Auto merge of #51316 - oli-obk:const_err, r=nikomatsakis

Refactor the const eval diagnostic API

* no longer report "const eval error" for things that have typeck errors
* errors and lints have saner spans and messages
* unified the diagnostic logic (const eval errors were slightly different depending on where they were reported, and there was also code duplication between the different reporters)
* report errors if an erroneous constant is used inside a promoted (fixes most of #50814)

Author: bors

src/librustc/ich/impls_ty.rs

@@ -510,6 +510,7 @@ impl_stable_hash_for!(struct ::middle::const_val::ConstEvalErr<'tcx> {
 
 impl_stable_hash_for!(struct ::middle::const_val::FrameInfo {
     span,
+    lint_root,
     location
 });
 
@@ -523,21 +524,11 @@ for ::middle::const_val::ErrKind<'gcx> {
         mem::discriminant(self).hash_stable(hcx, hasher);
 
         match *self {
-            NonConstPath |
             TypeckError |
+            CouldNotResolve |
             CheckMatchError => {
                 // nothing to do
             }
-            UnimplementedConstVal(s) => {
-                s.hash_stable(hcx, hasher);
-            }
-            IndexOutOfBounds { len, index } => {
-                len.hash_stable(hcx, hasher);
-                index.hash_stable(hcx, hasher);
-            }
-            LayoutError(ref layout_error) => {
-                layout_error.hash_stable(hcx, hasher);
-            }
             Miri(ref err, ref trace) => {
                 err.hash_stable(hcx, hasher);
                 trace.hash_stable(hcx, hasher);
@@ -608,8 +599,8 @@ for ::mir::interpret::EvalErrorKind<'gcx, O> {
             RemainderByZero |
             DivisionByZero |
             GeneratorResumedAfterReturn |
-            GeneratorResumedAfterPanic |
-            ReferencedConstant => {}
+            GeneratorResumedAfterPanic => {}
+            ReferencedConstant(ref err) => err.hash_stable(hcx, hasher),
             MachineError(ref err) => err.hash_stable(hcx, hasher),
             FunctionPointerTyMismatch(a, b) => {
                 a.hash_stable(hcx, hasher);

src/librustc/lint/context.rs

@@ -655,6 +655,9 @@ impl<'a, 'tcx> LateContext<'a, 'tcx> {
         f(self);
         self.param_env = old_param_env;
     }
+    pub fn current_lint_root(&self) -> ast::NodeId {
+        self.last_ast_node_with_lint_attrs
+    }
 }
 
 impl<'a, 'tcx> LayoutOf for &'a LateContext<'a, 'tcx> {

src/librustc/middle/const_val.rs

@@ -9,13 +9,15 @@
 // except according to those terms.
 
 use hir::def_id::DefId;
-use ty::{self, TyCtxt, layout};
+use ty;
 use ty::subst::Substs;
+use ty::maps::TyCtxtAt;
 use mir::interpret::ConstValue;
 use errors::DiagnosticBuilder;
 
 use graphviz::IntoCow;
 use syntax_pos::Span;
+use syntax::ast;
 
 use std::borrow::Cow;
 use rustc_data_structures::sync::Lrc;
@@ -28,30 +30,26 @@ pub enum ConstVal<'tcx> {
     Value(ConstValue<'tcx>),
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
 pub struct ConstEvalErr<'tcx> {
     pub span: Span,
     pub kind: Lrc<ErrKind<'tcx>>,
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
 pub enum ErrKind<'tcx> {
 
-    NonConstPath,
-    UnimplementedConstVal(&'static str),
-    IndexOutOfBounds { len: u64, index: u64 },
-
-    LayoutError(layout::LayoutError<'tcx>),
-
+    CouldNotResolve,
     TypeckError,
     CheckMatchError,
     Miri(::mir::interpret::EvalError<'tcx>, Vec<FrameInfo>),
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
 pub struct FrameInfo {
     pub span: Span,
     pub location: String,
+    pub lint_root: Option<ast::NodeId>,
 }
 
 #[derive(Clone, Debug)]
@@ -83,81 +81,98 @@ impl<'a, 'gcx, 'tcx> ConstEvalErr<'tcx> {
         }
 
         match *self.kind {
-            NonConstPath        => simple!("non-constant path in constant expression"),
-            UnimplementedConstVal(what) =>
-                simple!("unimplemented constant expression: {}", what),
-            IndexOutOfBounds { len, index } => {
-                simple!("index out of bounds: the len is {} but the index is {}",
-                        len, index)
-            }
-
-            LayoutError(ref err) => Simple(err.to_string().into_cow()),
-
+            CouldNotResolve => simple!("could not resolve"),
             TypeckError => simple!("type-checking failed"),
             CheckMatchError => simple!("match-checking failed"),
             Miri(ref err, ref trace) => Backtrace(err, trace),
         }
     }
 
     pub fn struct_error(&self,
-        tcx: TyCtxt<'a, 'gcx, 'tcx>,
-        primary_span: Span,
-        primary_kind: &str)
-        -> DiagnosticBuilder<'gcx>
+        tcx: TyCtxtAt<'a, 'gcx, 'tcx>,
+        message: &str)
+        -> Option<DiagnosticBuilder<'tcx>>
     {
-        let mut diag = struct_error(tcx, self.span, "constant evaluation error");
-        self.note(tcx, primary_span, primary_kind, &mut diag);
-        diag
+        self.struct_generic(tcx, message, None, true)
     }
 
-    pub fn note(&self,
-        _tcx: TyCtxt<'a, 'gcx, 'tcx>,
-        primary_span: Span,
-        primary_kind: &str,
-        diag: &mut DiagnosticBuilder)
-    {
-        match self.description() {
-            ConstEvalErrDescription::Simple(message) => {
-                diag.span_label(self.span, message);
-            }
-            ConstEvalErrDescription::Backtrace(miri, frames) => {
-                diag.span_label(self.span, format!("{}", miri));
-                for frame in frames {
-                    diag.span_label(frame.span, format!("inside call to `{}`", frame.location));
-                }
-            }
+    pub fn report_as_error(&self,
+        tcx: TyCtxtAt<'a, 'gcx, 'tcx>,
+        message: &str
+    ) {
+        let err = self.struct_generic(tcx, message, None, true);
+        if let Some(mut err) = err {
+            err.emit();
         }
+    }
 
-        if !primary_span.contains(self.span) {
-            diag.span_note(primary_span,
-                        &format!("for {} here", primary_kind));
+    pub fn report_as_lint(&self,
+        tcx: TyCtxtAt<'a, 'gcx, 'tcx>,
+        message: &str,
+        lint_root: ast::NodeId,
+    ) {
+        let lint = self.struct_generic(
+            tcx,
+            message,
+            Some(lint_root),
+            false,
+        );
+        if let Some(mut lint) = lint {
+            lint.emit();
         }
     }
 
-    pub fn report(&self,
-        tcx: TyCtxt<'a, 'gcx, 'tcx>,
-        primary_span: Span,
-        primary_kind: &str)
-    {
-        match *self.kind {
-            ErrKind::TypeckError | ErrKind::CheckMatchError => return,
-            ErrKind::Miri(ref miri, _) => {
+    fn struct_generic(
+        &self,
+        tcx: TyCtxtAt<'a, 'gcx, 'tcx>,
+        message: &str,
+        lint_root: Option<ast::NodeId>,
+        as_err: bool,
+    ) -> Option<DiagnosticBuilder<'tcx>> {
+        let (msg, frames): (_, &[_]) = match *self.kind {
+            ErrKind::TypeckError | ErrKind::CheckMatchError => return None,
+            ErrKind::Miri(ref miri, ref frames) => {
                 match miri.kind {
                     ::mir::interpret::EvalErrorKind::TypeckError |
-                    ::mir::interpret::EvalErrorKind::Layout(_) => return,
-                    _ => {},
+                    ::mir::interpret::EvalErrorKind::Layout(_) => return None,
+                    ::mir::interpret::EvalErrorKind::ReferencedConstant(ref inner) => {
+                        inner.struct_generic(tcx, "referenced constant", lint_root, as_err)?.emit();
+                        (miri.to_string(), frames)
+                    },
+                    _ => (miri.to_string(), frames),
                 }
             }
-            _ => {}
+            _ => (self.description().into_oneline().to_string(), &[]),
+        };
+        trace!("reporting const eval failure at {:?}", self.span);
+        let mut err = if as_err {
+            struct_error(tcx, message)
+        } else {
+            let node_id = frames
+                .iter()
+                .rev()
+                .filter_map(|frame| frame.lint_root)
+                .next()
+                .or(lint_root)
+                .expect("some part of a failing const eval must be local");
+            tcx.struct_span_lint_node(
+                ::rustc::lint::builtin::CONST_ERR,
+                node_id,
+                tcx.span,
+                message,
+            )
+        };
+        err.span_label(self.span, msg);
+        for FrameInfo { span, location, .. } in frames {
+            err.span_label(*span, format!("inside call to `{}`", location));
         }
-        self.struct_error(tcx, primary_span, primary_kind).emit();
+        Some(err)
     }
 }
 
 pub fn struct_error<'a, 'gcx, 'tcx>(
-    tcx: TyCtxt<'a, 'gcx, 'tcx>,
-    span: Span,
+    tcx: TyCtxtAt<'a, 'gcx, 'tcx>,
     msg: &str,
-) -> DiagnosticBuilder<'gcx> {
-    struct_span_err!(tcx.sess, span, E0080, "{}", msg)
+) -> DiagnosticBuilder<'tcx> {
+    struct_span_err!(tcx.sess, tcx.span, E0080, "{}", msg)
 }

src/librustc/mir/interpret/error.rs

@@ -1,6 +1,7 @@
 use std::{fmt, env};
 
 use mir;
+use middle::const_val::ConstEvalErr;
 use ty::{FnSig, Ty, layout};
 use ty::layout::{Size, Align};
 
@@ -10,21 +11,50 @@ use super::{
 
 use backtrace::Backtrace;
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, RustcEncodable, RustcDecodable)]
 pub struct EvalError<'tcx> {
     pub kind: EvalErrorKind<'tcx, u64>,
-    pub backtrace: Option<Backtrace>,
 }
 
 impl<'tcx> From<EvalErrorKind<'tcx, u64>> for EvalError<'tcx> {
     fn from(kind: EvalErrorKind<'tcx, u64>) -> Self {
-        let backtrace = match env::var("MIRI_BACKTRACE") {
-            Ok(ref val) if !val.is_empty() => Some(Backtrace::new_unresolved()),
-            _ => None
-        };
+        match env::var("MIRI_BACKTRACE") {
+            Ok(ref val) if !val.is_empty() => {
+                let backtrace = Backtrace::new();
+
+                use std::fmt::Write;
+                let mut trace_text = "\n\nAn error occurred in miri:\n".to_string();
+                write!(trace_text, "backtrace frames: {}\n", backtrace.frames().len()).unwrap();
+                'frames: for (i, frame) in backtrace.frames().iter().enumerate() {
+                    if frame.symbols().is_empty() {
+                        write!(trace_text, "{}: no symbols\n", i).unwrap();
+                    }
+                    for symbol in frame.symbols() {
+                        write!(trace_text, "{}: ", i).unwrap();
+                        if let Some(name) = symbol.name() {
+                            write!(trace_text, "{}\n", name).unwrap();
+                        } else {
+                            write!(trace_text, "<unknown>\n").unwrap();
+                        }
+                        write!(trace_text, "\tat ").unwrap();
+                        if let Some(file_path) = symbol.filename() {
+                            write!(trace_text, "{}", file_path.display()).unwrap();
+                        } else {
+                            write!(trace_text, "<unknown_file>").unwrap();
+                        }
+                        if let Some(line) = symbol.lineno() {
+                            write!(trace_text, ":{}\n", line).unwrap();
+                        } else {
+                            write!(trace_text, "\n").unwrap();
+                        }
+                    }
+                }
+                error!("{}", trace_text);
+            },
+            _ => {},
+        }
         EvalError {
             kind,
-            backtrace,
         }
     }
 }
@@ -122,7 +152,7 @@ pub enum EvalErrorKind<'tcx, O> {
     TypeckError,
     /// Cannot compute this constant because it depends on another one
     /// which already produced an error
-    ReferencedConstant,
+    ReferencedConstant(ConstEvalErr<'tcx>),
     GeneratorResumedAfterReturn,
     GeneratorResumedAfterPanic,
 }
@@ -238,7 +268,7 @@ impl<'tcx, O> EvalErrorKind<'tcx, O> {
                 "there were unresolved type arguments during trait selection",
             TypeckError =>
                 "encountered constants with type errors, stopping evaluation",
-            ReferencedConstant =>
+            ReferencedConstant(_) =>
                 "referenced constant has errors",
             Overflow(mir::BinOp::Add) => "attempt to add with overflow",
             Overflow(mir::BinOp::Sub) => "attempt to subtract with overflow",

src/librustc/traits/error_reporting.rs

@@ -827,10 +827,13 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             }
 
             ConstEvalFailure(ref err) => {
-                if let ::middle::const_val::ErrKind::TypeckError = *err.kind {
-                    return;
+                match err.struct_error(
+                    self.tcx.at(span),
+                    "could not evaluate constant expression",
+                ) {
+                    Some(err) => err,
+                    None => return,
                 }
-                err.struct_error(self.tcx, span, "constant expression")
             }
 
             Overflow => {

src/librustc/traits/fulfill.rs

@@ -534,8 +534,7 @@ fn process_predicate<'a, 'gcx, 'tcx>(
                             } else {
                                 Err(CodeSelectionError(ConstEvalFailure(ConstEvalErr {
                                     span: obligation.cause.span,
-                                    kind: ErrKind::UnimplementedConstVal("could not resolve")
-                                        .into(),
+                                    kind: ErrKind::CouldNotResolve.into(),
                                 })))
                             }
                         },

src/librustc/ty/mod.rs

@@ -2075,15 +2075,17 @@ impl<'a, 'gcx, 'tcx> AdtDef {
                 } else {
                     info!("invalid enum discriminant: {:#?}", val);
                     ::middle::const_val::struct_error(
-                        tcx,
-                        tcx.def_span(expr_did),
+                        tcx.at(tcx.def_span(expr_did)),
                         "constant evaluation of enum discriminant resulted in non-integer",
                     ).emit();
                     None
                 }
             }
             Err(err) => {
-                err.report(tcx, tcx.def_span(expr_did), "enum discriminant");
+                err.report_as_error(
+                    tcx.at(tcx.def_span(expr_did)),
+                    "could not evaluate enum discriminant",
+                );
                 if !expr_did.is_local() {
                     span_bug!(tcx.def_span(expr_did),
                         "variant discriminant evaluation succeeded \