rust-clippy/clippy_lints/src/format.rs

use crate::utils::paths;
use crate::utils::{
    in_macro_or_desugar, is_expn_of, last_path_segment, match_def_path, match_type, resolve_node, snippet,
    span_lint_and_then, walk_ptrs_ty,
};
use if_chain::if_chain;
use rustc::hir::*;
use rustc::lint::{LateContext, LateLintPass, LintArray, LintContext, LintPass};
use rustc::ty;
use rustc::{declare_lint_pass, declare_tool_lint};
use rustc_errors::Applicability;
use syntax::ast::LitKind;
use syntax::source_map::Span;

declare_clippy_lint! {
    /// **What it does:** Checks for the use of `format!("string literal with no
    /// argument")` and `format!("{}", foo)` where `foo` is a string.
    ///
    /// **Why is this bad?** There is no point of doing that. `format!("foo")` can
    /// be replaced by `"foo".to_owned()` if you really need a `String`. The even
    /// worse `&format!("foo")` is often encountered in the wild. `format!("{}",
    /// foo)` can be replaced by `foo.clone()` if `foo: String` or `foo.to_owned()`
    /// if `foo: &str`.
    ///
    /// **Known problems:** None.
    ///
    /// **Examples:**
    /// ```rust
    /// format!("foo")
    /// format!("{}", foo)
    /// ```
    pub USELESS_FORMAT,
    complexity,
    "useless use of `format!`"
}

declare_lint_pass!(UselessFormat => [USELESS_FORMAT]);

impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UselessFormat {
    fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
        if let Some(span) = is_expn_of(expr.span, "format") {
            if in_macro_or_desugar(span) {
                return;
            }
            match expr.node {
                // `format!("{}", foo)` expansion
                ExprKind::Call(ref fun, ref args) => {
                    if_chain! {
                        if let ExprKind::Path(ref qpath) = fun.node;
                        if let Some(fun_def_id) = resolve_node(cx, qpath, fun.hir_id).opt_def_id();
                        let new_v1 = match_def_path(cx, fun_def_id, &paths::FMT_ARGUMENTS_NEWV1);
                        let new_v1_fmt = match_def_path(cx,
                            fun_def_id,
                            &paths::FMT_ARGUMENTS_NEWV1FORMATTED
                        );
                        if new_v1 || new_v1_fmt;
                        if check_single_piece(&args[0]);
                        if let Some(format_arg) = get_single_string_arg(cx, &args[1]);
                        if new_v1 || check_unformatted(&args[2]);
                        if let ExprKind::AddrOf(_, ref format_arg) = format_arg.node;
                        then {
                            let (message, sugg) = if_chain! {
                                if let ExprKind::MethodCall(ref path, _, _) = format_arg.node;
                                if path.ident.as_interned_str().as_symbol() == sym!(to_string);
                                then {
                                    ("`to_string()` is enough",
                                    snippet(cx, format_arg.span, "<arg>").to_string())
                                } else {
                                    ("consider using .to_string()",
                                    format!("{}.to_string()", snippet(cx, format_arg.span, "<arg>")))
                                }
                            };

                            span_useless_format(cx, span, message, sugg);
                        }
                    }
                },
                // `format!("foo")` expansion contains `match () { () => [], }`
                ExprKind::Match(ref matchee, _, _) => {
                    if let ExprKind::Tup(ref tup) = matchee.node {
                        if tup.is_empty() {
                            let actual_snippet = snippet(cx, expr.span, "<expr>").to_string();
                            let actual_snippet = actual_snippet.replace("{{}}", "{}");
                            let sugg = format!("{}.to_string()", actual_snippet);
                            span_useless_format(cx, span, "consider using .to_string()", sugg);
                        }
                    }
                },
                _ => (),
            }
        }
    }
}

fn span_useless_format<'a, 'tcx: 'a, T: LintContext<'tcx>>(cx: &'a T, span: Span, help: &str, mut sugg: String) {
    let to_replace = span.source_callsite();

    // The callsite span contains the statement semicolon for some reason.
    let snippet = snippet(cx, to_replace, "..");
    if snippet.ends_with(';') {
        sugg.push(';');
    }

    span_lint_and_then(cx, USELESS_FORMAT, span, "useless use of `format!`", |db| {
        db.span_suggestion(
            to_replace,
            help,
            sugg,
            Applicability::MachineApplicable, // snippet
        );
    });
}

/// Checks if the expressions matches `&[""]`
fn check_single_piece(expr: &Expr) -> bool {
    if_chain! {
        if let ExprKind::AddrOf(_, ref expr) = expr.node; // &[""]
        if let ExprKind::Array(ref exprs) = expr.node; // [""]
        if exprs.len() == 1;
        if let ExprKind::Lit(ref lit) = exprs[0].node;
        if let LitKind::Str(ref lit, _) = lit.node;
        then {
            return lit.as_str().is_empty();
        }
    }

    false
}

/// Checks if the expressions matches
/// ```rust,ignore
/// &match (&"arg",) {
/// (__arg0,) => [::std::fmt::ArgumentV1::new(__arg0,
/// ::std::fmt::Display::fmt)],
/// }
/// ```
/// and that the type of `__arg0` is `&str` or `String`,
/// then returns the span of first element of the matched tuple.
fn get_single_string_arg<'a>(cx: &LateContext<'_, '_>, expr: &'a Expr) -> Option<&'a Expr> {
    if_chain! {
        if let ExprKind::AddrOf(_, ref expr) = expr.node;
        if let ExprKind::Match(ref match_expr, ref arms, _) = expr.node;
        if arms.len() == 1;
        if arms[0].pats.len() == 1;
        if let PatKind::Tuple(ref pat, None) = arms[0].pats[0].node;
        if pat.len() == 1;
        if let ExprKind::Array(ref exprs) = arms[0].body.node;
        if exprs.len() == 1;
        if let ExprKind::Call(_, ref args) = exprs[0].node;
        if args.len() == 2;
        if let ExprKind::Path(ref qpath) = args[1].node;
        if let Some(fun_def_id) = resolve_node(cx, qpath, args[1].hir_id).opt_def_id();
        if match_def_path(cx, fun_def_id, &paths::DISPLAY_FMT_METHOD);
        then {
            let ty = walk_ptrs_ty(cx.tables.pat_ty(&pat[0]));
            if ty.sty == ty::Str || match_type(cx, ty, &paths::STRING) {
                if let ExprKind::Tup(ref values) = match_expr.node {
                    return Some(&values[0]);
                }
            }
        }
    }

    None
}

/// Checks if the expression matches
/// ```rust,ignore
/// &[_ {
///    format: _ {
///         width: _::Implied,
///         ...
///    },
///    ...,
/// }]
/// ```
fn check_unformatted(expr: &Expr) -> bool {
    if_chain! {
        if let ExprKind::AddrOf(_, ref expr) = expr.node;
        if let ExprKind::Array(ref exprs) = expr.node;
        if exprs.len() == 1;
        if let ExprKind::Struct(_, ref fields, _) = exprs[0].node;
        if let Some(format_field) = fields.iter().find(|f| f.ident.name == sym!(format));
        if let ExprKind::Struct(_, ref fields, _) = format_field.expr.node;
        if let Some(width_field) = fields.iter().find(|f| f.ident.name == sym!(width));
        if let ExprKind::Path(ref width_qpath) = width_field.expr.node;
        if last_path_segment(width_qpath).ident.name == sym!(Implied);
        if let Some(precision_field) = fields.iter().find(|f| f.ident.name == sym!(precision));
        if let ExprKind::Path(ref precision_path) = precision_field.expr.node;
        if last_path_segment(precision_path).ident.name == sym!(Implied);
        then {
            return true;
        }
    }

    false
}