rust/compiler/rustc_const_eval/src/const_eval/error.rs

use std::error::Error;
use std::fmt;

use rustc_errors::Diagnostic;
use rustc_hir as hir;
use rustc_middle::mir::AssertKind;
use rustc_middle::ty::{layout::LayoutError, query::TyCtxtAt, ConstInt};
use rustc_span::{Span, Symbol};

use super::InterpCx;
use crate::interpret::{
    struct_error, ErrorHandled, FrameInfo, InterpError, InterpErrorInfo, Machine, MachineStopType,
};

/// The CTFE machine has some custom error kinds.
#[derive(Clone, Debug)]
pub enum ConstEvalErrKind {
    NeedsRfc(String),
    ConstAccessesStatic,
    ModifiedGlobal,
    AssertFailure(AssertKind<ConstInt>),
    Panic { msg: Symbol, line: u32, col: u32, file: Symbol },
    Abort(String),
}

impl MachineStopType for ConstEvalErrKind {
    fn is_hard_err(&self) -> bool {
        matches!(self, Self::Panic { .. })
    }
}

// The errors become `MachineStop` with plain strings when being raised.
// `ConstEvalErr` (in `librustc_middle/mir/interpret/error.rs`) knows to
// handle these.
impl<'tcx> Into<InterpErrorInfo<'tcx>> for ConstEvalErrKind {
    fn into(self) -> InterpErrorInfo<'tcx> {
        err_machine_stop!(self).into()
    }
}

impl fmt::Display for ConstEvalErrKind {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        use self::ConstEvalErrKind::*;
        match *self {
            NeedsRfc(ref msg) => {
                write!(f, "\"{}\" needs an rfc before being allowed inside constants", msg)
            }
            ConstAccessesStatic => write!(f, "constant accesses static"),
            ModifiedGlobal => {
                write!(f, "modifying a static's initial value from another static's initializer")
            }
            AssertFailure(ref msg) => write!(f, "{:?}", msg),
            Panic { msg, line, col, file } => {
                write!(f, "the evaluated program panicked at '{}', {}:{}:{}", msg, file, line, col)
            }
            Abort(ref msg) => write!(f, "{}", msg),
        }
    }
}

impl Error for ConstEvalErrKind {}

/// When const-evaluation errors, this type is constructed with the resulting information,
/// and then used to emit the error as a lint or hard error.
#[derive(Debug)]
pub struct ConstEvalErr<'tcx> {
    pub span: Span,
    pub error: InterpError<'tcx>,
    pub stacktrace: Vec<FrameInfo<'tcx>>,
}

impl<'tcx> ConstEvalErr<'tcx> {
    /// Turn an interpreter error into something to report to the user.
    /// As a side-effect, if RUSTC_CTFE_BACKTRACE is set, this prints the backtrace.
    /// Should be called only if the error is actually going to to be reported!
    pub fn new<'mir, M: Machine<'mir, 'tcx>>(
        ecx: &InterpCx<'mir, 'tcx, M>,
        error: InterpErrorInfo<'tcx>,
        span: Option<Span>,
    ) -> ConstEvalErr<'tcx>
    where
        'tcx: 'mir,
    {
        error.print_backtrace();
        let stacktrace = ecx.generate_stacktrace();
        ConstEvalErr {
            error: error.into_kind(),
            stacktrace,
            span: span.unwrap_or_else(|| ecx.cur_span()),
        }
    }

    pub fn struct_error(
        &self,
        tcx: TyCtxtAt<'tcx>,
        message: &str,
        decorate: impl FnOnce(&mut Diagnostic),
    ) -> ErrorHandled {
        self.struct_generic(tcx, message, decorate, None)
    }

    pub fn report_as_error(&self, tcx: TyCtxtAt<'tcx>, message: &str) -> ErrorHandled {
        self.struct_error(tcx, message, |_| {})
    }

    pub fn report_as_lint(
        &self,
        tcx: TyCtxtAt<'tcx>,
        message: &str,
        lint_root: hir::HirId,
        span: Option<Span>,
    ) -> ErrorHandled {
        self.struct_generic(
            tcx,
            message,
            |lint: &mut Diagnostic| {
                // Apply the span.
                if let Some(span) = span {
                    let primary_spans = lint.span.primary_spans().to_vec();
                    // point at the actual error as the primary span
                    lint.replace_span_with(span);
                    // point to the `const` statement as a secondary span
                    // they don't have any label
                    for sp in primary_spans {
                        if sp != span {
                            lint.span_label(sp, "");
                        }
                    }
                }
            },
            Some(lint_root),
        )
    }

    /// Create a diagnostic for this const eval error.
    ///
    /// Sets the message passed in via `message` and adds span labels with detailed error
    /// information before handing control back to `decorate` to do any final annotations,
    /// after which the diagnostic is emitted.
    ///
    /// If `lint_root.is_some()` report it as a lint, else report it as a hard error.
    /// (Except that for some errors, we ignore all that -- see `must_error` below.)
    #[instrument(skip(self, tcx, decorate, lint_root), level = "debug")]
    fn struct_generic(
        &self,
        tcx: TyCtxtAt<'tcx>,
        message: &str,
        decorate: impl FnOnce(&mut Diagnostic),
        lint_root: Option<hir::HirId>,
    ) -> ErrorHandled {
        let finish = |err: &mut Diagnostic, span_msg: Option<String>| {
            trace!("reporting const eval failure at {:?}", self.span);
            if let Some(span_msg) = span_msg {
                err.span_label(self.span, span_msg);
            }
            // Add spans for the stacktrace. Don't print a single-line backtrace though.
            if self.stacktrace.len() > 1 {
                // Helper closure to print duplicated lines.
                let mut flush_last_line = |last_frame, times| {
                    if let Some((line, span)) = last_frame {
                        err.span_label(span, &line);
                        // Don't print [... additional calls ...] if the number of lines is small
                        if times < 3 {
                            for _ in 0..times {
                                err.span_label(span, &line);
                            }
                        } else {
                            err.span_label(
                                span,
                                format!("[... {} additional calls {} ...]", times, &line),
                            );
                        }
                    }
                };

                let mut last_frame = None;
                let mut times = 0;
                for frame_info in &self.stacktrace {
                    let frame = (frame_info.to_string(), frame_info.span);
                    if last_frame.as_ref() == Some(&frame) {
                        times += 1;
                    } else {
                        flush_last_line(last_frame, times);
                        last_frame = Some(frame);
                        times = 0;
                    }
                }
                flush_last_line(last_frame, times);
            }
            // Let the caller attach any additional information it wants.
            decorate(err);
        };

        debug!("self.error: {:?}", self.error);
        // Special handling for certain errors
        match &self.error {
            // Don't emit a new diagnostic for these errors
            err_inval!(Layout(LayoutError::Unknown(_))) | err_inval!(TooGeneric) => {
                return ErrorHandled::TooGeneric;
            }
            err_inval!(AlreadyReported(error_reported)) => {
                return ErrorHandled::Reported(*error_reported);
            }
            err_inval!(Layout(LayoutError::SizeOverflow(_))) => {
                // We must *always* hard error on these, even if the caller wants just a lint.
                // The `message` makes little sense here, this is a more serious error than the
                // caller thinks anyway.
                // See <https://github.com/rust-lang/rust/pull/63152>.
                let mut err = struct_error(tcx, &self.error.to_string());
                finish(&mut err, None);
                return ErrorHandled::Reported(err.emit());
            }
            _ => {}
        };

        let err_msg = self.error.to_string();

        // Regular case - emit a lint.
        if let Some(lint_root) = lint_root {
            // Report as lint.
            let hir_id =
                self.stacktrace.iter().rev().find_map(|frame| frame.lint_root).unwrap_or(lint_root);
            tcx.struct_span_lint_hir(
                rustc_session::lint::builtin::CONST_ERR,
                hir_id,
                tcx.span,
                |lint| {
                    let mut lint = lint.build(message);
                    finish(&mut lint, Some(err_msg));
                    lint.emit();
                },
            );
            ErrorHandled::Linted
        } else {
            // Report as hard error.
            let mut err = struct_error(tcx, message);
            finish(&mut err, Some(err_msg));
            ErrorHandled::Reported(err.emit())
        }
    }
}