warn about each skipped feature gate

Author: RalfJung

src/librustc_mir/transform/check_consts/validation.rs

@@ -253,13 +253,7 @@ impl Validator<'mir, 'tcx> {
         let is_unleashable = O::IS_SUPPORTED_IN_MIRI;
 
         if is_unleashable && self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
-            // Use `def_span` to deduplicate all warnings for the same const.
-            self.tcx.sess.span_warn(self.tcx.def_span(self.def_id), "skipping const checks");
-            if let Some(feature) = O::feature_gate() {
-                // We'd like to use `delay_span_bug` here, but we cannot as that ICEs
-                // before codegen has the chance to emit errors.  So we use a custom system instead.
-                self.tcx.sess.miri_unleashed_feature(feature);
-            }
+            self.tcx.sess.miri_unleashed_feature(span, O::feature_gate());
             return;
         }
 

src/librustc_session/session.rs

@@ -27,7 +27,6 @@ use rustc_target::spec::{PanicStrategy, RelocModel, RelroLevel, Target, TargetTr
 
 use std::cell::{self, RefCell};
 use std::env;
-use std::fmt::Write as _;
 use std::io::Write;
 use std::num::NonZeroU32;
 use std::path::PathBuf;
@@ -144,9 +143,11 @@ pub struct Session {
     /// and immediately printing the backtrace to stderr.
     pub ctfe_backtrace: Lock<CtfeBacktrace>,
 
-    /// This tracks whether `-Zunleash-the-miri-inside-of-you` was used to get around a
-    /// feature gate.  If yes, this file must fail to compile.
-    miri_unleashed_features: Lock<FxHashSet<Symbol>>,
+    /// This tracks where `-Zunleash-the-miri-inside-of-you` was used to get around a
+    /// const check, optionally with the relevant feature gate.  We use this to
+    /// warn about unleashing, but with a single diagnostic instead of dozens that
+    /// drown everything else in noise.
+    miri_unleashed_features: Lock<Vec<(Span, Option<Symbol>)>>,
 
     /// Base directory containing the `src/` for the Rust standard library, and
     /// potentially `rustc` as well, if we can can find it. Right now it's always
@@ -195,29 +196,34 @@ impl From<&'static lint::Lint> for DiagnosticMessageId {
 }
 
 impl Session {
-    pub fn miri_unleashed_feature(&self, s: Symbol) {
-        self.miri_unleashed_features.lock().insert(s);
+    pub fn miri_unleashed_feature(&self, span: Span, feature_gate: Option<Symbol>) {
+        self.miri_unleashed_features.lock().push((span, feature_gate));
     }
 
     fn check_miri_unleashed_features(&self) {
-        if !self.has_errors_or_delayed_span_bugs() {
-            let unleashed_features = self.miri_unleashed_features.lock();
-            if !unleashed_features.is_empty() {
-                // Join the strings (itertools has it but libstd does not...)
-                let mut list = String::new();
-                for feature in unleashed_features.iter() {
-                    if !list.is_empty() {
-                        list.push_str(", ");
-                    }
-                    write!(&mut list, "{}", feature).unwrap();
+        let unleashed_features = self.miri_unleashed_features.lock();
+        if !unleashed_features.is_empty() {
+            let mut must_err = false;
+            // Create a diagnostic pointing at where things got unleashed.
+            let mut diag = self.struct_warn("skipping const checks");
+            for &(span, feature_gate) in unleashed_features.iter() {
+                // FIXME: `span_label` doesn't do anything, so we use "help" as a hack.
+                if let Some(feature_gate) = feature_gate {
+                    diag.span_help(span, &format!("skipping check for `{}` feature", feature_gate));
+                    // The unleash flag must *not* be used to just "hack around" feature gates.
+                    must_err = true;
+                } else {
+                    diag.span_help(span, "skipping check that does not even have a feature gate");
                 }
+            }
+            diag.emit();
+            // If we should err, make sure we did.
+            if must_err && !self.has_errors() {
                 // We have skipped a feature gate, and not run into other errors... reject.
-                self.err(&format!(
+                self.err(
                     "`-Zunleash-the-miri-inside-of-you` may not be used to circumvent feature \
-                    gates, except when testing error paths in the CTFE engine.\n\
-                    The following feature flags are missing from this crate: {}",
-                    list,
-                ));
+                     gates, except when testing error paths in the CTFE engine"
+                );
             }
         }
     }

src/test/ui/consts/const-eval/const_fn_ptr.rs

@@ -8,15 +8,15 @@ const fn double_const(x: usize) -> usize { x * 2 }
 const X: fn(usize) -> usize = double;
 const X_CONST: fn(usize) -> usize = double_const;
 
-const fn bar(x: usize) -> usize { //~ WARNING skipping const checks
+const fn bar(x: usize) -> usize {
     X(x)
 }
 
-const fn bar_const(x: usize) -> usize { //~ WARNING skipping const checks
+const fn bar_const(x: usize) -> usize {
     X_CONST(x)
 }
 
-const fn foo(x: fn(usize) -> usize, y: usize)  -> usize { //~ WARNING skipping const checks
+const fn foo(x: fn(usize) -> usize, y: usize)  -> usize {
     x(y)
 }
 

src/test/ui/consts/const-eval/const_fn_ptr.stderr

@@ -1,26 +1,20 @@
 warning: skipping const checks
-  --> $DIR/const_fn_ptr.rs:11:1
    |
-LL | / const fn bar(x: usize) -> usize {
-LL | |     X(x)
-LL | | }
-   | |_^
-
-warning: skipping const checks
-  --> $DIR/const_fn_ptr.rs:15:1
+help: skipping check that does not even have a feature gate
+  --> $DIR/const_fn_ptr.rs:12:5
    |
-LL | / const fn bar_const(x: usize) -> usize {
-LL | |     X_CONST(x)
-LL | | }
-   | |_^
-
-warning: skipping const checks
-  --> $DIR/const_fn_ptr.rs:19:1
+LL |     X(x)
+   |     ^^^^
+help: skipping check that does not even have a feature gate
+  --> $DIR/const_fn_ptr.rs:16:5
+   |
+LL |     X_CONST(x)
+   |     ^^^^^^^^^^
+help: skipping check that does not even have a feature gate
+  --> $DIR/const_fn_ptr.rs:20:5
    |
-LL | / const fn foo(x: fn(usize) -> usize, y: usize)  -> usize {
-LL | |     x(y)
-LL | | }
-   | |_^
+LL |     x(y)
+   |     ^^^^
 
-warning: 3 warnings emitted
+warning: 1 warning emitted
 

src/test/ui/consts/const-eval/const_fn_ptr_fail.rs

@@ -6,7 +6,7 @@
 fn double(x: usize) -> usize { x * 2 }
 const X: fn(usize) -> usize = double;
 
-const fn bar(x: usize) -> usize { //~ WARNING skipping const checks
+const fn bar(x: usize) -> usize {
     X(x) // FIXME: this should error someday
 }
 

src/test/ui/consts/const-eval/const_fn_ptr_fail.stderr

@@ -1,10 +1,10 @@
 warning: skipping const checks
-  --> $DIR/const_fn_ptr_fail.rs:9:1
    |
-LL | / const fn bar(x: usize) -> usize {
-LL | |     X(x) // FIXME: this should error someday
-LL | | }
-   | |_^
+help: skipping check that does not even have a feature gate
+  --> $DIR/const_fn_ptr_fail.rs:10:5
+   |
+LL |     X(x) // FIXME: this should error someday
+   |     ^^^^
 
 warning: 1 warning emitted
 

src/test/ui/consts/const-eval/const_fn_ptr_fail2.rs

@@ -9,7 +9,7 @@ fn double(x: usize) -> usize {
 }
 const X: fn(usize) -> usize = double;
 
-const fn bar(x: fn(usize) -> usize, y: usize) -> usize { //~ WARN skipping const checks
+const fn bar(x: fn(usize) -> usize, y: usize) -> usize {
     x(y)
 }
 

src/test/ui/consts/const-eval/const_fn_ptr_fail2.stderr

@@ -1,11 +1,3 @@
-warning: skipping const checks
-  --> $DIR/const_fn_ptr_fail2.rs:12:1
-   |
-LL | / const fn bar(x: fn(usize) -> usize, y: usize) -> usize {
-LL | |     x(y)
-LL | | }
-   | |_^
-
 error[E0080]: evaluation of constant expression failed
   --> $DIR/const_fn_ptr_fail2.rs:20:5
    |
@@ -26,6 +18,14 @@ LL |     assert_eq!(Z, 4);
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
+warning: skipping const checks
+   |
+help: skipping check that does not even have a feature gate
+  --> $DIR/const_fn_ptr_fail2.rs:13:5
+   |
+LL |     x(y)
+   |     ^^^^
+
 error: aborting due to 2 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/const-points-to-static.rs

@@ -3,8 +3,9 @@
 #![allow(dead_code)]
 
 const TEST: &u8 = &MY_STATIC;
-//~^ skipping const checks
-//~| it is undefined behavior to use this value
+//~^ ERROR it is undefined behavior to use this value
+//~| NOTE  encountered a reference pointing to a static variable
+//~| NOTE
 
 static MY_STATIC: u8 = 4;
 

src/test/ui/consts/const-points-to-static.stderr

@@ -1,9 +1,3 @@
-warning: skipping const checks
-  --> $DIR/const-points-to-static.rs:5:1
-   |
-LL | const TEST: &u8 = &MY_STATIC;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/const-points-to-static.rs:5:1
    |
@@ -12,6 +6,14 @@ LL | const TEST: &u8 = &MY_STATIC;
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
 
+warning: skipping const checks
+   |
+help: skipping check that does not even have a feature gate
+  --> $DIR/const-points-to-static.rs:5:20
+   |
+LL | const TEST: &u8 = &MY_STATIC;
+   |                    ^^^^^^^^^
+
 error: aborting due to previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0080`.