Make new type param suggestion more targetted

src/librustc_resolve/build_reduced_graph.rs

@@ -485,6 +485,7 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
                             module_path.push(Segment {
                                 ident: Ident { name: kw::PathRoot, span: source.ident.span },
                                 id: Some(self.r.next_node_id()),
+                                has_args: false,
                             });
                             source.ident.name = crate_name;
                         }

src/librustc_resolve/late/diagnostics.rs

@@ -919,20 +919,45 @@ impl<'a> LateResolutionVisitor<'a, '_, '_> {
         &self,
         path: &[Segment],
     ) -> Option<(Span, &'static str, String, Applicability)> {
-        let ident = match path {
-            [segment] => segment.ident,
+        let (ident, span) = match path {
+            [segment] if !segment.has_args => (segment.ident.to_string(), segment.ident.span),
             _ => return None,
         };
-        match (
-            self.diagnostic_metadata.current_item,
-            self.diagnostic_metadata.currently_processing_generics,
-        ) {
-            (Some(Item { kind: ItemKind::Fn(..), ident, .. }), true) if ident.name == sym::main => {
+        let mut iter = ident.chars().map(|c| c.is_uppercase());
+        let single_uppercase_char =
+            matches!(iter.next(), Some(true)) && matches!(iter.next(), None);
+        if !self.diagnostic_metadata.currently_processing_generics && !single_uppercase_char {
+            return None;
+        }
+        match (self.diagnostic_metadata.current_item, single_uppercase_char) {
+            (Some(Item { kind: ItemKind::Fn(..), ident, .. }), _) if ident.name == sym::main => {
                 // Ignore `fn main()` as we don't want to suggest `fn main<T>()`
             }
-            (Some(Item { kind, .. }), true) => {
+            (
+                Some(Item {
+                    kind:
+                        kind @ ItemKind::Fn(..)
+                        | kind @ ItemKind::Enum(..)
+                        | kind @ ItemKind::Struct(..)
+                        | kind @ ItemKind::Union(..),
+                    ..
+                }),
+                true,
+            )
+            | (Some(Item { kind, .. }), false) => {
                 // Likely missing type parameter.
                 if let Some(generics) = kind.generics() {
+                    if span.overlaps(generics.span) {
+                        // Avoid the following:
+                        // error[E0405]: cannot find trait `A` in this scope
+                        //  --> $DIR/typo-suggestion-named-underscore.rs:CC:LL
+                        //   |
+                        // L | fn foo<T: A>(x: T) {} // Shouldn't suggest underscore
+                        //   |           ^- help: you might be missing a type parameter: `, A`
+                        //   |           |
+                        //   |           not found in this scope
+                        return None;
+                    }
                     let msg = "you might be missing a type parameter";
                     let (span, sugg) = if let [.., param] = &generics.params[..] {
                         let span = if let [.., bound] = &param.bounds[..] {

src/librustc_resolve/lib.rs

@@ -225,13 +225,15 @@ enum VisResolutionError<'a> {
     ModuleOnly(Span),
 }
 
-// A minimal representation of a path segment. We use this in resolve because
-// we synthesize 'path segments' which don't have the rest of an AST or HIR
-// `PathSegment`.
+/// A minimal representation of a path segment. We use this in resolve because we synthesize 'path
+/// segments' which don't have the rest of an AST or HIR `PathSegment`.
 #[derive(Clone, Copy, Debug)]
 pub struct Segment {
     ident: Ident,
     id: Option<NodeId>,
+    /// Signals whether this `PathSegment` has generic arguments. Used to avoid providing
+    /// nonsensical suggestions.
+    has_args: bool,
 }
 
 impl Segment {
@@ -240,7 +242,7 @@ impl Segment {
     }
 
     fn from_ident(ident: Ident) -> Segment {
-        Segment { ident, id: None }
+        Segment { ident, id: None, has_args: false }
     }
 
     fn names_to_string(segments: &[Segment]) -> String {
@@ -250,7 +252,7 @@ impl Segment {
 
 impl<'a> From<&'a ast::PathSegment> for Segment {
     fn from(seg: &'a ast::PathSegment) -> Segment {
-        Segment { ident: seg.ident, id: Some(seg.id) }
+        Segment { ident: seg.ident, id: Some(seg.id), has_args: seg.args.is_some() }
     }
 }
 
@@ -2017,7 +2019,7 @@ impl<'a> Resolver<'a> {
             path, opt_ns, record_used, path_span, crate_lint,
         );
 
-        for (i, &Segment { ident, id }) in path.iter().enumerate() {
+        for (i, &Segment { ident, id, has_args: _ }) in path.iter().enumerate() {
             debug!("resolve_path ident {} {:?} {:?}", i, ident, id);
             let record_segment_res = |this: &mut Self, res| {
                 if record_used {

src/test/ui/associated-types/associated-types-eq-1.stderr

@@ -4,7 +4,16 @@ error[E0412]: cannot find type `A` in this scope
 LL | fn foo2<I: Foo>(x: I) {
    |         - similarly named type parameter `I` defined here
 LL |     let _: A = x.boo();
-   |            ^ help: a type parameter with a similar name exists: `I`
+   |            ^
+   |
+help: a type parameter with a similar name exists
+   |
+LL |     let _: I = x.boo();
+   |            ^
+help: you might be missing a type parameter
+   |
+LL | fn foo2<I: Foo, A>(x: I) {
+   |               ^^^
 
 error: aborting due to previous error
 

src/test/ui/suggestions/type-not-found-in-adt-field.rs

@@ -0,0 +1,9 @@
+struct Struct {
+    m: Vec<Someunknownname<String, ()>>, //~ ERROR cannot find type `Someunknownname` in this scope
+    //~^ NOTE not found in this scope
+}
+struct OtherStruct { //~ HELP you might be missing a type parameter
+    m: K, //~ ERROR cannot find type `K` in this scope
+    //~^ NOTE not found in this scope
+}
+fn main() {}

src/test/ui/suggestions/type-not-found-in-adt-field.stderr

@@ -0,0 +1,17 @@
+error[E0412]: cannot find type `Someunknownname` in this scope
+  --> $DIR/type-not-found-in-adt-field.rs:2:12
+   |
+LL |     m: Vec<Someunknownname<String, ()>>,
+   |            ^^^^^^^^^^^^^^^ not found in this scope
+
+error[E0412]: cannot find type `K` in this scope
+  --> $DIR/type-not-found-in-adt-field.rs:6:8
+   |
+LL | struct OtherStruct {
+   |                   - help: you might be missing a type parameter: `<K>`
+LL |     m: K,
+   |        ^ not found in this scope
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0412`.