2 files changed, 127 insertions, 0 deletions

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"];