subscriber: warn if trying to enable a statically disabled level

This implements the warning in `tracing-subscriber` only,
as mentioned in tokio-rs#975 (comment).
It warns whenever directives are parsed, including programmatically and
through environment variables. It does not include the suggested new API
which returns the filters that wouldn't be parsed.

- List filters that would be ignored
- Mention 'static max level'
- Describe how to enable the logging

Example output:

```
$ RUST_LOG=off,debug,silenced[x]=trace cargo run -q
warning: some trace filter directives would enable traces that are disabled statically
 | `debug` would enable the DEBUG level for all targets
 | `silenced[x]=trace` would enable the TRACE level for the `silenced` target
 | the static max level is info
note: to enable DEBUG logging, remove the `max_level_info` feature
```

Fixes: tokio-rs#975

tracing-core/src/metadata.rs

@@ -94,7 +94,7 @@ pub struct Metadata<'a> {
 pub struct Kind(KindInner);
 
 /// Describes the level of verbosity of a span or event.
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub struct Level(LevelInner);
 
 /// A filter comparable to a verbosity `Level`.
@@ -107,7 +107,7 @@ pub struct Level(LevelInner);
 /// addition of an `OFF` level that completely disables all trace
 /// instrumentation.
 #[repr(transparent)]
-#[derive(Clone, Eq, PartialEq)]
+#[derive(Copy, Clone, Eq, PartialEq)]
 pub struct LevelFilter(Option<Level>);
 
 /// Indicates that a string could not be parsed to a valid level.

tracing-subscriber/Cargo.toml

@@ -24,7 +24,7 @@ keywords = ["logging", "tracing", "metrics", "subscriber"]
 [features]
 
 default = ["env-filter", "smallvec", "fmt", "ansi", "chrono", "tracing-log", "json"]
-env-filter = ["matchers", "regex", "lazy_static"]
+env-filter = ["matchers", "regex", "lazy_static", "tracing"]
 fmt = ["registry"]
 ansi = ["fmt", "ansi_term"]
 registry = ["sharded-slab", "thread_local"]
@@ -34,6 +34,7 @@ json = ["tracing-serde", "serde", "serde_json"]
 tracing-core = { path = "../tracing-core", version = "0.1.16" }
 
 # only required by the filter feature
+tracing = { optional = true, path = "../tracing", version = "0.1.16" }
 matchers = { optional = true, version = "0.0.1" }
 regex = { optional = true, version = "1", default-features = false, features = ["std"] }
 smallvec = { optional = true, version = "1" }

tracing-subscriber/src/filter/env/directive.rs

@@ -9,10 +9,10 @@ use tracing_core::{span, Level, Metadata};
 // TODO(eliza): add a builder for programmatically constructing directives?
 #[derive(Debug, Eq, PartialEq)]
 pub struct Directive {
-    target: Option<String>,
+    pub(crate) target: Option<String>,
     in_span: Option<String>,
     fields: FilterVec<field::Match>,
-    level: LevelFilter,
+    pub(crate) level: LevelFilter,
 }
 
 /// A directive which will statically enable or disable a given callsite.

tracing-subscriber/src/filter/env/mod.rs

@@ -242,6 +242,42 @@ impl EnvFilter {
     }
 
     fn from_directives(directives: impl IntoIterator<Item = Directive>) -> Self {
+        use tracing::level_filters::STATIC_MAX_LEVEL;
+        use tracing::Level;
+
+        let directives: Vec<_> = directives.into_iter().collect();
+
+        let disabled: Vec<_> = directives.iter()
+            .filter(|directive| directive.level > STATIC_MAX_LEVEL)
+            .collect();
+
+        if !disabled.is_empty() {
+            eprintln!("warning: some trace filter directives would enable traces that are disabled statically");
+            for directive in disabled {
+                let target = if let Some(target) = &directive.target {
+                    format!("the `{}` target", target)
+                } else {
+                    "all targets".into()
+                };
+                let level = directive.level.into_level().expect("=off would not have enabled any filters");
+                eprintln!(" | `{}` would enable the {} level for {}", directive, level, target);
+            }
+            eprintln!(" | the static max level is {}", STATIC_MAX_LEVEL);
+            let help_msg = || {
+                let (feature, filter) = match STATIC_MAX_LEVEL.into_level() {
+                    Some(Level::TRACE) => unreachable!("if the max level is trace, no static filtering features are enabled"),
+                    Some(Level::DEBUG) => ("max_level_debug", Level::TRACE),
+                    Some(Level::INFO) => ("max_level_info", Level::DEBUG),
+                    Some(Level::WARN) => ("max_level_warn", Level::INFO),
+                    Some(Level::ERROR) => ("max_level_error", Level::WARN),
+                    None => return ("max_level_off", String::new()),
+                };
+                (feature, format!("{} ", filter))
+            };
+            let (feature, earlier_level) = help_msg();
+            eprintln!("note: to enable {}logging, remove the `{}` feature", earlier_level, feature);
+        }
+
         let (dynamics, mut statics) = Directive::make_tables(directives);
         let has_dynamics = !dynamics.is_empty();