"soft" (warn instead of error) feature-gate for #[must_use] on functions

Before `#[must_use]` for functions was implemented, a `#[must_use]` attribute
on a function was a no-op. To avoid a breaking change in this behavior, we add
an option for "this-and-such feature is experimental" feature-gate messages to
be a mere warning rather than a compilation-halting failure (so old code that
used to have a useless no-op `#[must_use]` attribute now warns rather than
breaking). When we're on stable, we add a help note to clarify that the feature
isn't "on."

This is in support of #43302.

src/libsyntax/feature_gate.rs

@@ -918,20 +918,27 @@ struct Context<'a> {
 }
 
 macro_rules! gate_feature_fn {
-    ($cx: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr) => {{
+    ($cx: expr, $has_feature: expr, $span: expr, $name: expr, $explain: expr, $level: expr) => {{
-        let (cx, has_feature, span, name, explain) = ($cx, $has_feature, $span, $name, $explain);
+        let (cx, has_feature, span,
+             name, explain, level) = ($cx, $has_feature, $span, $name, $explain, $level);
         let has_feature: bool = has_feature(&$cx.features);
         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", name, span, has_feature);
         if !has_feature && !span.allows_unstable() {
-            emit_feature_err(cx.parse_sess, name, span, GateIssue::Language, explain);
+            leveled_feature_err(cx.parse_sess, name, span, GateIssue::Language, explain, level)
+                .emit();
         }
     }}
 }
 
 macro_rules! gate_feature {
     ($cx: expr, $feature: ident, $span: expr, $explain: expr) => {
-        gate_feature_fn!($cx, |x:&Features| x.$feature, $span, stringify!($feature), $explain)
+        gate_feature_fn!($cx, |x:&Features| x.$feature, $span,
-    }
+                         stringify!($feature), $explain, GateStrength::Hard)
+    };
+    ($cx: expr, $feature: ident, $span: expr, $explain: expr, $level: expr) => {
+        gate_feature_fn!($cx, |x:&Features| x.$feature, $span,
+                         stringify!($feature), $explain, $level)
+    };
 }
 
 impl<'a> Context<'a> {
@@ -941,7 +948,7 @@ impl<'a> Context<'a> {
         for &(n, ty, ref gateage) in BUILTIN_ATTRIBUTES {
             if name == n {
                 if let Gated(_, name, desc, ref has_feature) = *gateage {
-                    gate_feature_fn!(self, has_feature, attr.span, name, desc);
+                    gate_feature_fn!(self, has_feature, attr.span, name, desc, GateStrength::Hard);
                 }
                 debug!("check_attribute: {:?} is builtin, {:?}, {:?}", attr.path, ty, gateage);
                 return;
@@ -1011,6 +1018,14 @@ pub enum GateIssue {
     Library(Option<u32>)
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum GateStrength {
+    /// A hard error. (Most feature gates should use this.)
+    Hard,
+    /// Only a warning. (Use this only as backwards-compatibility demands.)
+    Soft,
+}
+
 pub fn emit_feature_err(sess: &ParseSess, feature: &str, span: Span, issue: GateIssue,
                         explain: &str) {
     feature_err(sess, feature, span, issue, explain).emit();
@@ -1018,6 +1033,11 @@ pub fn emit_feature_err(sess: &ParseSess, feature: &str, span: Span, issue: Gate
 
 pub fn feature_err<'a>(sess: &'a ParseSess, feature: &str, span: Span, issue: GateIssue,
                        explain: &str) -> DiagnosticBuilder<'a> {
+    leveled_feature_err(sess, feature, span, issue, explain, GateStrength::Hard)
+}
+
+fn leveled_feature_err<'a>(sess: &'a ParseSess, feature: &str, span: Span, issue: GateIssue,
+                           explain: &str, level: GateStrength) -> DiagnosticBuilder<'a> {
     let diag = &sess.span_diagnostic;
 
     let issue = match issue {
@@ -1025,10 +1045,15 @@ pub fn feature_err<'a>(sess: &'a ParseSess, feature: &str, span: Span, issue: Ga
         GateIssue::Library(lib) => lib,
     };
 
-    let mut err = if let Some(n) = issue {
+    let explanation = if let Some(n) = issue {
-        diag.struct_span_err(span, &format!("{} (see issue #{})", explain, n))
+        format!("{} (see issue #{})", explain, n)
     } else {
-        diag.struct_span_err(span, explain)
+        explain.to_owned()
+    };
+
+    let mut err = match level {
+        GateStrength::Hard => diag.struct_span_err(span, &explanation),
+        GateStrength::Soft => diag.struct_span_warn(span, &explanation),
     };
 
     // #23973: do not suggest `#![feature(...)]` if we are in beta/stable
@@ -1038,7 +1063,15 @@ pub fn feature_err<'a>(sess: &'a ParseSess, feature: &str, span: Span, issue: Ga
                           feature));
     }
 
+    // If we're on stable and only emitting a "soft" warning, add a note to
+    // clarify that the feature isn't "on" (rather than being on but
+    // warning-worthy).
+    if !sess.unstable_features.is_nightly_build() && level == GateStrength::Soft {
+        err.help("a nightly build of the compiler is required to enable this feature");
+    }
+
     err
+
 }
 
 const EXPLAIN_BOX_SYNTAX: &'static str =
@@ -1095,6 +1128,12 @@ macro_rules! gate_feature_post {
         if !span.allows_unstable() {
             gate_feature!(cx.context, $feature, span, $explain)
         }
+    }};
+    ($cx: expr, $feature: ident, $span: expr, $explain: expr, $level: expr) => {{
+        let (cx, span) = ($cx, $span);
+        if !span.allows_unstable() {
+            gate_feature!(cx.context, $feature, span, $explain, $level)
+        }
     }}
 }
 
@@ -1239,7 +1278,8 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                 }
                 if attr::contains_name(&i.attrs[..], "must_use") {
                     gate_feature_post!(&self, fn_must_use, i.span,
-                                       "`#[must_use]` on functions is experimental");
+                                       "`#[must_use]` on functions is experimental",
+                                       GateStrength::Soft);
                 }
             }
 

src/test/compile-fail/feature-gate-fn_must_use.rs

@@ -8,7 +8,17 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#[must_use]
+#![feature(rustc_attrs)]
-fn need_to_use_it() -> bool { true } //~ ERROR `#[must_use]` on functions is experimental
 
-fn main() {}
+#[must_use]
+fn need_to_use_it() -> bool { true } //~ WARN `#[must_use]` on functions is experimental
+
+
+// Feature gates are tidy-required to have a specially named (or
+// comment-annotated) compile-fail test (which MUST fail), but for
+// backwards-compatibility reasons, we want `#[must_use]` on functions to be
+// compilable even if the `fn_must_use` feature is absent, thus necessitating
+// the usage of `#[rustc_error]` here, pragmatically if awkwardly solving this
+// dilemma until a superior solution can be devised.
+#[rustc_error]
+fn main() {} //~ ERROR compilation successful

src/test/compile-fail/feature-gate/issue-43106-gating-of-builtin-attrs.rs

@@ -680,7 +680,7 @@ mod must_use {
     mod inner { #![must_use="1400"] }
 
     #[must_use = "1400"] fn f() { }
-    //~^ ERROR `#[must_use]` on functions is experimental
+    //~^ WARN `#[must_use]` on functions is experimental
 
     #[must_use = "1400"] struct S;