4 files changed, 621 insertions, 2 deletions

diff --git a/crates/cfg/Cargo.toml b/crates/cfg/Cargo.toml
index a6785ee8e..c68e391c1 100644
--- a/crates/cfg/Cargo.toml
+++ b/crates/cfg/Cargo.toml
@@ -17,3 +17,4 @@ tt = { path = "../tt", version = "0.0.0" }
 [dev-dependencies]
 mbe = { path = "../mbe" }
 syntax = { path = "../syntax" }
+expect-test = "1.0"
diff --git a/crates/cfg/src/cfg_expr.rs b/crates/cfg/src/cfg_expr.rs
index db3655b74..3f12a3fa8 100644
--- a/crates/cfg/src/cfg_expr.rs
+++ b/crates/cfg/src/cfg_expr.rs
@@ -2,12 +2,12 @@
 //!
 //! See: https://doc.rust-lang.org/reference/conditional-compilation.html#conditional-compilation
 
-use std::slice::Iter as SliceIter;
+use std::{fmt, slice::Iter as SliceIter};
 
 use tt::SmolStr;
 
 /// A simple configuration value passed in from the outside.
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash, Ord, PartialOrd)]
 pub enum CfgAtom {
     /// eg. `#[cfg(test)]`
     Flag(SmolStr),
@@ -18,6 +18,37 @@ pub enum CfgAtom {
     KeyValue { key: SmolStr, value: SmolStr },
 }
 
+impl CfgAtom {
+    /// Returns `true` when the atom comes from the target specification.
+    ///
+    /// If this returns `true`, then changing this atom requires changing the compilation target. If
+    /// it returns `false`, the atom might come from a build script or the build system.
+    pub fn is_target_defined(&self) -> bool {
+        match self {
+            CfgAtom::Flag(flag) => matches!(&**flag, "unix" | "windows"),
+            CfgAtom::KeyValue { key, value: _ } => matches!(
+                &**key,
+                "target_arch"
+                    | "target_os"
+                    | "target_env"
+                    | "target_family"
+                    | "target_endian"
+                    | "target_pointer_width"
+                    | "target_vendor" // NOTE: `target_feature` is left out since it can be configured via `-Ctarget-feature`
+            ),
+        }
+    }
+}
+
+impl fmt::Display for CfgAtom {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            CfgAtom::Flag(name) => write!(f, "{}", name),
+            CfgAtom::KeyValue { key, value } => write!(f, "{} = \"{}\"", key, value),
+        }
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum CfgExpr {
     Invalid,
diff --git a/crates/cfg/src/dnf.rs b/crates/cfg/src/dnf.rs
new file mode 100644
index 000000000..35f946e6f
--- /dev/null
+++ b/crates/cfg/src/dnf.rs
@@ -0,0 +1,477 @@
+//! Disjunctive Normal Form construction.
+//!
+//! Algorithm from <https://www.cs.drexel.edu/~jjohnson/2015-16/fall/CS270/Lectures/3/dnf.pdf>,
+//! which would have been much easier to read if it used pattern matching. It's also missing the
+//! entire "distribute ANDs over ORs" part, which is not trivial. Oh well.
+//!
+//! This is currently both messy and inefficient. Feel free to improve, there are unit tests.
+
+use std::fmt;
+
+use rustc_hash::FxHashSet;
+
+use crate::{CfgAtom, CfgDiff, CfgExpr, CfgOptions, InactiveReason};
+
+/// A `#[cfg]` directive in Disjunctive Normal Form (DNF).
+pub struct DnfExpr {
+    conjunctions: Vec<Conjunction>,
+}
+
+impl DnfExpr {
+    pub fn new(expr: CfgExpr) -> Self {
+        let builder = Builder { expr: DnfExpr { conjunctions: Vec::new() } };
+
+        builder.lower(expr.clone())
+    }
+
+    /// Computes a list of present or absent atoms in `opts` that cause this expression to evaluate
+    /// to `false`.
+    ///
+    /// Note that flipping a subset of these atoms might be sufficient to make the whole expression
+    /// evaluate to `true`. For that, see `compute_enable_hints`.
+    ///
+    /// Returns `None` when `self` is already true, or contains errors.
+    pub fn why_inactive(&self, opts: &CfgOptions) -> Option<InactiveReason> {
+        let mut res = InactiveReason { enabled: Vec::new(), disabled: Vec::new() };
+
+        for conj in &self.conjunctions {
+            let mut conj_is_true = true;
+            for lit in &conj.literals {
+                let atom = lit.var.as_ref()?;
+                let enabled = opts.enabled.contains(atom);
+                if lit.negate == enabled {
+                    // Literal is false, but needs to be true for this conjunction.
+                    conj_is_true = false;
+
+                    if enabled {
+                        res.enabled.push(atom.clone());
+                    } else {
+                        res.disabled.push(atom.clone());
+                    }
+                }
+            }
+
+            if conj_is_true {
+                // This expression is not actually inactive.
+                return None;
+            }
+        }
+
+        res.enabled.sort_unstable();
+        res.enabled.dedup();
+        res.disabled.sort_unstable();
+        res.disabled.dedup();
+        Some(res)
+    }
+
+    /// Returns `CfgDiff` objects that would enable this directive if applied to `opts`.
+    pub fn compute_enable_hints<'a>(
+        &'a self,
+        opts: &'a CfgOptions,
+    ) -> impl Iterator<Item = CfgDiff> + 'a {
+        // A cfg is enabled if any of `self.conjunctions` evaluate to `true`.
+
+        self.conjunctions.iter().filter_map(move |conj| {
+            let mut enable = FxHashSet::default();
+            let mut disable = FxHashSet::default();
+            for lit in &conj.literals {
+                let atom = lit.var.as_ref()?;
+                let enabled = opts.enabled.contains(atom);
+                if lit.negate && enabled {
+                    disable.insert(atom.clone());
+                }
+                if !lit.negate && !enabled {
+                    enable.insert(atom.clone());
+                }
+            }
+
+            // Check that this actually makes `conj` true.
+            for lit in &conj.literals {
+                let atom = lit.var.as_ref()?;
+                let enabled = enable.contains(atom)
+                    || (opts.enabled.contains(atom) && !disable.contains(atom));
+                if enabled == lit.negate {
+                    return None;
+                }
+            }
+
+            if enable.is_empty() && disable.is_empty() {
+                return None;
+            }
+
+            let mut diff = CfgDiff {
+                enable: enable.into_iter().collect(),
+                disable: disable.into_iter().collect(),
+            };
+
+            // Undo the FxHashMap randomization for consistent output.
+            diff.enable.sort_unstable();
+            diff.disable.sort_unstable();
+
+            Some(diff)
+        })
+    }
+}
+
+impl fmt::Display for DnfExpr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.conjunctions.len() != 1 {
+            write!(f, "any(")?;
+        }
+        for (i, conj) in self.conjunctions.iter().enumerate() {
+            if i != 0 {
+                f.write_str(", ")?;
+            }
+
+            write!(f, "{}", conj)?;
+        }
+        if self.conjunctions.len() != 1 {
+            write!(f, ")")?;
+        }
+
+        Ok(())
+    }
+}
+
+struct Conjunction {
+    literals: Vec<Literal>,
+}
+
+impl Conjunction {
+    fn new(parts: Vec<CfgExpr>) -> Self {
+        let mut literals = Vec::new();
+        for part in parts {
+            match part {
+                CfgExpr::Invalid | CfgExpr::Atom(_) | CfgExpr::Not(_) => {
+                    literals.push(Literal::new(part));
+                }
+                CfgExpr::All(conj) => {
+                    // Flatten.
+                    literals.extend(Conjunction::new(conj).literals);
+                }
+                CfgExpr::Any(_) => unreachable!("disjunction in conjunction"),
+            }
+        }
+
+        Self { literals }
+    }
+}
+
+impl fmt::Display for Conjunction {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.literals.len() != 1 {
+            write!(f, "all(")?;
+        }
+        for (i, lit) in self.literals.iter().enumerate() {
+            if i != 0 {
+                f.write_str(", ")?;
+            }
+
+            write!(f, "{}", lit)?;
+        }
+        if self.literals.len() != 1 {
+            write!(f, ")")?;
+        }
+
+        Ok(())
+    }
+}
+
+struct Literal {
+    negate: bool,
+    var: Option<CfgAtom>, // None = Invalid
+}
+
+impl Literal {
+    fn new(expr: CfgExpr) -> Self {
+        match expr {
+            CfgExpr::Invalid => Self { negate: false, var: None },
+            CfgExpr::Atom(atom) => Self { negate: false, var: Some(atom) },
+            CfgExpr::Not(expr) => match *expr {
+                CfgExpr::Invalid => Self { negate: true, var: None },
+                CfgExpr::Atom(atom) => Self { negate: true, var: Some(atom) },
+                _ => unreachable!("non-atom {:?}", expr),
+            },
+            CfgExpr::Any(_) | CfgExpr::All(_) => unreachable!("non-literal {:?}", expr),
+        }
+    }
+}
+
+impl fmt::Display for Literal {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.negate {
+            write!(f, "not(")?;
+        }
+
+        match &self.var {
+            Some(var) => write!(f, "{}", var)?,
+            None => f.write_str("<invalid>")?,
+        }
+
+        if self.negate {
+            write!(f, ")")?;
+        }
+
+        Ok(())
+    }
+}
+
+struct Builder {
+    expr: DnfExpr,
+}
+
+impl Builder {
+    fn lower(mut self, expr: CfgExpr) -> DnfExpr {
+        let expr = make_nnf(expr);
+        let expr = make_dnf(expr);
+
+        match expr {
+            CfgExpr::Invalid | CfgExpr::Atom(_) | CfgExpr::Not(_) => {
+                self.expr.conjunctions.push(Conjunction::new(vec![expr]));
+            }
+            CfgExpr::All(conj) => {
+                self.expr.conjunctions.push(Conjunction::new(conj));
+            }
+            CfgExpr::Any(mut disj) => {
+                disj.reverse();
+                while let Some(conj) = disj.pop() {
+                    match conj {
+                        CfgExpr::Invalid | CfgExpr::Atom(_) | CfgExpr::All(_) | CfgExpr::Not(_) => {
+                            self.expr.conjunctions.push(Conjunction::new(vec![conj]));
+                        }
+                        CfgExpr::Any(inner_disj) => {
+                            // Flatten.
+                            disj.extend(inner_disj.into_iter().rev());
+                        }
+                    }
+                }
+            }
+        }
+
+        self.expr
+    }
+}
+
+fn make_dnf(expr: CfgExpr) -> CfgExpr {
+    match expr {
+        CfgExpr::Invalid | CfgExpr::Atom(_) | CfgExpr::Not(_) => expr,
+        CfgExpr::Any(e) => CfgExpr::Any(e.into_iter().map(|expr| make_dnf(expr)).collect()),
+        CfgExpr::All(e) => {
+            let e = e.into_iter().map(|expr| make_nnf(expr)).collect::<Vec<_>>();
+
+            CfgExpr::Any(distribute_conj(&e))
+        }
+    }
+}
+
+/// Turns a conjunction of expressions into a disjunction of expressions.
+fn distribute_conj(conj: &[CfgExpr]) -> Vec<CfgExpr> {
+    fn go(out: &mut Vec<CfgExpr>, with: &mut Vec<CfgExpr>, rest: &[CfgExpr]) {
+        match rest {
+            [head, tail @ ..] => match head {
+                CfgExpr::Any(disj) => {
+                    for part in disj {
+                        with.push(part.clone());
+                        go(out, with, tail);
+                        with.pop();
+                    }
+                }
+                _ => {
+                    with.push(head.clone());
+                    go(out, with, tail);
+                    with.pop();
+                }
+            },
+            _ => {
+                // Turn accumulated parts into a new conjunction.
+                out.push(CfgExpr::All(with.clone()));
+            }
+        }
+    }
+
+    let mut out = Vec::new();
+    let mut with = Vec::new();
+
+    go(&mut out, &mut with, conj);
+
+    out
+}
+
+fn make_nnf(expr: CfgExpr) -> CfgExpr {
+    match expr {
+        CfgExpr::Invalid | CfgExpr::Atom(_) => expr,
+        CfgExpr::Any(expr) => CfgExpr::Any(expr.into_iter().map(|expr| make_nnf(expr)).collect()),
+        CfgExpr::All(expr) => CfgExpr::All(expr.into_iter().map(|expr| make_nnf(expr)).collect()),
+        CfgExpr::Not(operand) => match *operand {
+            CfgExpr::Invalid | CfgExpr::Atom(_) => CfgExpr::Not(operand.clone()), // Original negated expr
+            CfgExpr::Not(expr) => {
+                // Remove double negation.
+                make_nnf(*expr)
+            }
+            // Convert negated conjunction/disjunction using DeMorgan's Law.
+            CfgExpr::Any(inner) => CfgExpr::All(
+                inner.into_iter().map(|expr| make_nnf(CfgExpr::Not(Box::new(expr)))).collect(),
+            ),
+            CfgExpr::All(inner) => CfgExpr::Any(
+                inner.into_iter().map(|expr| make_nnf(CfgExpr::Not(Box::new(expr)))).collect(),
+            ),
+        },
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use expect_test::{expect, Expect};
+    use mbe::ast_to_token_tree;
+    use syntax::{ast, AstNode};
+
+    use super::*;
+
+    fn check_dnf(input: &str, expect: Expect) {
+        let (tt, _) = {
+            let source_file = ast::SourceFile::parse(input).ok().unwrap();
+            let tt = source_file.syntax().descendants().find_map(ast::TokenTree::cast).unwrap();
+            ast_to_token_tree(&tt).unwrap()
+        };
+        let cfg = CfgExpr::parse(&tt);
+        let actual = format!("#![cfg({})]", DnfExpr::new(cfg));
+        expect.assert_eq(&actual);
+    }
+
+    fn check_why_inactive(input: &str, opts: &CfgOptions, expect: Expect) {
+        let (tt, _) = {
+            let source_file = ast::SourceFile::parse(input).ok().unwrap();
+            let tt = source_file.syntax().descendants().find_map(ast::TokenTree::cast).unwrap();
+            ast_to_token_tree(&tt).unwrap()
+        };
+        let cfg = CfgExpr::parse(&tt);
+        let dnf = DnfExpr::new(cfg);
+        let why_inactive = dnf.why_inactive(opts).unwrap().to_string();
+        expect.assert_eq(&why_inactive);
+    }
+
+    #[track_caller]
+    fn check_enable_hints(input: &str, opts: &CfgOptions, expected_hints: &[&str]) {
+        let (tt, _) = {
+            let source_file = ast::SourceFile::parse(input).ok().unwrap();
+            let tt = source_file.syntax().descendants().find_map(ast::TokenTree::cast).unwrap();
+            ast_to_token_tree(&tt).unwrap()
+        };
+        let cfg = CfgExpr::parse(&tt);
+        let dnf = DnfExpr::new(cfg);
+        let hints = dnf.compute_enable_hints(opts).map(|diff| diff.to_string()).collect::<Vec<_>>();
+        assert_eq!(hints, expected_hints);
+    }
+
+    #[test]
+    fn smoke() {
+        check_dnf("#![cfg(test)]", expect![[r#"#![cfg(test)]"#]]);
+        check_dnf("#![cfg(not(test))]", expect![[r#"#![cfg(not(test))]"#]]);
+        check_dnf("#![cfg(not(not(test)))]", expect![[r#"#![cfg(test)]"#]]);
+
+        check_dnf("#![cfg(all(a, b))]", expect![[r#"#![cfg(all(a, b))]"#]]);
+        check_dnf("#![cfg(any(a, b))]", expect![[r#"#![cfg(any(a, b))]"#]]);
+
+        check_dnf("#![cfg(not(a))]", expect![[r#"#![cfg(not(a))]"#]]);
+    }
+
+    #[test]
+    fn distribute() {
+        check_dnf("#![cfg(all(any(a, b), c))]", expect![[r#"#![cfg(any(all(a, c), all(b, c)))]"#]]);
+        check_dnf("#![cfg(all(c, any(a, b)))]", expect![[r#"#![cfg(any(all(c, a), all(c, b)))]"#]]);
+        check_dnf(
+            "#![cfg(all(any(a, b), any(c, d)))]",
+            expect![[r#"#![cfg(any(all(a, c), all(a, d), all(b, c), all(b, d)))]"#]],
+        );
+
+        check_dnf(
+            "#![cfg(all(any(a, b, c), any(d, e, f), g))]",
+            expect![[
+                r#"#![cfg(any(all(a, d, g), all(a, e, g), all(a, f, g), all(b, d, g), all(b, e, g), all(b, f, g), all(c, d, g), all(c, e, g), all(c, f, g)))]"#
+            ]],
+        );
+    }
+
+    #[test]
+    fn demorgan() {
+        check_dnf("#![cfg(not(all(a, b)))]", expect![[r#"#![cfg(any(not(a), not(b)))]"#]]);
+        check_dnf("#![cfg(not(any(a, b)))]", expect![[r#"#![cfg(all(not(a), not(b)))]"#]]);
+
+        check_dnf("#![cfg(not(all(not(a), b)))]", expect![[r#"#![cfg(any(a, not(b)))]"#]]);
+        check_dnf("#![cfg(not(any(a, not(b))))]", expect![[r#"#![cfg(all(not(a), b))]"#]]);
+    }
+
+    #[test]
+    fn nested() {
+        check_dnf(
+            "#![cfg(all(any(a), not(all(any(b)))))]",
+            expect![[r#"#![cfg(all(a, not(b)))]"#]],
+        );
+
+        check_dnf("#![cfg(any(any(a, b)))]", expect![[r#"#![cfg(any(a, b))]"#]]);
+        check_dnf("#![cfg(not(any(any(a, b))))]", expect![[r#"#![cfg(all(not(a), not(b)))]"#]]);
+        check_dnf("#![cfg(all(all(a, b)))]", expect![[r#"#![cfg(all(a, b))]"#]]);
+        check_dnf("#![cfg(not(all(all(a, b))))]", expect![[r#"#![cfg(any(not(a), not(b)))]"#]]);
+    }
+
+    #[test]
+    fn hints() {
+        let mut opts = CfgOptions::default();
+
+        check_enable_hints("#![cfg(test)]", &opts, &["enable test"]);
+        check_enable_hints("#![cfg(not(test))]", &opts, &[]);
+
+        check_enable_hints("#![cfg(any(a, b))]", &opts, &["enable a", "enable b"]);
+        check_enable_hints("#![cfg(any(b, a))]", &opts, &["enable b", "enable a"]);
+
+        check_enable_hints("#![cfg(all(a, b))]", &opts, &["enable a and b"]);
+
+        opts.insert_atom("test".into());
+
+        check_enable_hints("#![cfg(test)]", &opts, &[]);
+        check_enable_hints("#![cfg(not(test))]", &opts, &["disable test"]);
+    }
+
+    /// Tests that we don't suggest hints for cfgs that express an inconsistent formula.
+    #[test]
+    fn hints_impossible() {
+        let mut opts = CfgOptions::default();
+
+        check_enable_hints("#![cfg(all(test, not(test)))]", &opts, &[]);
+
+        opts.insert_atom("test".into());
+
+        check_enable_hints("#![cfg(all(test, not(test)))]", &opts, &[]);
+    }
+
+    #[test]
+    fn why_inactive() {
+        let mut opts = CfgOptions::default();
+        opts.insert_atom("test".into());
+        opts.insert_atom("test2".into());
+
+        check_why_inactive("#![cfg(a)]", &opts, expect![["a is disabled"]]);
+        check_why_inactive("#![cfg(not(test))]", &opts, expect![["test is enabled"]]);
+
+        check_why_inactive(
+            "#![cfg(all(not(test), not(test2)))]",
+            &opts,
+            expect![["test and test2 are enabled"]],
+        );
+        check_why_inactive(
+            "#![cfg(all(not(test), a))]",
+            &opts,
+            expect![["test is enabled and a is disabled"]],
+        );
+        check_why_inactive(
+            "#![cfg(all(not(test), test2, a))]",
+            &opts,
+            expect![["test is enabled and a is disabled"]],
+        );
+        check_why_inactive(
+            "#![cfg(all(not(test), not(test2), a))]",
+            &opts,
+            expect![["test and test2 are enabled and a is disabled"]],
+        );
+    }
+}
diff --git a/crates/cfg/src/lib.rs b/crates/cfg/src/lib.rs
index 35f540ac3..0b0734213 100644
--- a/crates/cfg/src/lib.rs
+++ b/crates/cfg/src/lib.rs
@@ -1,11 +1,15 @@
 //! cfg defines conditional compiling options, `cfg` attibute parser and evaluator
 
 mod cfg_expr;
+mod dnf;
+
+use std::fmt;
 
 use rustc_hash::FxHashSet;
 use tt::SmolStr;
 
 pub use cfg_expr::{CfgAtom, CfgExpr};
+pub use dnf::DnfExpr;
 
 /// Configuration options used for conditional compilition on items with `cfg` attributes.
 /// We have two kind of options in different namespaces: atomic options like `unix`, and
@@ -40,4 +44,110 @@ impl CfgOptions {
             self.enabled.insert(atom.clone());
         }
     }
+
+    pub fn apply_diff(&mut self, diff: CfgDiff) {
+        for atom in diff.enable {
+            self.enabled.insert(atom);
+        }
+
+        for atom in diff.disable {
+            self.enabled.remove(&atom);
+        }
+    }
+}
+
+pub struct CfgDiff {
+    // Invariants: No duplicates, no atom that's both in `enable` and `disable`.
+    enable: Vec<CfgAtom>,
+    disable: Vec<CfgAtom>,
+}
+
+impl CfgDiff {
+    /// Returns the total number of atoms changed by this diff.
+    pub fn len(&self) -> usize {
+        self.enable.len() + self.disable.len()
+    }
+}
+
+impl fmt::Display for CfgDiff {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if !self.enable.is_empty() {
+            f.write_str("enable ")?;
+            for (i, atom) in self.enable.iter().enumerate() {
+                let sep = match i {
+                    0 => "",
+                    _ if i == self.enable.len() - 1 => " and ",
+                    _ => ", ",
+                };
+                f.write_str(sep)?;
+
+                write!(f, "{}", atom)?;
+            }
+
+            if !self.disable.is_empty() {
+                f.write_str("; ")?;
+            }
+        }
+
+        if !self.disable.is_empty() {
+            f.write_str("disable ")?;
+            for (i, atom) in self.disable.iter().enumerate() {
+                let sep = match i {
+                    0 => "",
+                    _ if i == self.enable.len() - 1 => " and ",
+                    _ => ", ",
+                };
+                f.write_str(sep)?;
+
+                write!(f, "{}", atom)?;
+            }
+        }
+
+        Ok(())
+    }
+}
+
+pub struct InactiveReason {
+    enabled: Vec<CfgAtom>,
+    disabled: Vec<CfgAtom>,
+}
+
+impl fmt::Display for InactiveReason {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if !self.enabled.is_empty() {
+            for (i, atom) in self.enabled.iter().enumerate() {
+                let sep = match i {
+                    0 => "",
+                    _ if i == self.enabled.len() - 1 => " and ",
+                    _ => ", ",
+                };
+                f.write_str(sep)?;
+
+                write!(f, "{}", atom)?;
+            }
+            let is_are = if self.enabled.len() == 1 { "is" } else { "are" };
+            write!(f, " {} enabled", is_are)?;
+
+            if !self.disabled.is_empty() {
+                f.write_str(" and ")?;
+            }
+        }
+
+        if !self.disabled.is_empty() {
+            for (i, atom) in self.disabled.iter().enumerate() {
+                let sep = match i {
+                    0 => "",
+                    _ if i == self.enabled.len() - 1 => " and ",
+                    _ => ", ",
+                };
+                f.write_str(sep)?;
+
+                write!(f, "{}", atom)?;
+            }
+            let is_are = if self.disabled.len() == 1 { "is" } else { "are" };
+            write!(f, " {} disabled", is_are)?;
+        }
+
+        Ok(())
+    }
 }