diff --git a/Cargo.lock b/Cargo.lock
index 8bd1e5f3308..32a6f5fce74 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -74,7 +74,7 @@ name = "arena"
 version = "0.0.0"
 dependencies = [
  "rustc_data_structures",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -492,7 +492,7 @@ dependencies = [
  "regex-syntax",
  "semver",
  "serde",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
  "toml",
  "unicode-normalization",
  "url 2.1.0",
@@ -2428,7 +2428,7 @@ dependencies = [
  "cloudabi",
  "libc",
  "redox_syscall",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
  "winapi 0.3.8",
 ]
 
@@ -3151,7 +3151,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "81dfcfbb0ddfd533abf8c076e3b49d1e5042d1962526a12ce2c66d514b24cca3"
 dependencies = [
  "rustc-ap-rustc_data_structures",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3174,7 +3174,7 @@ dependencies = [
  "rustc-ap-rustc_span",
  "rustc-ap-serialize",
  "scoped-tls",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3223,7 +3223,7 @@ dependencies = [
  "rustc-ap-rustc_session",
  "rustc-ap-rustc_span",
  "rustc-ap-serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3249,7 +3249,7 @@ dependencies = [
  "rustc-hash",
  "rustc-rayon",
  "rustc-rayon-core",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
  "stable_deref_trait",
  "winapi 0.3.8",
 ]
@@ -3291,7 +3291,7 @@ dependencies = [
  "rustc-ap-rustc_session",
  "rustc-ap-rustc_span",
  "rustc-ap-serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3318,7 +3318,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "32220c3e6cdf226f38e4474b747dca15f3106bb680c74f10b299af3f6cdb1663"
 dependencies = [
  "rustc-ap-serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3358,7 +3358,7 @@ dependencies = [
  "rustc-ap-rustc_lexer",
  "rustc-ap-rustc_session",
  "rustc-ap-rustc_span",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
  "unicode-normalization",
 ]
 
@@ -3423,7 +3423,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "756e8f526ec7906e132188bf25e3c10a6ee42ab77294ecb3b3602647f0508eef"
 dependencies = [
  "indexmap",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3507,7 +3507,7 @@ dependencies = [
  "serde",
  "serde_json",
  "smallvec 0.6.10",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
  "syn 0.15.35",
  "url 2.1.0",
  "winapi 0.3.8",
@@ -3518,7 +3518,7 @@ name = "rustc_apfloat"
 version = "0.0.0"
 dependencies = [
  "bitflags",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3533,7 +3533,7 @@ dependencies = [
  "rustc_span",
  "scoped-tls",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3551,7 +3551,7 @@ dependencies = [
  "rustc_session",
  "rustc_span",
  "rustc_target",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3612,7 +3612,7 @@ dependencies = [
  "rustc_session",
  "rustc_span",
  "rustc_target",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3641,7 +3641,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3694,7 +3694,7 @@ dependencies = [
  "rustc-rayon-core",
  "rustc_index",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
  "stable_deref_trait",
  "winapi 0.3.8",
 ]
@@ -3768,7 +3768,7 @@ dependencies = [
  "rustc_session",
  "rustc_span",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3797,7 +3797,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3833,7 +3833,7 @@ name = "rustc_index"
 version = "0.0.0"
 dependencies = [
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3853,7 +3853,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -3895,7 +3895,7 @@ dependencies = [
  "rustc_ty",
  "rustc_typeck",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
  "tempfile",
  "winapi 0.3.8",
 ]
@@ -3968,7 +3968,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
  "stable_deref_trait",
  "winapi 0.3.8",
 ]
@@ -4000,7 +4000,7 @@ dependencies = [
  "rustc_target",
  "scoped-tls",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -4029,7 +4029,7 @@ dependencies = [
  "rustc_target",
  "rustc_trait_selection",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -4053,7 +4053,7 @@ dependencies = [
  "rustc_target",
  "rustc_trait_selection",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -4134,7 +4134,7 @@ dependencies = [
  "rustc_index",
  "rustc_span",
  "serialize",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -4157,7 +4157,7 @@ dependencies = [
  "rustc_middle",
  "rustc_session",
  "rustc_span",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -4272,7 +4272,7 @@ dependencies = [
  "rustc_session",
  "rustc_span",
  "rustc_target",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -4287,7 +4287,7 @@ dependencies = [
  "rustc_middle",
  "rustc_span",
  "rustc_trait_selection",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -4324,7 +4324,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "rustc_trait_selection",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -4568,7 +4568,7 @@ name = "serialize"
 version = "0.0.0"
 dependencies = [
  "indexmap",
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -4635,9 +4635,9 @@ checksum = "ab606a9c5e214920bb66c458cd7be8ef094f813f20fe77a54cc7dbfff220d4b7"
 
 [[package]]
 name = "smallvec"
-version = "1.0.0"
+version = "1.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4ecf3b85f68e8abaa7555aa5abdb1153079387e60b718283d732f03897fcfc86"
+checksum = "c7cb5678e1615754284ec264d9bb5b4c27d2018577fd90ac0ceb578591ed5ee4"
 
 [[package]]
 name = "socket2"
@@ -5359,11 +5359,11 @@ dependencies = [
 
 [[package]]
 name = "unicode-normalization"
-version = "0.1.11"
+version = "0.1.12"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b561e267b2326bb4cebfc0ef9e68355c7abe6c6f522aeac2f5bf95d56c59bdcf"
+checksum = "5479532badd04e128284890390c1e876ef7a993d0570b3597ae43dfa1d59afa4"
 dependencies = [
- "smallvec 1.0.0",
+ "smallvec 1.4.0",
 ]
 
 [[package]]
@@ -5374,10 +5374,11 @@ checksum = "5b2c5c29e805da6817f5af6a627d65adb045cebf05cccd5a3493d6109454391c"
 
 [[package]]
 name = "unicode-security"
-version = "0.0.2"
+version = "0.0.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c49d35967fa037b881acc34ef717c38c4b5560eba10e3685271b3f530bb19634"
+checksum = "a5f9011bbed9c13372bc8df618b55a38138445199caf3b61d432c6859c36dee0"
 dependencies = [
+ "unicode-normalization",
  "unicode-script",
 ]
 
diff --git a/src/librustc_expand/proc_macro_server.rs b/src/librustc_expand/proc_macro_server.rs
index 5f21ff503d5..10baffb28ac 100644
--- a/src/librustc_expand/proc_macro_server.rs
+++ b/src/librustc_expand/proc_macro_server.rs
@@ -141,10 +141,10 @@ impl FromInternal<(TreeAndJoint, &'_ ParseSess, &'_ mut Vec<Self>)>
             SingleQuote => op!('\''),
 
             Ident(name, false) if name == kw::DollarCrate => tt!(Ident::dollar_crate()),
-            Ident(name, is_raw) => tt!(Ident::new(name, is_raw)),
+            Ident(name, is_raw) => tt!(Ident::new(sess, name, is_raw)),
             Lifetime(name) => {
                 let ident = ast::Ident::new(name, span).without_first_quote();
-                stack.push(tt!(Ident::new(ident.name, false)));
+                stack.push(tt!(Ident::new(sess, ident.name, false)));
                 tt!(Punct::new('\'', true))
             }
             Literal(lit) => tt!(Literal { lit }),
@@ -322,7 +322,7 @@ impl Ident {
             false
         }
     }
-    fn new(sym: Symbol, is_raw: bool, span: Span) -> Ident {
+    fn new(sess: &ParseSess, sym: Symbol, is_raw: bool, span: Span) -> Ident {
         let sym = nfc_normalize(&sym.as_str());
         let string = sym.as_str();
         if !Self::is_valid(&string) {
@@ -331,6 +331,7 @@ impl Ident {
         if is_raw && !sym.can_be_raw() {
             panic!("`{}` cannot be a raw identifier", string);
         }
+        sess.symbol_gallery.insert(sym, span);
         Ident { sym, is_raw, span }
     }
     fn dollar_crate(span: Span) -> Ident {
@@ -495,7 +496,7 @@ impl server::Punct for Rustc<'_> {
 
 impl server::Ident for Rustc<'_> {
     fn new(&mut self, string: &str, span: Self::Span, is_raw: bool) -> Self::Ident {
-        Ident::new(Symbol::intern(string), is_raw, span)
+        Ident::new(self.sess, Symbol::intern(string), is_raw, span)
     }
     fn span(&mut self, ident: Self::Ident) -> Self::Span {
         ident.span
diff --git a/src/librustc_lint/Cargo.toml b/src/librustc_lint/Cargo.toml
index b238a3156fa..ada6f2a9381 100644
--- a/src/librustc_lint/Cargo.toml
+++ b/src/librustc_lint/Cargo.toml
@@ -10,7 +10,7 @@ path = "lib.rs"
 
 [dependencies]
 log = "0.4"
-unicode-security = "0.0.2"
+unicode-security = "0.0.3"
 rustc_middle = { path = "../librustc_middle" }
 rustc_ast_pretty = { path = "../librustc_ast_pretty" }
 rustc_attr = { path = "../librustc_attr" }
diff --git a/src/librustc_lint/levels.rs b/src/librustc_lint/levels.rs
index 7e8e4f59707..293dd326d3b 100644
--- a/src/librustc_lint/levels.rs
+++ b/src/librustc_lint/levels.rs
@@ -388,6 +388,11 @@ impl<'s> LintLevelsBuilder<'s> {
         self.cur = push.prev;
     }
 
+    /// Find the lint level for a lint.
+    pub fn lint_level(&self, lint: &'static Lint) -> (Level, LintSource) {
+        self.sets.get_lint_level(lint, self.cur, None, self.sess)
+    }
+
     /// Used to emit a lint-related diagnostic based on the current state of
     /// this lint context.
     pub fn struct_lint(
@@ -396,7 +401,7 @@ impl<'s> LintLevelsBuilder<'s> {
         span: Option<MultiSpan>,
         decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a>),
     ) {
-        let (level, src) = self.sets.get_lint_level(lint, self.cur, None, self.sess);
+        let (level, src) = self.lint_level(lint);
         struct_lint_level(self.sess, lint, level, src, span, decorate)
     }
 
diff --git a/src/librustc_lint/non_ascii_idents.rs b/src/librustc_lint/non_ascii_idents.rs
index 470fac2675b..522f292dc94 100644
--- a/src/librustc_lint/non_ascii_idents.rs
+++ b/src/librustc_lint/non_ascii_idents.rs
@@ -1,5 +1,7 @@
 use crate::{EarlyContext, EarlyLintPass, LintContext};
 use rustc_ast::ast;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_span::symbol::SymbolStr;
 
 declare_lint! {
     pub NON_ASCII_IDENTS,
@@ -13,9 +15,101 @@ declare_lint! {
     "detects uncommon Unicode codepoints in identifiers"
 }
 
-declare_lint_pass!(NonAsciiIdents => [NON_ASCII_IDENTS, UNCOMMON_CODEPOINTS]);
+// FIXME: Change this to warn.
+declare_lint! {
+    pub CONFUSABLE_IDENTS,
+    Allow,
+    "detects visually confusable pairs between identifiers"
+}
+
+declare_lint_pass!(NonAsciiIdents => [NON_ASCII_IDENTS, UNCOMMON_CODEPOINTS, CONFUSABLE_IDENTS]);
+
+use std::hash::{Hash, Hasher};
+use std::ops::Deref;
+
+enum CowBoxSymStr {
+    Interned(SymbolStr),
+    Owned(Box<str>),
+}
+
+impl Deref for CowBoxSymStr {
+    type Target = str;
+
+    fn deref(&self) -> &str {
+        match self {
+            CowBoxSymStr::Interned(interned) => interned,
+            CowBoxSymStr::Owned(ref owned) => owned,
+        }
+    }
+}
+
+impl Hash for CowBoxSymStr {
+    #[inline]
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        Hash::hash(&**self, state)
+    }
+}
+
+impl PartialEq<CowBoxSymStr> for CowBoxSymStr {
+    #[inline]
+    fn eq(&self, other: &CowBoxSymStr) -> bool {
+        PartialEq::eq(&**self, &**other)
+    }
+}
+
+impl Eq for CowBoxSymStr {}
+
+fn calc_skeleton(symbol_str: SymbolStr, buffer: &'_ mut String) -> CowBoxSymStr {
+    use std::mem::swap;
+    use unicode_security::confusable_detection::skeleton;
+    buffer.clear();
+    buffer.extend(skeleton(&symbol_str));
+    if symbol_str == *buffer {
+        CowBoxSymStr::Interned(symbol_str)
+    } else {
+        let mut owned = String::new();
+        swap(buffer, &mut owned);
+        CowBoxSymStr::Owned(owned.into_boxed_str())
+    }
+}
 
 impl EarlyLintPass for NonAsciiIdents {
+    fn check_crate(&mut self, cx: &EarlyContext<'_>, _: &ast::Crate) {
+        use rustc_session::lint::Level;
+        if cx.builder.lint_level(CONFUSABLE_IDENTS).0 == Level::Allow {
+            return;
+        }
+        let symbols = cx.sess.parse_sess.symbol_gallery.symbols.lock();
+        let mut symbol_strs_and_spans = Vec::with_capacity(symbols.len());
+        for (symbol, sp) in symbols.iter() {
+            let symbol_str = symbol.as_str();
+            symbol_strs_and_spans.push((symbol_str, *sp));
+        }
+        drop(symbols);
+        symbol_strs_and_spans.sort_by_key(|x| x.0.clone());
+        let mut skeleton_map =
+            FxHashMap::with_capacity_and_hasher(symbol_strs_and_spans.len(), Default::default());
+        let mut str_buf = String::new();
+        for (symbol_str, sp) in symbol_strs_and_spans {
+            let skeleton = calc_skeleton(symbol_str.clone(), &mut str_buf);
+            skeleton_map
+                .entry(skeleton)
+                .and_modify(|(existing_symbolstr, existing_span)| {
+                    cx.struct_span_lint(CONFUSABLE_IDENTS, sp, |lint| {
+                        lint.build(&format!(
+                            "identifier pair considered confusable between `{}` and `{}`",
+                            existing_symbolstr, symbol_str
+                        ))
+                        .span_label(
+                            *existing_span,
+                            "this is where the previous identifier occurred",
+                        )
+                        .emit();
+                    });
+                })
+                .or_insert((symbol_str, sp));
+        }
+    }
     fn check_ident(&mut self, cx: &EarlyContext<'_>, ident: ast::Ident) {
         use unicode_security::GeneralSecurityProfile;
         let name_str = ident.name.as_str();
diff --git a/src/librustc_parse/lexer/mod.rs b/src/librustc_parse/lexer/mod.rs
index 96321ef2145..f676a34a1d1 100644
--- a/src/librustc_parse/lexer/mod.rs
+++ b/src/librustc_parse/lexer/mod.rs
@@ -222,8 +222,9 @@ impl<'a> StringReader<'a> {
                     ident_start = ident_start + BytePos(2);
                 }
                 let sym = nfc_normalize(self.str_from(ident_start));
+                let span = self.mk_sp(start, self.pos);
+                self.sess.symbol_gallery.insert(sym, span);
                 if is_raw_ident {
-                    let span = self.mk_sp(start, self.pos);
                     if !sym.can_be_raw() {
                         self.err_span(span, &format!("`{}` cannot be a raw identifier", sym));
                     }
diff --git a/src/librustc_session/parse.rs b/src/librustc_session/parse.rs
index 387d35422c4..69d3e99b745 100644
--- a/src/librustc_session/parse.rs
+++ b/src/librustc_session/parse.rs
@@ -60,6 +60,20 @@ impl GatedSpans {
     }
 }
 
+#[derive(Default)]
+pub struct SymbolGallery {
+    /// All symbols occurred and their first occurrance span.
+    pub symbols: Lock<FxHashMap<Symbol, Span>>,
+}
+
+impl SymbolGallery {
+    /// Insert a symbol and its span into symbol gallery.
+    /// If the symbol has occurred before, ignore the new occurance.
+    pub fn insert(&self, symbol: Symbol, span: Span) {
+        self.symbols.lock().entry(symbol).or_insert(span);
+    }
+}
+
 /// Construct a diagnostic for a language feature error due to the given `span`.
 /// The `feature`'s `Symbol` is the one you used in `active.rs` and `rustc_span::symbols`.
 pub fn feature_err<'a>(
@@ -118,6 +132,7 @@ pub struct ParseSess {
     pub ambiguous_block_expr_parse: Lock<FxHashMap<Span, Span>>,
     pub injected_crate_name: Once<Symbol>,
     pub gated_spans: GatedSpans,
+    pub symbol_gallery: SymbolGallery,
     /// The parser has reached `Eof` due to an unclosed brace. Used to silence unnecessary errors.
     pub reached_eof: Lock<bool>,
 }
@@ -143,6 +158,7 @@ impl ParseSess {
             ambiguous_block_expr_parse: Lock::new(FxHashMap::default()),
             injected_crate_name: Once::new(),
             gated_spans: GatedSpans::default(),
+            symbol_gallery: SymbolGallery::default(),
             reached_eof: Lock::new(false),
         }
     }
diff --git a/src/test/ui/lint/rfc-2457-non-ascii-idents/lint-confusable-idents.rs b/src/test/ui/lint/rfc-2457-non-ascii-idents/lint-confusable-idents.rs
new file mode 100644
index 00000000000..12093837d26
--- /dev/null
+++ b/src/test/ui/lint/rfc-2457-non-ascii-idents/lint-confusable-idents.rs
@@ -0,0 +1,9 @@
+#![feature(non_ascii_idents)]
+#![deny(confusable_idents)]
+#![allow(uncommon_codepoints, non_upper_case_globals)]
+
+const ｓ: usize = 42; //~ ERROR identifier pair considered confusable
+
+fn main() {
+    let s = "rust";
+}
diff --git a/src/test/ui/lint/rfc-2457-non-ascii-idents/lint-confusable-idents.stderr b/src/test/ui/lint/rfc-2457-non-ascii-idents/lint-confusable-idents.stderr
new file mode 100644
index 00000000000..40ee18acb3c
--- /dev/null
+++ b/src/test/ui/lint/rfc-2457-non-ascii-idents/lint-confusable-idents.stderr
@@ -0,0 +1,17 @@
+error: identifier pair considered confusable between `s` and `ｓ`
+  --> $DIR/lint-confusable-idents.rs:5:7
+   |
+LL | const ｓ: usize = 42;
+   |       ^^
+...
+LL |     let s = "rust";
+   |         - this is where the previous identifier occurred
+   |
+note: the lint level is defined here
+  --> $DIR/lint-confusable-idents.rs:2:9
+   |
+LL | #![deny(confusable_idents)]
+   |         ^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+