src/tools/clippy/clippy_lints/src/methods/from_iter_instead_of_collect.rs

use clippy_utils::diagnostics::span_lint_and_sugg;
use clippy_utils::source::snippet_opt;
use clippy_utils::ty::implements_trait;
use clippy_utils::{is_path_diagnostic_item, sugg};
use if_chain::if_chain;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_lint::LateContext;
use rustc_middle::ty::Ty;
use rustc_span::sym;

use super::FROM_ITER_INSTEAD_OF_COLLECT;

pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, args: &[hir::Expr<'_>], func: &hir::Expr<'_>) {
    if_chain! {
        if is_path_diagnostic_item(cx, func, sym::from_iter_fn);
        let ty = cx.typeck_results().expr_ty(expr);
        let arg_ty = cx.typeck_results().expr_ty(&args[0]);
        if let Some(iter_id) = cx.tcx.get_diagnostic_item(sym::Iterator);

        if implements_trait(cx, arg_ty, iter_id, &[]);
        then {
            // `expr` implements `FromIterator` trait
            let iter_expr = sugg::Sugg::hir(cx, &args[0], "..").maybe_par();
            let turbofish = extract_turbofish(cx, expr, ty);
            let sugg = format!("{iter_expr}.collect::<{turbofish}>()");
            span_lint_and_sugg(
                cx,
                FROM_ITER_INSTEAD_OF_COLLECT,
                expr.span,
                "usage of `FromIterator::from_iter`",
                "use `.collect()` instead of `::from_iter()`",
                sugg,
                Applicability::MaybeIncorrect,
            );
        }
    }
}

fn extract_turbofish(cx: &LateContext<'_>, expr: &hir::Expr<'_>, ty: Ty<'_>) -> String {
    fn strip_angle_brackets(s: &str) -> Option<&str> {
        s.strip_prefix('<')?.strip_suffix('>')
    }

    let call_site = expr.span.source_callsite();
    if_chain! {
        if let Some(snippet) = snippet_opt(cx, call_site);
        let snippet_split = snippet.split("::").collect::<Vec<_>>();
        if let Some((_, elements)) = snippet_split.split_last();

        then {
            if_chain! {
                if let [type_specifier, _] = snippet_split.as_slice();
                if let Some(type_specifier) = strip_angle_brackets(type_specifier);
                if let Some((type_specifier, ..)) = type_specifier.split_once(" as ");
                then {
                    type_specifier.to_string()
                } else {
                    // is there a type specifier? (i.e.: like `<u32>` in `collections::BTreeSet::<u32>::`)
                    if let Some(type_specifier) = snippet_split.iter().find(|e| strip_angle_brackets(e).is_some()) {
                        // remove the type specifier from the path elements
                        let without_ts = elements.iter().filter_map(|e| {
                            if e == type_specifier { None } else { Some((*e).to_string()) }
                        }).collect::<Vec<_>>();
                        // join and add the type specifier at the end (i.e.: `collections::BTreeSet<u32>`)
                        format!("{}{type_specifier}", without_ts.join("::"))
                    } else {
                        // type is not explicitly specified so wildcards are needed
                        // i.e.: 2 wildcards in `std::collections::BTreeMap<&i32, &char>`
                        let ty_str = ty.to_string();
                        let start = ty_str.find('<').unwrap_or(0);
                        let end = ty_str.find('>').unwrap_or(ty_str.len());
                        let nb_wildcard = ty_str[start..end].split(',').count();
                        let wildcards = format!("_{}", ", _".repeat(nb_wildcard - 1));
                        format!("{}<{wildcards}>", elements.join("::"))
                    }
                }
            }
        } else {
            ty.to_string()
        }
    }
}