCapturing) => {
                suggest_change_to_explicit_bound.push(info);
            }

            // E.g. `+ 'a`, `+ use<'a>`.
            (ExplicitBound, OutlivesBound | PreciseCapturing) => {
                suggest_change_to_mixed_explicit_anonymous.push(info);
                suggest_change_to_explicit_anonymous.push(info);
            }

            (_, Other) => {
                panic!("This syntax / source combination has already been skipped");
            }
        }

        if matches!(syntax_source, (_, Path { .. } | OutlivesBound | PreciseCapturing)) {
            allow_suggesting_implicit = false;
        }

        match syntax_source {
            (_, Reference) => saw_a_reference = true,
            (_, Path { .. }) => saw_a_path = true,
            _ => {}
        }
    }

    let make_implicit_suggestions =
        |infos: &[&Info<'_>]| infos.iter().map(|i| i.removing_span()).collect::<Vec<_>>();

    let inputs = input_info.iter().map(|info| info.reporting_span()).collect();
    let outputs = output_info.iter().map(|info| info.reporting_span()).collect();

    let explicit_bound_suggestion = bound_lifetime.map(|info| {
        build_mismatch_suggestion(info.lifetime_name(), &suggest_change_to_explicit_bound)
    });

    let is_bound_static = bound_lifetime.is_some_and(|info| info.is_static());

    tracing::debug!(?bound_lifetime, ?explicit_bound_suggestion, ?is_bound_static);

    let should_suggest_mixed =
        // Do we have a mixed case?
        (saw_a_reference && saw_a_path) &&
        // Is there anything to change?
        (!suggest_change_to_mixed_implicit.is_empty() ||
         !suggest_change_to_mixed_explicit_anonymous.is_empty()) &&
        // If we have `'static`, we don't want to remove it.
        !is_bound_static;

    let mixed_suggestion = should_suggest_mixed.then(|| {
        let implicit_suggestions = make_implicit_suggestions(&suggest_change_to_mixed_implicit);

        let explicit_anonymous_suggestions = suggest_change_to_mixed_explicit_anonymous
            .iter()
            .map(|info| info.suggestion("'_"))
            .collect();

        lints::MismatchedLifetimeSyntaxesSuggestion::Mixed {
            implicit_suggestions,
            explicit_anonymous_suggestions,
            tool_only: false,
        }
    });

    tracing::debug!(
        ?suggest_change_to_mixed_implicit,
        ?suggest_change_to_mixed_explicit_anonymous,
        ?mixed_suggestion,
    );

    let should_suggest_implicit =
        // Is there anything to change?
        !suggest_change_to_implicit.is_empty() &&
        // We never want to hide the lifetime in a path (or similar).
        allow_suggesting_implicit &&
        // If we have `'static`, we don't want to remove it.
        !is_bound_static;

    let implicit_suggestion = should_suggest_implicit.then(|| {
        let suggestions = make_implicit_suggestions(&suggest_change_to_implicit);

        lints::MismatchedLifetimeSyntaxesSuggestion::Implicit { suggestions, tool_only: false }
    });

    tracing::debug!(
        ?should_suggest_implicit,
        ?suggest_change_to_implicit,
        allow_suggesting_implicit,
        ?implicit_suggestion,
    );

    let should_suggest_explicit_anonymous =
        // Is there anything to change?
        !suggest_change_to_explicit_anonymous.is_empty() &&
        // If we have `'static`, we don't want to remove it.
        !is_bound_static;

    let explicit_anonymous_suggestion = should_suggest_explicit_anonymous
        .then(|| build_mismatch_suggestion("'_", &suggest_change_to_explicit_anonymous));

    tracing::debug!(
        ?should_suggest_explicit_anonymous,
        ?suggest_change_to_explicit_anonymous,
        ?explicit_anonymous_suggestion,
    );

    let lifetime_name = bound_lifetime.map(|info| info.lifetime_name()).unwrap_or("'_").to_owned();

    // We can produce a number of suggestions which may overwhelm
    // the user. Instead, we order the suggestions based on Rust
    // idioms. The "best" choice is shown to the user and the
    // remaining choices are shown to tools only.
    let mut suggestions = Vec::new();
    suggestions.extend(explicit_bound_suggestion);
    suggestions.extend(mixed_suggestion);
    suggestions.extend(implicit_suggestion);
    suggestions.extend(explicit_anonymous_suggestion);

    cx.emit_span_lint(
        MISMATCHED_LIFETIME_SYNTAXES,
        Vec::clone(&inputs),
        lints::MismatchedLifetimeSyntaxes { lifetime_name, inputs, outputs, suggestions },
    );
}

fn build_mismatch_suggestion(
    lifetime_name: &str,
    infos: &[&Info<'_>],
) -> lints::MismatchedLifetimeSyntaxesSuggestion {
    let lifetime_name = lifetime_name.to_owned();

    let suggestions = infos.iter().map(|info| info.suggestion(&lifetime_name)).collect();

    lints::MismatchedLifetimeSyntaxesSuggestion::Explicit {
        lifetime_name,
        suggestions,
        tool_only: false,
    }
}

#[derive(Debug)]
struct Info<'tcx> {
    type_span: Span,
    referenced_type_span: Option<Span>,
    lifetime: &'tcx hir::Lifetime,
}

impl<'tcx> Info<'tcx> {
    fn lifetime_name(&self) -> &str {
        self.lifetime.ident.as_str()
    }

    fn is_static(&self) -> bool {
        self.lifetime.is_static()
    }

    /// When reporting a lifetime that is implicit, we expand the span
    /// to include the type. Otherwise we end up pointing at nothing,
    /// which is a bit confusing.
    fn reporting_span(&self) -> Span {
        if self.lifetime.is_implicit() { self.type_span } else { self.lifetime.ident.span }
    }

    /// When removing an explicit lifetime from a reference,
    /// we want to remove the whitespace after the lifetime.
    ///
    /// ```rust
    /// fn x(a: &'_ u8) {}
    /// ```
    ///
    /// Should become:
    ///
    /// ```rust
    /// fn x(a: &u8) {}
    /// ```
    // FIXME: Ideally, we'd also remove the lifetime declaration.
    fn removing_span(&self) -> Span {
        let mut span = self.suggestion("'dummy").0;

        if let Some(referenced_type_span) = self.referenced_type_span {
            span = span.until(referenced_type_span);
        }

        span
    }

    fn suggestion(&self, lifetime_name: &str) -> (Span, String) {
        self.lifetime.suggestion(lifetime_name)
    }
}

type LifetimeInfoMap<'tcx> = FxIndexMap<&'tcx hir::LifetimeKind, Vec<Info<'tcx>>>;

struct LifetimeInfoCollector<'a, 'tcx> {
    type_span: Span,
    referenced_type_span: Option<Span>,
    map: &'a mut LifetimeInfoMap<'tcx>,
}

impl<'a, 'tcx> LifetimeInfoCollector<'a, 'tcx> {
    fn collect(ty: &'tcx hir::Ty<'tcx>, map: &'a mut LifetimeInfoMap<'tcx>) {
        let mut this = Self { type_span: ty.span, referenced_type_span: None, map };

        intravisit::walk_unambig_ty(&mut this, ty);
    }
}

impl<'a, 'tcx> Visitor<'tcx> for LifetimeInfoCollector<'a, 'tcx> {
    #[instrument(skip(self))]
    fn visit_lifetime(&mut self, lifetime: &'tcx hir::Lifetime) {
        let type_span = self.type_span;
        let referenced_type_span = self.referenced_type_span;

        let info = Info { type_span, referenced_type_span, lifetime };

        self.map.entry(&lifetime.kind).or_default().push(info);
    }

    #[instrument(skip(self))]
    fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx, hir::AmbigArg>) -> Self::Result {
        let old_type_span = self.type_span;
        let old_referenced_type_span = self.referenced_type_span;

        self.type_span = ty.span;
        if let hir::TyKind::Ref(_, ty) = ty.kind {
            self.referenced_type_span = Some(ty.ty.span);
        }

        intravisit::walk_ty(self, ty);

        self.type_span = old_type_span;
        self.referenced_type_span = old_referenced_type_span;
    }
}