error.rs

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

use rustc_errors::{DiagnosticBuilder, ErrorReported};
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,
};

/// 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 },
}

// 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.to_string()).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)
            }
        }
    }
}

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.kind, stacktrace, span: span.unwrap_or_else(|| ecx.cur_span()) }
    }

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

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

    pub fn report_as_lint(
        &self,
        tcx: TyCtxtAt<'tcx>,
        message: &str,
        lint_root: hir::HirId,
        span: Option<Span>,
    ) -> ErrorHandled {
        self.struct_generic(
            tcx,
            message,
            |mut lint: DiagnosticBuilder<'_>| {
                // 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, "");
                        }
                    }
                }
                lint.emit();
            },
            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 `emit` to do any final processing.
    /// It's the caller's responsibility to call emit(), stash(), etc. within the `emit`
    /// function to dispose of the diagnostic properly.
    ///
    /// 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.)
    fn struct_generic(
        &self,
        tcx: TyCtxtAt<'tcx>,
        message: &str,
        emit: impl FnOnce(DiagnosticBuilder<'_>),
        lint_root: Option<hir::HirId>,
    ) -> ErrorHandled {
        let must_error = match self.error {
            err_inval!(Layout(LayoutError::Unknown(_))) | err_inval!(TooGeneric) => {
                return ErrorHandled::TooGeneric;
            }
            err_inval!(TypeckError(error_reported)) => {
                return ErrorHandled::Reported(error_reported);
            }
            // We must *always* hard error on these, even if the caller wants just a lint.
            err_inval!(Layout(LayoutError::SizeOverflow(_))) => true,
            _ => false,
        };
        trace!("reporting const eval failure at {:?}", self.span);

        let err_msg = match &self.error {
            InterpError::MachineStop(msg) => {
                // A custom error (`ConstEvalErrKind` in `librustc_mir/interp/const_eval/error.rs`).
                // Should be turned into a string by now.
                msg.downcast_ref::<String>().expect("invalid MachineStop payload").clone()
            }
            err => err.to_string(),
        };

        let finish = |mut err: DiagnosticBuilder<'_>, span_msg: Option<String>| {
            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 {
                for frame_info in &self.stacktrace {
                    err.span_label(frame_info.span, frame_info.to_string());
                }
            }
            // Let the caller finish the job.
            emit(err)
        };

        if must_error {
            // 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>.
            finish(struct_error(tcx, &err_msg), None);
            ErrorHandled::Reported(ErrorReported)
        } else {
            // Regular case.
            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| finish(lint.build(message), Some(err_msg)),
                );
                ErrorHandled::Linted
            } else {
                // Report as hard error.
                finish(struct_error(tcx, message), Some(err_msg));
                ErrorHandled::Reported(ErrorReported)
            }
        }
    }
}