rust-clippy/clippy_lints/src/explicit_write.rs

use clippy_utils::diagnostics::{span_lint, span_lint_and_sugg};
use clippy_utils::{is_expn_of, match_function_call, paths};
use if_chain::if_chain;
use rustc_ast::ast::LitKind;
use rustc_errors::Applicability;
use rustc_hir::{BorrowKind, Expr, ExprKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::sym;

declare_clippy_lint! {
    /// **What it does:** Checks for usage of `write!()` / `writeln()!` which can be
    /// replaced with `(e)print!()` / `(e)println!()`
    ///
    /// **Why is this bad?** Using `(e)println! is clearer and more concise
    ///
    /// **Known problems:** None.
    ///
    /// **Example:**
    /// ```rust
    /// # use std::io::Write;
    /// # let bar = "furchtbar";
    /// // this would be clearer as `eprintln!("foo: {:?}", bar);`
    /// writeln!(&mut std::io::stderr(), "foo: {:?}", bar).unwrap();
    /// ```
    pub EXPLICIT_WRITE,
    complexity,
    "using the `write!()` family of functions instead of the `print!()` family of functions, when using the latter would work"
}

declare_lint_pass!(ExplicitWrite => [EXPLICIT_WRITE]);

impl<'tcx> LateLintPass<'tcx> for ExplicitWrite {
    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
        if_chain! {
            // match call to unwrap
            if let ExprKind::MethodCall(unwrap_fun, _, unwrap_args, _) = expr.kind;
            if unwrap_fun.ident.name == sym::unwrap;
            // match call to write_fmt
            if !unwrap_args.is_empty();
            if let ExprKind::MethodCall(write_fun, _, write_args, _) =
                unwrap_args[0].kind;
            if write_fun.ident.name == sym!(write_fmt);
            // match calls to std::io::stdout() / std::io::stderr ()
            if !write_args.is_empty();
            if let Some(dest_name) = if match_function_call(cx, &write_args[0], &paths::STDOUT).is_some() {
                Some("stdout")
            } else if match_function_call(cx, &write_args[0], &paths::STDERR).is_some() {
                Some("stderr")
            } else {
                None
            };
            then {
                let write_span = unwrap_args[0].span;
                let calling_macro =
                    // ordering is important here, since `writeln!` uses `write!` internally
                    if is_expn_of(write_span, "writeln").is_some() {
                        Some("writeln")
                    } else if is_expn_of(write_span, "write").is_some() {
                        Some("write")
                    } else {
                        None
                    };
                let prefix = if dest_name == "stderr" {
                    "e"
                } else {
                    ""
                };

                // We need to remove the last trailing newline from the string because the
                // underlying `fmt::write` function doesn't know whether `println!` or `print!` was
                // used.
                if let Some(mut write_output) = write_output_string(write_args) {
                    if write_output.ends_with('\n') {
                        write_output.pop();
                    }

                    if let Some(macro_name) = calling_macro {
                        span_lint_and_sugg(
                            cx,
                            EXPLICIT_WRITE,
                            expr.span,
                            &format!(
                                "use of `{}!({}(), ...).unwrap()`",
                                macro_name,
                                dest_name
                            ),
                            "try this",
                            format!("{}{}!(\"{}\")", prefix, macro_name.replace("write", "print"), write_output.escape_default()),
                            Applicability::MachineApplicable
                        );
                    } else {
                        span_lint_and_sugg(
                            cx,
                            EXPLICIT_WRITE,
                            expr.span,
                            &format!("use of `{}().write_fmt(...).unwrap()`", dest_name),
                            "try this",
                            format!("{}print!(\"{}\")", prefix, write_output.escape_default()),
                            Applicability::MachineApplicable
                        );
                    }
                } else {
                    // We don't have a proper suggestion
                    if let Some(macro_name) = calling_macro {
                        span_lint(
                            cx,
                            EXPLICIT_WRITE,
                            expr.span,
                            &format!(
                                "use of `{}!({}(), ...).unwrap()`. Consider using `{}{}!` instead",
                                macro_name,
                                dest_name,
                                prefix,
                                macro_name.replace("write", "print")
                            )
                        );
                    } else {
                        span_lint(
                            cx,
                            EXPLICIT_WRITE,
                            expr.span,
                            &format!("use of `{}().write_fmt(...).unwrap()`. Consider using `{}print!` instead", dest_name, prefix),
                        );
                    }
                }

            }
        }
    }
}

// Extract the output string from the given `write_args`.
fn write_output_string(write_args: &[Expr<'_>]) -> Option<String> {
    if_chain! {
        // Obtain the string that should be printed
        if write_args.len() > 1;
        if let ExprKind::Call(_, output_args) = write_args[1].kind;
        if !output_args.is_empty();
        if let ExprKind::AddrOf(BorrowKind::Ref, _, output_string_expr) = output_args[0].kind;
        if let ExprKind::Array(string_exprs) = output_string_expr.kind;
        // we only want to provide an automatic suggestion for simple (non-format) strings
        if string_exprs.len() == 1;
        if let ExprKind::Lit(ref lit) = string_exprs[0].kind;
        if let LitKind::Str(ref write_output, _) = lit.node;
        then {
            return Some(write_output.to_string())
        }
    }
    None
}