Improve error message when shadowing builtin type

Closes #3109

crates/compiler/can/src/def.rs

@@ -2302,11 +2302,6 @@ fn to_pending_alias_or_opaque<'a>(
     opt_derived: Option<&'a Loc<ast::HasAbilities<'a>>>,
     kind: AliasKind,
 ) -> PendingTypeDef<'a> {
-    let shadow_kind = match kind {
-        AliasKind::Structural => ShadowKind::Alias,
-        AliasKind::Opaque => ShadowKind::Opaque,
-    };
-
     let region = Region::span_across(&name.region, &ann.region);
 
     match scope.introduce_without_shadow_symbol(&Ident::from(name.value), region) {
@@ -2361,7 +2356,12 @@ fn to_pending_alias_or_opaque<'a>(
             }
         }
 
-        Err((original_region, loc_shadowed_symbol)) => {
+        Err((original_sym, original_region, loc_shadowed_symbol)) => {
+            let shadow_kind = match kind {
+                AliasKind::Structural => ShadowKind::Alias(original_sym),
+                AliasKind::Opaque => ShadowKind::Opaque(original_sym),
+            };
+
             env.problem(Problem::Shadowing {
                 original_region,
                 shadow: loc_shadowed_symbol,
@@ -2422,11 +2422,11 @@ fn to_pending_type_def<'a>(
                 .introduce_without_shadow_symbol(&Ident::from(name.value), name.region)
             {
                 Ok(symbol) => Loc::at(name.region, symbol),
-                Err((original_region, shadowed_symbol)) => {
+                Err((original_symbol, original_region, shadowed_symbol)) => {
                     env.problem(Problem::Shadowing {
                         original_region,
                         shadow: shadowed_symbol,
-                        kind: ShadowKind::Ability,
+                        kind: ShadowKind::Ability(original_symbol),
                     });
                     return PendingTypeDef::AbilityShadows;
                 }

crates/compiler/can/src/scope.rs

@@ -284,14 +284,14 @@ impl Scope {
         &mut self,
         ident: &Ident,
         region: Region,
-    ) -> Result<Symbol, (Region, Loc<Ident>)> {
+    ) -> Result<Symbol, (Symbol, Region, Loc<Ident>)> {
         match self.introduce_help(ident.as_str(), region) {
-            Err((_, original_region)) => {
+            Err((symbol, original_region)) => {
                 let shadow = Loc {
                     value: ident.clone(),
                     region,
                 };
-                Err((original_region, shadow))
+                Err((symbol, original_region, shadow))
             }
             Ok(symbol) => Ok(symbol),
         }

crates/compiler/problem/src/can.rs

@@ -22,9 +22,9 @@ pub enum BadPattern {
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum ShadowKind {
     Variable,
-    Alias,
+    Alias(Symbol),
-    Opaque,
+    Opaque(Symbol),
-    Ability,
+    Ability(Symbol),
 }
 
 /// Problems that can occur in the course of canonicalization.

crates/reporting/src/error/canonicalize.rs

@@ -257,9 +257,11 @@ pub fn can_problem<'b>(
             shadow,
             kind,
         } => {
-            doc = report_shadowing(alloc, lines, original_region, shadow, kind);
+            let (res_title, res_doc) =
+                report_shadowing(alloc, lines, original_region, shadow, kind);
 
-            title = DUPLICATE_NAME.to_string();
+            doc = res_doc;
+            title = res_title.to_string();
             severity = Severity::RuntimeError;
         }
         Problem::CyclicAlias(symbol, region, others, alias_kind) => {
@@ -1371,28 +1373,48 @@ fn report_shadowing<'b>(
     original_region: Region,
     shadow: Loc<Ident>,
     kind: ShadowKind,
-) -> RocDocBuilder<'b> {
+) -> (&'static str, RocDocBuilder<'b>) {
-    let what = match kind {
+    let (what, what_plural, is_builtin) = match kind {
-        ShadowKind::Variable => "variables",
+        ShadowKind::Variable => ("variable", "variables", false),
-        ShadowKind::Alias => "aliases",
+        ShadowKind::Alias(sym) => ("alias", "aliases", sym.is_builtin()),
-        ShadowKind::Opaque => "opaques",
+        ShadowKind::Opaque(sym) => ("opaque type", "opaque types", sym.is_builtin()),
-        ShadowKind::Ability => "abilities",
+        ShadowKind::Ability(sym) => ("ability", "abilities", sym.is_builtin()),
     };
 
-    alloc.stack([
+    let doc = if is_builtin {
-        alloc
+        alloc.stack([
-            .text("The ")
+            alloc.concat([
-            .append(alloc.ident(shadow.value))
+                alloc.reflow("This "),
-            .append(alloc.reflow(" name is first defined here:")),
+                alloc.reflow(what),
-        alloc.region(lines.convert_region(original_region)),
+                alloc.reflow(" has the same name as a builtin:"),
-        alloc.reflow("But then it's defined a second time here:"),
+            ]),
-        alloc.region(lines.convert_region(shadow.region)),
+            alloc.region(lines.convert_region(shadow.region)),
-        alloc.concat([
+            alloc.concat([
-            alloc.reflow("Since these "),
+                alloc.reflow("All builtin "),
-            alloc.reflow(what),
+                alloc.reflow(what_plural),
-            alloc.reflow(" have the same name, it's easy to use the wrong one on accident. Give one of them a new name."),
+                alloc.reflow(" are in scope by default, so I need this "),
-        ]),
+                alloc.reflow(what),
-    ])
+                alloc.reflow(" to have a different name!"),
+            ]),
+        ])
+    } else {
+        alloc.stack([
+            alloc
+                .text("The ")
+                .append(alloc.ident(shadow.value))
+                .append(alloc.reflow(" name is first defined here:")),
+            alloc.region(lines.convert_region(original_region)),
+            alloc.reflow("But then it's defined a second time here:"),
+            alloc.region(lines.convert_region(shadow.region)),
+            alloc.concat([
+                alloc.reflow("Since these "),
+                alloc.reflow(what_plural),
+                alloc.reflow(" have the same name, it's easy to use the wrong one on accident. Give one of them a new name."),
+            ]),
+        ])
+    };
+
+    (DUPLICATE_NAME, doc)
 }
 
 fn pretty_runtime_error<'b>(
@@ -1420,8 +1442,7 @@ fn pretty_runtime_error<'b>(
             shadow,
             kind,
         } => {
-            doc = report_shadowing(alloc, lines, original_region, shadow, kind);
+            (title, doc) = report_shadowing(alloc, lines, original_region, shadow, kind);
-            title = DUPLICATE_NAME;
         }
 
         RuntimeError::LookupNotInScope(loc_name, options) => {

crates/reporting/tests/test_reporting.rs

@@ -6198,18 +6198,13 @@ All branches in an `if` must have the same type!
         @r###"
     ── DUPLICATE NAME ──────────────────────────────────────── /code/proj/Main.roc ─
 
-    The `Result` name is first defined here:
+    This alias has the same name as a builtin:
-
-    1│  app "test" provides [main] to "./platform"
-      
-
-    But then it's defined a second time here:
 
     4│      Result a b : [Ok a, Err b]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-    Since these aliases have the same name, it's easy to use the wrong one
+    All builtin aliases are in scope by default, so I need this alias to
-    on accident. Give one of them a new name.
+    have a different name!
 
     ── TOO FEW TYPE ARGUMENTS ──────────────────────────────── /code/proj/Main.roc ─
 
@@ -6241,18 +6236,13 @@ All branches in an `if` must have the same type!
         @r###"
     ── DUPLICATE NAME ──────────────────────────────────────── /code/proj/Main.roc ─
 
-    The `Result` name is first defined here:
+    This alias has the same name as a builtin:
-
-    1│  app "test" provides [main] to "./platform"
-      
-
-    But then it's defined a second time here:
 
     4│      Result a b : [Ok a, Err b]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-    Since these aliases have the same name, it's easy to use the wrong one
+    All builtin aliases are in scope by default, so I need this alias to
-    on accident. Give one of them a new name.
+    have a different name!
 
     ── TOO MANY TYPE ARGUMENTS ─────────────────────────────── /code/proj/Main.roc ─
 
@@ -7435,18 +7425,13 @@ All branches in an `if` must have the same type!
         @r###"
     ── DUPLICATE NAME ──────────────────────────────────────── /code/proj/Main.roc ─
 
-    The `Result` name is first defined here:
+    This alias has the same name as a builtin:
-
-    1│  app "test" provides [main] to "./platform"
-      
-
-    But then it's defined a second time here:
 
     4│      Result a b : [Ok a, Err b]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-    Since these aliases have the same name, it's easy to use the wrong one
+    All builtin aliases are in scope by default, so I need this alias to
-    on accident. Give one of them a new name.
+    have a different name!
     "###
     );
 
@@ -10710,4 +10695,26 @@ All branches in an `if` must have the same type!
     be safely removed!
     "###
     );
+
+    test_report!(
+        custom_type_conflicts_with_builtin,
+        indoc!(
+            r#"
+            Nat := [ S Nat, Z ]
+
+            ""
+            "#
+        ),
+    @r###"
+    ── DUPLICATE NAME ──────────────────────────────────────── /code/proj/Main.roc ─
+
+    This opaque type has the same name as a builtin:
+
+    4│      Nat := [ S Nat, Z ]
+            ^^^^^^^^^^^^^^^^^^^
+
+    All builtin opaque types are in scope by default, so I need this
+    opaque type to have a different name!
+    "###
+    );
 }