8 files changed, 179 insertions, 0 deletions

diff --git a/crates/base_db/src/fixture.rs b/crates/base_db/src/fixture.rs
index 6ce377710..7d5d12e63 100644
--- a/crates/base_db/src/fixture.rs
+++ b/crates/base_db/src/fixture.rs
@@ -128,6 +128,7 @@ impl ChangeFixture {
                     file_id,
                     meta.edition,
                     Some(crate_name.clone().into()),
+                    meta.cfg.clone(),
                     meta.cfg,
                     meta.env,
                     Default::default(),
@@ -157,6 +158,7 @@ impl ChangeFixture {
                 crate_root,
                 Edition::Edition2018,
                 Some(CrateName::new("test").unwrap().into()),
+                default_cfg.clone(),
                 default_cfg,
                 Env::default(),
                 Default::default(),
@@ -186,6 +188,7 @@ impl ChangeFixture {
                 Edition::Edition2021,
                 Some(CrateDisplayName::from_canonical_name("core".to_string())),
                 CfgOptions::default(),
+                CfgOptions::default(),
                 Env::default(),
                 Vec::new(),
             );
diff --git a/crates/base_db/src/input.rs b/crates/base_db/src/input.rs
index 23cb0c839..0c51a59a0 100644
--- a/crates/base_db/src/input.rs
+++ b/crates/base_db/src/input.rs
@@ -189,6 +189,7 @@ pub struct CrateData {
     /// `Dependency` matters), this name should only be used for UI.
     pub display_name: Option<CrateDisplayName>,
     pub cfg_options: CfgOptions,
+    pub potential_cfg_options: CfgOptions,
     pub env: Env,
     pub dependencies: Vec<Dependency>,
     pub proc_macro: Vec<ProcMacro>,
@@ -219,6 +220,7 @@ impl CrateGraph {
         edition: Edition,
         display_name: Option<CrateDisplayName>,
         cfg_options: CfgOptions,
+        potential_cfg_options: CfgOptions,
         env: Env,
         proc_macro: Vec<ProcMacro>,
     ) -> CrateId {
@@ -227,6 +229,7 @@ impl CrateGraph {
             edition,
             display_name,
             cfg_options,
+            potential_cfg_options,
             env,
             proc_macro,
             dependencies: Vec::new(),
@@ -504,6 +507,7 @@ mod tests {
             Edition2018,
             None,
             CfgOptions::default(),
+            CfgOptions::default(),
             Env::default(),
             Default::default(),
         );
@@ -512,6 +516,7 @@ mod tests {
             Edition2018,
             None,
             CfgOptions::default(),
+            CfgOptions::default(),
             Env::default(),
             Default::default(),
         );
@@ -520,6 +525,7 @@ mod tests {
             Edition2018,
             None,
             CfgOptions::default(),
+            CfgOptions::default(),
             Env::default(),
             Default::default(),
         );
@@ -536,6 +542,7 @@ mod tests {
             Edition2018,
             None,
             CfgOptions::default(),
+            CfgOptions::default(),
             Env::default(),
             Default::default(),
         );
@@ -544,6 +551,7 @@ mod tests {
             Edition2018,
             None,
             CfgOptions::default(),
+            CfgOptions::default(),
             Env::default(),
             Default::default(),
         );
@@ -559,6 +567,7 @@ mod tests {
             Edition2018,
             None,
             CfgOptions::default(),
+            CfgOptions::default(),
             Env::default(),
             Default::default(),
         );
@@ -567,6 +576,7 @@ mod tests {
             Edition2018,
             None,
             CfgOptions::default(),
+            CfgOptions::default(),
             Env::default(),
             Default::default(),
         );
@@ -575,6 +585,7 @@ mod tests {
             Edition2018,
             None,
             CfgOptions::default(),
+            CfgOptions::default(),
             Env::default(),
             Default::default(),
         );
@@ -590,6 +601,7 @@ mod tests {
             Edition2018,
             None,
             CfgOptions::default(),
+            CfgOptions::default(),
             Env::default(),
             Default::default(),
         );
@@ -598,6 +610,7 @@ mod tests {
             Edition2018,
             None,
             CfgOptions::default(),
+            CfgOptions::default(),
             Env::default(),
             Default::default(),
         );
diff --git a/crates/cfg/src/lib.rs b/crates/cfg/src/lib.rs
index 916d39a0b..9a4baa636 100644
--- a/crates/cfg/src/lib.rs
+++ b/crates/cfg/src/lib.rs
@@ -50,6 +50,26 @@ impl CfgOptions {
             self.enabled.remove(&atom);
         }
     }
+
+    pub fn get_cfg_keys(&self) -> Vec<&SmolStr> {
+        self.enabled
+            .iter()
+            .map(|x| match x {
+                CfgAtom::Flag(key) => key,
+                CfgAtom::KeyValue { key, .. } => key,
+            })
+            .collect()
+    }
+
+    pub fn get_cfg_values(&self, cfg_key: &str) -> Vec<&SmolStr> {
+        self.enabled
+            .iter()
+            .filter_map(|x| match x {
+                CfgAtom::KeyValue { key, value } if cfg_key == key => Some(value),
+                _ => None,
+            })
+            .collect()
+    }
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index 88490fea9..30cc34403 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -233,6 +233,10 @@ impl Crate {
     pub fn cfg(&self, db: &dyn HirDatabase) -> CfgOptions {
         db.crate_graph()[self.id].cfg_options.clone()
     }
+
+    pub fn potential_cfg(&self, db: &dyn HirDatabase) -> CfgOptions {
+        db.crate_graph()[self.id].potential_cfg_options.clone()
+    }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
diff --git a/crates/ide/src/lib.rs b/crates/ide/src/lib.rs
index 3798f32cc..aac084012 100644
--- a/crates/ide/src/lib.rs
+++ b/crates/ide/src/lib.rs
@@ -217,6 +217,7 @@ impl Analysis {
             file_id,
             Edition::Edition2018,
             None,
+            cfg_options.clone(),
             cfg_options,
             Env::default(),
             Default::default(),
diff --git a/crates/ide_completion/src/completions/attribute.rs b/crates/ide_completion/src/completions/attribute.rs
index 78fc30e16..cc4f4b2af 100644
--- a/crates/ide_completion/src/completions/attribute.rs
+++ b/crates/ide_completion/src/completions/attribute.rs
@@ -15,6 +15,7 @@ use crate::{
     Completions,
 };
 
+mod cfg;
 mod derive;
 mod lint;
 mod repr;
@@ -30,6 +31,9 @@ pub(crate) fn complete_attribute(acc: &mut Completions, ctx: &CompletionContext)
                 lint::complete_lint(acc, ctx, token_tree.clone(), DEFAULT_LINTS);
                 lint::complete_lint(acc, ctx, token_tree, CLIPPY_LINTS);
             }
+            "cfg" => {
+                cfg::complete_cfg(acc, ctx);
+            }
             _ => (),
         },
         (None, Some(_)) => (),
@@ -852,4 +856,15 @@ mod tests {
             "#]],
         );
     }
+
+    #[test]
+    fn test_cfg() {
+        check(
+            r#"#[cfg(target_endian = $0"#,
+            expect![[r#"
+                at little
+                at big
+"#]],
+        );
+    }
 }
diff --git a/crates/ide_completion/src/completions/attribute/cfg.rs b/crates/ide_completion/src/completions/attribute/cfg.rs
new file mode 100644
index 000000000..847e6529a
--- /dev/null
+++ b/crates/ide_completion/src/completions/attribute/cfg.rs
@@ -0,0 +1,112 @@
+//! Completion for cfg
+
+use std::iter;
+
+use syntax::SyntaxKind;
+
+use crate::{
+    completions::Completions, context::CompletionContext, item::CompletionKind, CompletionItem,
+    CompletionItemKind,
+};
+
+pub(crate) fn complete_cfg(acc: &mut Completions, ctx: &CompletionContext) {
+    let add_completion = |item: &&str| {
+        let mut completion =
+            CompletionItem::new(CompletionKind::Attribute, ctx.source_range(), *item);
+        completion.insert_text(format!(r#""{}""#, item));
+        completion.kind(CompletionItemKind::Attribute);
+        acc.add(completion.build());
+    };
+
+    let previous = iter::successors(ctx.original_token.prev_token(), |t| {
+        (matches!(t.kind(), SyntaxKind::EQ) || t.kind().is_trivia())
+            .then(|| t.prev_token())
+            .flatten()
+    })
+    .find(|t| matches!(t.kind(), SyntaxKind::IDENT));
+
+    match previous.as_ref().map(|p| p.text()) {
+        Some("target_arch") => KNOWN_ARCH.iter().for_each(add_completion),
+        Some("target_env") => KNOWN_ENV.iter().for_each(add_completion),
+        Some("target_os") => KNOWN_OS.iter().for_each(add_completion),
+        Some("target_vendor") => KNOWN_VENDOR.iter().for_each(add_completion),
+        Some("target_endian") => ["little", "big"].iter().for_each(add_completion),
+        Some(name) => {
+            ctx.krate.map(|krate| {
+                krate.potential_cfg(ctx.db).get_cfg_values(&name).iter().for_each(|s| {
+                    let mut item = CompletionItem::new(
+                        CompletionKind::Attribute,
+                        ctx.source_range(),
+                        s.as_str(),
+                    );
+                    item.insert_text(format!(r#""{}""#, s));
+
+                    acc.add(item.build());
+                })
+            });
+        }
+        None => {
+            ctx.krate.map(|krate| {
+                krate.potential_cfg(ctx.db).get_cfg_keys().iter().for_each(|s| {
+                    let item = CompletionItem::new(
+                        CompletionKind::Attribute,
+                        ctx.source_range(),
+                        s.as_str(),
+                    );
+                    acc.add(item.build());
+                })
+            });
+        }
+    };
+}
+
+const KNOWN_ARCH: [&'static str; 19] = [
+    "aarch64",
+    "arm",
+    "avr",
+    "hexagon",
+    "mips",
+    "mips64",
+    "msp430",
+    "nvptx64",
+    "powerpc",
+    "powerpc64",
+    "riscv32",
+    "riscv64",
+    "s390x",
+    "sparc",
+    "sparc64",
+    "wasm32",
+    "wasm64",
+    "x86",
+    "x86_64",
+];
+
+const KNOWN_ENV: [&'static str; 7] =
+    ["eabihf", "gnu", "gnueabihf", "msvc", "relibc", "sgx", "uclibc"];
+
+const KNOWN_OS: [&'static str; 20] = [
+    "cuda",
+    "dragonfly",
+    "emscripten",
+    "freebsd",
+    "fuchsia",
+    "haiku",
+    "hermit",
+    "illumos",
+    "l4re",
+    "linux",
+    "netbsd",
+    "none",
+    "openbsd",
+    "psp",
+    "redox",
+    "solaris",
+    "uefi",
+    "unknown",
+    "vxworks",
+    "windows",
+];
+
+const KNOWN_VENDOR: [&'static str; 8] =
+    ["apple", "fortanix", "nvidia", "pc", "sony", "unknown", "wrs", "uwp"];
diff --git a/crates/project_model/src/workspace.rs b/crates/project_model/src/workspace.rs
index d8217f714..e67ba2bd9 100644
--- a/crates/project_model/src/workspace.rs
+++ b/crates/project_model/src/workspace.rs
@@ -384,6 +384,7 @@ fn project_json_to_crate_graph(
                     file_id,
                     krate.edition,
                     krate.display_name.clone(),
+                    cfg_options.clone(),
                     cfg_options,
                     env,
                     proc_macro.unwrap_or_default(),
@@ -580,6 +581,7 @@ fn detached_files_to_crate_graph(
             Edition::Edition2018,
             display_name,
             cfg_options.clone(),
+            cfg_options.clone(),
             Env::default(),
             Vec::new(),
         );
@@ -719,11 +721,19 @@ fn add_target_crate_root(
         .unwrap_or_default();
 
     let display_name = CrateDisplayName::from_canonical_name(cargo_name.to_string());
+    let mut potential_cfg_options = cfg_options.clone();
+    potential_cfg_options.extend(
+        pkg.features
+            .iter()
+            .map(|feat| CfgFlag::KeyValue { key: "feature".into(), value: feat.0.into() }),
+    );
+
     let crate_id = crate_graph.add_crate_root(
         file_id,
         edition,
         Some(display_name),
         cfg_options,
+        potential_cfg_options,
         env,
         proc_macro,
     );
@@ -753,6 +763,7 @@ fn sysroot_to_crate_graph(
                 Edition::Edition2018,
                 Some(display_name),
                 cfg_options.clone(),
+                cfg_options.clone(),
                 env,
                 proc_macro,
             );