rust/clippy_lints/src/manual_let_else.rs

use clippy_utils::diagnostics::span_lint_and_then;
use clippy_utils::higher::IfLetOrMatch;
use clippy_utils::msrvs::{self, Msrv};
use clippy_utils::peel_blocks;
use clippy_utils::source::snippet_with_context;
use clippy_utils::ty::is_type_diagnostic_item;
use clippy_utils::visitors::{Descend, Visitable};
use if_chain::if_chain;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_errors::Applicability;
use rustc_hir::intravisit::{walk_expr, Visitor};
use rustc_hir::{Expr, ExprKind, HirId, ItemId, Local, MatchSource, Pat, PatKind, QPath, Stmt, StmtKind, Ty};
use rustc_lint::{LateContext, LateLintPass, LintContext};
use rustc_middle::lint::in_external_macro;
use rustc_session::{declare_tool_lint, impl_lint_pass};
use rustc_span::symbol::{sym, Symbol};
use rustc_span::Span;
use serde::Deserialize;
use std::ops::ControlFlow;
use std::slice;

declare_clippy_lint! {
    /// ### What it does
    ///
    /// Warn of cases where `let...else` could be used
    ///
    /// ### Why is this bad?
    ///
    /// `let...else` provides a standard construct for this pattern
    /// that people can easily recognize. It's also more compact.
    ///
    /// ### Example
    ///
    /// ```rust
    /// # let w = Some(0);
    /// let v = if let Some(v) = w { v } else { return };
    /// ```
    ///
    /// Could be written:
    ///
    /// ```rust
    /// # fn main () {
    /// # let w = Some(0);
    /// let Some(v) = w else { return };
    /// # }
    /// ```
    #[clippy::version = "1.67.0"]
    pub MANUAL_LET_ELSE,
    pedantic,
    "manual implementation of a let...else statement"
}

pub struct ManualLetElse {
    msrv: Msrv,
    matches_behaviour: MatchLintBehaviour,
}

impl ManualLetElse {
    #[must_use]
    pub fn new(msrv: Msrv, matches_behaviour: MatchLintBehaviour) -> Self {
        Self {
            msrv,
            matches_behaviour,
        }
    }
}

impl_lint_pass!(ManualLetElse => [MANUAL_LET_ELSE]);

impl<'tcx> LateLintPass<'tcx> for ManualLetElse {
    fn check_stmt(&mut self, cx: &LateContext<'_>, stmt: &'tcx Stmt<'tcx>) {
        if !self.msrv.meets(msrvs::LET_ELSE) || in_external_macro(cx.sess(), stmt.span) {
            return;
        }

        if let StmtKind::Local(local) = stmt.kind &&
            let Some(init) = local.init &&
            local.els.is_none() &&
            local.ty.is_none() &&
            init.span.ctxt() == stmt.span.ctxt() &&
            let Some(if_let_or_match) = IfLetOrMatch::parse(cx, init)
        {
            match if_let_or_match {
                IfLetOrMatch::IfLet(if_let_expr, let_pat, if_then, if_else) => if_chain! {
                    if let Some(ident_map) = expr_simple_identity_map(local.pat, let_pat, if_then);
                    if let Some(if_else) = if_else;
                    if expr_diverges(cx, if_else);
                    then {
                        emit_manual_let_else(cx, stmt.span, if_let_expr, &ident_map, let_pat, if_else);
                    }
                },
                IfLetOrMatch::Match(match_expr, arms, source) => {
                    if self.matches_behaviour == MatchLintBehaviour::Never {
                        return;
                    }
                    if source != MatchSource::Normal {
                        return;
                    }
                    // Any other number than two arms doesn't (necessarily)
                    // have a trivial mapping to let else.
                    if arms.len() != 2 {
                        return;
                    }
                    // Guards don't give us an easy mapping either
                    if arms.iter().any(|arm| arm.guard.is_some()) {
                        return;
                    }
                    let check_types = self.matches_behaviour == MatchLintBehaviour::WellKnownTypes;
                    let diverging_arm_opt = arms
                        .iter()
                        .enumerate()
                        .find(|(_, arm)| expr_diverges(cx, arm.body) && pat_allowed_for_else(cx, arm.pat, check_types));
                    let Some((idx, diverging_arm)) = diverging_arm_opt else { return; };
                    // If the non-diverging arm is the first one, its pattern can be reused in a let/else statement.
                    // However, if it arrives in second position, its pattern may cover some cases already covered
                    // by the diverging one.
                    // TODO: accept the non-diverging arm as a second position if patterns are disjointed.
                    if idx == 0 {
                        return;
                    }
                    let pat_arm = &arms[1 - idx];
                    let Some(ident_map) = expr_simple_identity_map(local.pat, pat_arm.pat, pat_arm.body) else {
                        return
                    };

                    emit_manual_let_else(cx, stmt.span, match_expr, &ident_map, pat_arm.pat, diverging_arm.body);
                },
            }
        };
    }

    extract_msrv_attr!(LateContext);
}

fn emit_manual_let_else(
    cx: &LateContext<'_>,
    span: Span,
    expr: &Expr<'_>,
    ident_map: &FxHashMap<Symbol, &Pat<'_>>,
    pat: &Pat<'_>,
    else_body: &Expr<'_>,
) {
    span_lint_and_then(
        cx,
        MANUAL_LET_ELSE,
        span,
        "this could be rewritten as `let...else`",
        |diag| {
            // This is far from perfect, for example there needs to be:
            // * renamings of the bindings for many `PatKind`s like slices, etc.
            // * limitations in the existing replacement algorithms
            // * unused binding collision detection with existing ones
            // for this to be machine applicable.
            let mut app = Applicability::HasPlaceholders;
            let (sn_expr, _) = snippet_with_context(cx, expr.span, span.ctxt(), "", &mut app);
            let (sn_else, _) = snippet_with_context(cx, else_body.span, span.ctxt(), "", &mut app);

            let else_bl = if matches!(else_body.kind, ExprKind::Block(..)) {
                sn_else.into_owned()
            } else {
                format!("{{ {sn_else} }}")
            };
            let sn_bl = replace_in_pattern(cx, span, ident_map, pat, &mut app, true);
            let sugg = format!("let {sn_bl} = {sn_expr} else {else_bl};");
            diag.span_suggestion(span, "consider writing", sugg, app);
        },
    );
}

/// Replaces the locals in the pattern
///
/// For this example:
///
/// ```ignore
/// let (a, FooBar { b, c }) = if let Bar { Some(a_i), b_i } = ex { (a_i, b_i) } else { return };
/// ```
///
/// We have:
///
/// ```ignore
/// pat: Bar { Some(a_i), b_i }
/// ident_map: (a_i) -> (a), (b_i) -> (FooBar { b, c })
/// ```
///
/// We return:
///
/// ```ignore
/// Bar { Some(a), b_i: FooBar { b, c } }
/// ```
fn replace_in_pattern(
    cx: &LateContext<'_>,
    span: Span,
    ident_map: &FxHashMap<Symbol, &Pat<'_>>,
    pat: &Pat<'_>,
    app: &mut Applicability,
    top_level: bool,
) -> String {
    // We put a labeled block here so that we can implement the fallback in this function.
    // As the function has multiple call sites, implementing the fallback via an Option<T>
    // return type and unwrap_or_else would cause repetition. Similarly, the function also
    // invokes the fall back multiple times.
    'a: {
        // If the ident map is empty, there is no replacement to do.
        // The code following this if assumes a non-empty ident_map.
        if ident_map.is_empty() {
            break 'a;
        }

        match pat.kind {
            PatKind::Binding(_ann, _id, binding_name, opt_subpt) => {
                let Some(pat_to_put) = ident_map.get(&binding_name.name) else { break 'a };
                let (sn_ptp, _) = snippet_with_context(cx, pat_to_put.span, span.ctxt(), "", app);
                if let Some(subpt) = opt_subpt {
                    let subpt = replace_in_pattern(cx, span, ident_map, subpt, app, false);
                    return format!("{sn_ptp} @ {subpt}");
                }
                return sn_ptp.to_string();
            },
            PatKind::Or(pats) => {
                let patterns = pats
                    .iter()
                    .map(|pat| replace_in_pattern(cx, span, ident_map, pat, app, false))
                    .collect::<Vec<_>>();
                let or_pat = patterns.join(" | ");
                if top_level {
                    return format!("({or_pat})");
                }
                return or_pat;
            },
            PatKind::Struct(path, fields, has_dot_dot) => {
                let fields = fields
                    .iter()
                    .map(|fld| {
                        if let PatKind::Binding(_, _, name, None) = fld.pat.kind &&
                            let Some(pat_to_put) = ident_map.get(&name.name)
                        {
                            let (sn_fld_name, _) = snippet_with_context(cx, fld.ident.span, span.ctxt(), "", app);
                            let (sn_ptp, _) = snippet_with_context(cx, pat_to_put.span, span.ctxt(), "", app);
                            // TODO: this is a bit of a hack, but it does its job. Ideally, we'd check if pat_to_put is
                            // a PatKind::Binding but that is also hard to get right.
                            if sn_fld_name == sn_ptp {
                                // Field init shorthand
                                return format!("{sn_fld_name}");
                            }
                            return format!("{sn_fld_name}: {sn_ptp}");
                        }
                        let (sn_fld, _) = snippet_with_context(cx, fld.span, span.ctxt(), "", app);
                        sn_fld.into_owned()
                    })
                    .collect::<Vec<_>>();
                let fields_string = fields.join(", ");

                let dot_dot_str = if has_dot_dot { " .." } else { "" };
                let (sn_pth, _) = snippet_with_context(cx, path.span(), span.ctxt(), "", app);
                return format!("{sn_pth} {{ {fields_string}{dot_dot_str} }}");
            },
            // Replace the variable name iff `TupleStruct` has one argument like `Variant(v)`.
            PatKind::TupleStruct(ref w, args, dot_dot_pos) => {
                let mut args = args
                    .iter()
                    .map(|pat| replace_in_pattern(cx, span, ident_map, pat, app, false))
                    .collect::<Vec<_>>();
                if let Some(pos) = dot_dot_pos.as_opt_usize() {
                    args.insert(pos, "..".to_owned());
                }
                let args = args.join(", ");
                let sn_wrapper = cx.sess().source_map().span_to_snippet(w.span()).unwrap_or_default();
                return format!("{sn_wrapper}({args})");
            },
            PatKind::Tuple(args, dot_dot_pos) => {
                let mut args = args
                    .iter()
                    .map(|pat| replace_in_pattern(cx, span, ident_map, pat, app, false))
                    .collect::<Vec<_>>();
                if let Some(pos) = dot_dot_pos.as_opt_usize() {
                    args.insert(pos, "..".to_owned());
                }
                let args = args.join(", ");
                return format!("({args})");
            },
            _ => {},
        }
    }
    let (sn_pat, _) = snippet_with_context(cx, pat.span, span.ctxt(), "", app);
    sn_pat.into_owned()
}

/// Check whether an expression is divergent. May give false negatives.
fn expr_diverges(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
    struct V<'cx, 'tcx> {
        cx: &'cx LateContext<'tcx>,
        res: ControlFlow<(), Descend>,
    }
    impl<'tcx> Visitor<'tcx> for V<'_, '_> {
        fn visit_expr(&mut self, e: &'tcx Expr<'tcx>) {
            fn is_never(cx: &LateContext<'_>, expr: &'_ Expr<'_>) -> bool {
                if let Some(ty) = cx.typeck_results().expr_ty_opt(expr) {
                    return ty.is_never();
                }
                false
            }

            if self.res.is_break() {
                return;
            }

            // We can't just call is_never on expr and be done, because the type system
            // sometimes coerces the ! type to something different before we can get
            // our hands on it. So instead, we do a manual search. We do fall back to
            // is_never in some places when there is no better alternative.
            self.res = match e.kind {
                ExprKind::Continue(_) | ExprKind::Break(_, _) | ExprKind::Ret(_) => ControlFlow::Break(()),
                ExprKind::Call(call, _) => {
                    if is_never(self.cx, e) || is_never(self.cx, call) {
                        ControlFlow::Break(())
                    } else {
                        ControlFlow::Continue(Descend::Yes)
                    }
                },
                ExprKind::MethodCall(..) => {
                    if is_never(self.cx, e) {
                        ControlFlow::Break(())
                    } else {
                        ControlFlow::Continue(Descend::Yes)
                    }
                },
                ExprKind::If(if_expr, if_then, if_else) => {
                    let else_diverges = if_else.map_or(false, |ex| expr_diverges(self.cx, ex));
                    let diverges =
                        expr_diverges(self.cx, if_expr) || (else_diverges && expr_diverges(self.cx, if_then));
                    if diverges {
                        ControlFlow::Break(())
                    } else {
                        ControlFlow::Continue(Descend::No)
                    }
                },
                ExprKind::Match(match_expr, match_arms, _) => {
                    let diverges = expr_diverges(self.cx, match_expr)
                        || match_arms.iter().all(|arm| {
                            let guard_diverges = arm.guard.as_ref().map_or(false, |g| expr_diverges(self.cx, g.body()));
                            guard_diverges || expr_diverges(self.cx, arm.body)
                        });
                    if diverges {
                        ControlFlow::Break(())
                    } else {
                        ControlFlow::Continue(Descend::No)
                    }
                },

                // Don't continue into loops or labeled blocks, as they are breakable,
                // and we'd have to start checking labels.
                ExprKind::Block(_, Some(_)) | ExprKind::Loop(..) => ControlFlow::Continue(Descend::No),

                // Default: descend
                _ => ControlFlow::Continue(Descend::Yes),
            };
            if let ControlFlow::Continue(Descend::Yes) = self.res {
                walk_expr(self, e);
            }
        }

        fn visit_local(&mut self, local: &'tcx Local<'_>) {
            // Don't visit the else block of a let/else statement as it will not make
            // the statement divergent even though the else block is divergent.
            if let Some(init) = local.init {
                self.visit_expr(init);
            }
        }

        // Avoid unnecessary `walk_*` calls.
        fn visit_ty(&mut self, _: &'tcx Ty<'tcx>) {}
        fn visit_pat(&mut self, _: &'tcx Pat<'tcx>) {}
        fn visit_qpath(&mut self, _: &'tcx QPath<'tcx>, _: HirId, _: Span) {}
        // Avoid monomorphising all `visit_*` functions.
        fn visit_nested_item(&mut self, _: ItemId) {}
    }

    let mut v = V {
        cx,
        res: ControlFlow::Continue(Descend::Yes),
    };
    expr.visit(&mut v);
    v.res.is_break()
}

fn pat_allowed_for_else(cx: &LateContext<'_>, pat: &'_ Pat<'_>, check_types: bool) -> bool {
    // Check whether the pattern contains any bindings, as the
    // binding might potentially be used in the body.
    // TODO: only look for *used* bindings.
    let mut has_bindings = false;
    pat.each_binding_or_first(&mut |_, _, _, _| has_bindings = true);
    if has_bindings {
        return false;
    }

    // If we shouldn't check the types, exit early.
    if !check_types {
        return true;
    }

    // Check whether any possibly "unknown" patterns are included,
    // because users might not know which values some enum has.
    // Well-known enums are excepted, as we assume people know them.
    // We do a deep check, to be able to disallow Err(En::Foo(_))
    // for usage of the En::Foo variant, as we disallow En::Foo(_),
    // but we allow Err(_).
    let typeck_results = cx.typeck_results();
    let mut has_disallowed = false;
    pat.walk_always(|pat| {
        // Only do the check if the type is "spelled out" in the pattern
        if !matches!(
            pat.kind,
            PatKind::Struct(..) | PatKind::TupleStruct(..) | PatKind::Path(..)
        ) {
            return;
        };
        let ty = typeck_results.pat_ty(pat);
        // Option and Result are allowed, everything else isn't.
        if !(is_type_diagnostic_item(cx, ty, sym::Option) || is_type_diagnostic_item(cx, ty, sym::Result)) {
            has_disallowed = true;
        }
    });
    !has_disallowed
}

/// Checks if the passed block is a simple identity referring to bindings created by the pattern,
/// and if yes, returns a mapping between the relevant sub-pattern and the identifier it corresponds
/// to.
///
/// We support patterns with multiple bindings and tuples, e.g.:
///
/// ```ignore
/// let (foo_o, bar_o) = if let (Some(foo), bar) = g() { (foo, bar) } else { ... }
/// ```
///
/// The expected params would be:
///
/// ```ignore
/// local_pat: (foo_o, bar_o)
/// let_pat: (Some(foo), bar)
/// expr: (foo, bar)
/// ```
///
/// We build internal `sub_pats` so that it looks like `[foo_o, bar_o]` and `paths` so that it looks
/// like `[foo, bar]`. Then we turn that into `FxHashMap [(foo) -> (foo_o), (bar) -> (bar_o)]` which
/// we return.
fn expr_simple_identity_map<'a, 'hir>(
    local_pat: &'a Pat<'hir>,
    let_pat: &'_ Pat<'hir>,
    expr: &'_ Expr<'hir>,
) -> Option<FxHashMap<Symbol, &'a Pat<'hir>>> {
    let peeled = peel_blocks(expr);
    let (sub_pats, paths) = match (local_pat.kind, peeled.kind) {
        (PatKind::Tuple(pats, _), ExprKind::Tup(exprs)) | (PatKind::Slice(pats, ..), ExprKind::Array(exprs)) => {
            (pats, exprs)
        },
        (_, ExprKind::Path(_)) => (slice::from_ref(local_pat), slice::from_ref(peeled)),
        _ => return None,
    };

    // There is some length mismatch, which indicates usage of .. in the patterns above e.g.:
    // let (a, ..) = if let [a, b, _c] = ex { (a, b) } else { ... };
    // We bail in these cases as they should be rare.
    if paths.len() != sub_pats.len() {
        return None;
    }

    let mut pat_bindings = FxHashSet::default();
    let_pat.each_binding_or_first(&mut |_ann, _hir_id, _sp, ident| {
        pat_bindings.insert(ident);
    });
    if pat_bindings.len() < paths.len() {
        // This rebinds some bindings from the outer scope, or it repeats some copy-able bindings multiple
        // times. We don't support these cases so we bail here. E.g.:
        // let foo = 0;
        // let (new_foo, bar, bar_copied) = if let Some(bar) = Some(0) { (foo, bar, bar) } else { .. };
        return None;
    }
    let mut ident_map = FxHashMap::default();
    for (sub_pat, path) in sub_pats.iter().zip(paths.iter()) {
        if let ExprKind::Path(QPath::Resolved(_ty, path)) = path.kind &&
            let [path_seg] = path.segments
        {
            let ident = path_seg.ident;
            if !pat_bindings.remove(&ident) {
                return None;
            }
            ident_map.insert(ident.name, sub_pat);
        } else {
            return None;
        }
    }
    Some(ident_map)
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Deserialize)]
pub enum MatchLintBehaviour {
    AllTypes,
    WellKnownTypes,
    Never,
}