collapsible_if.rs

//! Checks for if expressions that contain only an if expression.
//!
//! For example, the lint would catch:
//!
//! ```rust,ignore
//! if x {
//!     if y {
//!         println!("Hello world");
//!     }
//! }
//! ```
//!
//! This lint is **warn** by default

use if_chain::if_chain;
use rustc_ast::ast;
use rustc_lint::{EarlyContext, EarlyLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};

use crate::utils::sugg::Sugg;
use crate::utils::{snippet_block, snippet_block_with_applicability, span_lint_and_sugg, span_lint_and_then};
use rustc_errors::Applicability;

declare_clippy_lint! {
    /// **What it does:** Checks for nested `if` statements which can be collapsed
    /// by `&&`-combining their conditions and for `else { if ... }` expressions
    /// that
    /// can be collapsed to `else if ...`.
    ///
    /// **Why is this bad?** Each `if`-statement adds one level of nesting, which
    /// makes code look more complex than it really is.
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust,ignore
    /// if x {
    ///     if y {
    ///         …
    ///     }
    /// }
    ///
    /// // or
    ///
    /// if x {
    ///     …
    /// } else {
    ///     if y {
    ///         …
    ///     }
    /// }
    /// ```
    ///
    /// Should be written:
    ///
    /// ```rust.ignore
    /// if x && y {
    ///     …
    /// }
    ///
    /// // or
    ///
    /// if x {
    ///     …
    /// } else if y {
    ///     …
    /// }
    /// ```
    pub COLLAPSIBLE_IF,
    style,
    "`if`s that can be collapsed (e.g., `if x { if y { ... } }` and `else { if x { ... } }`)"
}

declare_lint_pass!(CollapsibleIf => [COLLAPSIBLE_IF]);

impl EarlyLintPass for CollapsibleIf {
    fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr) {
        if !expr.span.from_expansion() {
            check_if(cx, expr)
        }
    }
}

fn check_if(cx: &EarlyContext<'_>, expr: &ast::Expr) {
    if let ast::ExprKind::If(check, then, else_) = &expr.kind {
        if let Some(else_) = else_ {
            check_collapsible_maybe_if_let(cx, else_);
        } else if let ast::ExprKind::Let(..) = check.kind {
            // Prevent triggering on `if let a = b { if c { .. } }`.
        } else {
            check_collapsible_no_if_let(cx, expr, check, then);
        }
    }
}

fn block_starts_with_comment(cx: &EarlyContext<'_>, expr: &ast::Block) -> bool {
    // We trim all opening braces and whitespaces and then check if the next string is a comment.
    let trimmed_block_text = snippet_block(cx, expr.span, "..", None)
        .trim_start_matches(|c: char| c.is_whitespace() || c == '{')
        .to_owned();
    trimmed_block_text.starts_with("//") || trimmed_block_text.starts_with("/*")
}

fn check_collapsible_maybe_if_let(cx: &EarlyContext<'_>, else_: &ast::Expr) {
    if_chain! {
        if let ast::ExprKind::Block(ref block, _) = else_.kind;
        if !block_starts_with_comment(cx, block);
        if let Some(else_) = expr_block(block);
        if !else_.span.from_expansion();
        if let ast::ExprKind::If(..) = else_.kind;
        then {
            let mut applicability = Applicability::MachineApplicable;
            span_lint_and_sugg(
                cx,
                COLLAPSIBLE_IF,
                block.span,
                "this `else { if .. }` block can be collapsed",
                "try",
                snippet_block_with_applicability(cx, else_.span, "..", Some(block.span), &mut applicability).into_owned(),
                applicability,
            );
        }
    }
}

fn check_collapsible_no_if_let(cx: &EarlyContext<'_>, expr: &ast::Expr, check: &ast::Expr, then: &ast::Block) {
    if_chain! {
        if !block_starts_with_comment(cx, then);
        if let Some(inner) = expr_block(then);
        if let ast::ExprKind::If(ref check_inner, ref content, None) = inner.kind;
        then {
            if let ast::ExprKind::Let(..) = check_inner.kind {
                // Prevent triggering on `if c { if let a = b { .. } }`.
                return;
            }

            if expr.span.ctxt() != inner.span.ctxt() {
                return;
            }
            span_lint_and_then(cx, COLLAPSIBLE_IF, expr.span, "this `if` statement can be collapsed", |diag| {
                let lhs = Sugg::ast(cx, check, "..");
                let rhs = Sugg::ast(cx, check_inner, "..");
                diag.span_suggestion(
                    expr.span,
                    "try",
                    format!(
                        "if {} {}",
                        lhs.and(&rhs),
                        snippet_block(cx, content.span, "..", Some(expr.span)),
                    ),
                    Applicability::MachineApplicable, // snippet
                );
            });
        }
    }
}

/// If the block contains only one expression, return it.
fn expr_block(block: &ast::Block) -> Option<&ast::Expr> {
    let mut it = block.stmts.iter();

    if let (Some(stmt), None) = (it.next(), it.next()) {
        match stmt.kind {
            ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => Some(expr),
            _ => None,
        }
    } else {
        None
    }
}