diff --git a/crates/wit-parser/src/decoding.rs b/crates/wit-parser/src/decoding.rs
index aeddc85eb9..9a0b63819b 100644
--- a/crates/wit-parser/src/decoding.rs
+++ b/crates/wit-parser/src/decoding.rs
@@ -62,15 +62,10 @@ impl ComponentInfo {
                 Chunk::NeedMoreData(hint) => {
                     assert!(!eof); // otherwise an error would be returned
 
-                    // Use the hint to preallocate more space, then read
-                    // some more data into our buffer.
-                    //
-                    // Note that the buffer management here is not ideal,
-                    // but it's compact enough to fit in an example!
-                    let len = buffer.len();
-                    buffer.extend((0..hint).map(|_| 0u8));
-                    let n = reader.read(&mut buffer[len..])?;
-                    buffer.truncate(len + n);
+                    // Read up to the hint into our buffer, which grows with
+                    // what is read: the hint comes from the input, so it may
+                    // be far more than there is.
+                    let n = (&mut reader).take(hint as u64).read_to_end(&mut buffer)?;
                     eof = n == 0;
                     continue;
                 }
@@ -470,8 +465,12 @@ pub fn decode_world(wasm: &[u8]) -> Result<(Resolve, WorldId)> {
     let mut interfaces = IndexMap::default();
     let mut worlds = IndexMap::default();
     let ty = &types[world];
-    assert_eq!(ty.imports.len(), 0);
-    assert_eq!(ty.exports.len(), 1);
+    if ty.imports.len() != 0 || ty.exports.len() != 1 {
+        bail!(
+            "cannot represent this component in WIT: expected an exported component type with \
+             no imports and one export"
+        );
+    }
     let name = ty.exports.keys().nth(0).unwrap();
     let name = decoder.decode_world(
         name,
@@ -618,7 +617,7 @@ impl WitPackageDecoder<'_> {
     ) -> Result<PackageName> {
         let ty = match item.ty {
             ComponentEntityType::Component(ty) => &self.types[ty],
-            _ => unreachable!(),
+            _ => bail!("cannot represent this component in WIT: export `{name}` is not a world"),
         };
         let kebab_name = self
             .parse_component_name(name)
@@ -793,7 +792,10 @@ impl WitPackageDecoder<'_> {
                                     self.register_defined(id, &self.types[ty])?;
                                 }
                                 ComponentAnyTypeId::Resource(_) => {}
-                                _ => unreachable!(),
+                                _ => bail!(
+                                    "cannot represent this component in WIT: type `{name}` is a \
+                                     component, instance or function type"
+                                ),
                             }
                             let prev = self.type_map.insert(created, id);
                             assert!(prev.is_none());
@@ -1033,6 +1035,13 @@ impl WitPackageDecoder<'_> {
         item: &ComponentItem,
         owner: TypeOwner,
     ) -> Result<TypeId> {
+        // A WIT type has a plain name, not one like `a:b/c` or `[method]r.f`.
+        match self.parse_component_name(name)?.kind() {
+            ComponentNameKind::Plain(n) if n.is_bare() => {}
+            _ => bail!(
+                "cannot represent this component in WIT: type `{name}` doesn't have a plain name"
+            ),
+        }
         let (referenced, created) = match item.ty {
             ComponentEntityType::Type {
                 referenced,
@@ -1074,7 +1083,10 @@ impl WitPackageDecoder<'_> {
                         .convert_defined(&self.types[ty])
                         .context("failed to convert unaliased type")?,
                     ComponentAnyTypeId::Resource(_) => TypeDefKind::Resource,
-                    _ => unreachable!(),
+                    _ => bail!(
+                        "cannot represent this component in WIT: type `{name}` is a component, \
+                         instance or function type"
+                    ),
                 }
             }
         };
@@ -1222,7 +1234,19 @@ impl WitPackageDecoder<'_> {
 
                     let resource = plain
                         .resource()
-                        .map(|resource| self.resources[&owner][resource.as_str()]);
+                        .map(|resource| {
+                            self.resources
+                                .get(&owner)
+                                .and_then(|resources| resources.get(resource.as_str()))
+                                .copied()
+                                .with_context(|| {
+                                    format!(
+                                        "cannot represent this component in WIT: function \
+                                         `{name}` is not next to the definition of its resource"
+                                    )
+                                })
+                        })
+                        .transpose()?;
                     match (plain.resource_func, plain.accessor) {
                         (None, None) => {
                             if ty.async_ {
@@ -1273,8 +1297,11 @@ impl WitPackageDecoder<'_> {
                     }
                 }
 
-                // Functions shouldn't have ID-based names at this time.
-                ComponentNameKind::Interface(_) => unreachable!(),
+                // Functions can't have ID-based names in WIT.
+                ComponentNameKind::Interface(_) => bail!(
+                    "cannot represent this component in WIT: function `{name}` has an interface's \
+                     name"
+                ),
             },
 
             // Note that this name includes "name mangling" such as
diff --git a/crates/wit-parser/src/resolve/mod.rs b/crates/wit-parser/src/resolve/mod.rs
index 2680c1a011..e410bdbfa5 100644
--- a/crates/wit-parser/src/resolve/mod.rs
+++ b/crates/wit-parser/src/resolve/mod.rs
@@ -4442,20 +4442,30 @@ impl Remap {
     }
 
     fn type_has_borrow(&mut self, resolve: &Resolve, ty: &Type) -> bool {
-        let id = match ty {
-            Type::Id(id) => *id,
-            _ => return false,
+        // A chain of aliases is as long as the source makes it, so it is walked
+        // here and not recursed down.
+        let mut ty = *ty;
+        let mut walked = Vec::new();
+        let result = loop {
+            let id = match ty {
+                Type::Id(id) => id,
+                _ => break false,
+            };
+            if let Some(Some(has_borrow)) = self.type_has_borrow.get(id.index()) {
+                break *has_borrow;
+            }
+            walked.push(id);
+            match &resolve.types[id].kind {
+                TypeDefKind::Type(next) => ty = *next,
+                _ => break self.typedef_has_borrow(resolve, &resolve.types[id]),
+            }
         };
-
-        if let Some(Some(has_borrow)) = self.type_has_borrow.get(id.index()) {
-            return *has_borrow;
-        }
-
-        let result = self.typedef_has_borrow(resolve, &resolve.types[id]);
-        if self.type_has_borrow.len() <= id.index() {
-            self.type_has_borrow.resize(id.index() + 1, None);
+        for id in walked {
+            if self.type_has_borrow.len() <= id.index() {
+                self.type_has_borrow.resize(id.index() + 1, None);
+            }
+            self.type_has_borrow[id.index()] = Some(result);
         }
-        self.type_has_borrow[id.index()] = Some(result);
         result
     }
 
diff --git a/crates/wit-parser/tests/alias_chain.rs b/crates/wit-parser/tests/alias_chain.rs
new file mode 100644
index 0000000000..83d1f844ea
--- /dev/null
+++ b/crates/wit-parser/tests/alias_chain.rs
@@ -0,0 +1,53 @@
+//! A chain of type aliases is as long as the source makes it, and how deep the
+//! parser recurses must not depend on that: a stack overflow aborts the process.
+
+use wit_parser::Resolve;
+
+fn chain(aliases: usize) -> String {
+    let mut wit = String::from("package a:b;\ninterface i {\n  type t0 = u32;\n");
+    for i in 1..aliases {
+        wit.push_str(&format!("  type t{i} = t{};\n", i - 1));
+    }
+    let last = aliases - 1;
+    wit.push_str(&format!(
+        "  get: func(x: t{last}) -> t{last};\n}}\nworld w {{ import i; }}\n"
+    ));
+    wit
+}
+
+#[test]
+fn a_long_alias_chain_parses_on_a_small_stack() {
+    let wit = chain(200_000);
+    let parse = move || {
+        let mut resolve = Resolve::default();
+        resolve.push_str("chain.wit", &wit).unwrap();
+    };
+    // Without threads, as on wasm32-wasip1, parse on the main thread, whose
+    // stack is as small as the one asked for here.
+    if cfg!(target_family = "wasm") && !cfg!(target_feature = "atomics") {
+        parse();
+    } else {
+        std::thread::Builder::new()
+            .stack_size(1 << 20)
+            .spawn(parse)
+            .unwrap()
+            .join()
+            .unwrap();
+    }
+}
+
+#[test]
+fn a_borrow_at_the_end_of_an_alias_chain_is_still_found() {
+    let wit = "package a:b;
+        interface i {
+            resource r;
+            type b = borrow<r>;
+            type c = b;
+            type d = c;
+            get: func() -> d;
+            other: func() -> d;
+        }";
+    let mut resolve = Resolve::default();
+    let err = resolve.push_str("borrow.wit", wit).unwrap_err();
+    assert!(format!("{err:#}").contains("borrow<T>"), "{err:#}");
+}
diff --git a/crates/wit-parser/tests/decode_allocation.rs b/crates/wit-parser/tests/decode_allocation.rs
new file mode 100644
index 0000000000..4fbf3d4e83
--- /dev/null
+++ b/crates/wit-parser/tests/decode_allocation.rs
@@ -0,0 +1,38 @@
+//! A length in the input must not decide how much the decoder allocates.
+#![cfg(feature = "decoding")]
+#![allow(unsafe_code)]
+
+use std::alloc::{GlobalAlloc, Layout, System};
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+struct Largest;
+
+static LARGEST: AtomicUsize = AtomicUsize::new(0);
+
+unsafe impl GlobalAlloc for Largest {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        LARGEST.fetch_max(layout.size(), Ordering::Relaxed);
+        unsafe { System.alloc(layout) }
+    }
+    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, size: usize) -> *mut u8 {
+        LARGEST.fetch_max(size, Ordering::Relaxed);
+        unsafe { System.realloc(ptr, layout, size) }
+    }
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        unsafe { System.dealloc(ptr, layout) }
+    }
+}
+
+#[global_allocator]
+static ALLOCATOR: Largest = Largest;
+
+#[test]
+fn a_section_claiming_4_gib_allocates_little() {
+    // A component's header, then a custom section claiming nearly 4 GiB.
+    let bytes = [
+        0x00, 0x61, 0x73, 0x6d, 0x0d, 0x00, 0x01, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff, 0x0f,
+    ];
+    assert!(wit_parser::decoding::decode(&bytes).is_err());
+    let largest = LARGEST.load(Ordering::Relaxed);
+    assert!(largest < 1 << 20, "allocated {largest} bytes");
+}
diff --git a/tests/cli/component-wit-custom-section-world-exports-instance.wat b/tests/cli/component-wit-custom-section-world-exports-instance.wat
new file mode 100644
index 0000000000..14d9084643
--- /dev/null
+++ b/tests/cli/component-wit-custom-section-world-exports-instance.wat
@@ -0,0 +1,9 @@
+;; FAIL: component wit %
+;;
+;; The custom section holds the binary of:
+;;
+;;     (component (type $c (component (export "a:b/w" (instance)))) (export "w" (type $c)))
+
+(module
+    (@custom "component-type:x" "\00\61\73\6d\0d\00\01\00\07\10\01\41\02\01\42\00\04\00\05\61\3a\62\2f\77\05\00\0b\07\01\00\01\77\03\00\00\00\16\0e\63\6f\6d\70\6f\6e\65\6e\74\2d\6e\61\6d\65\01\05\03\01\00\01\63")
+)
diff --git a/tests/cli/component-wit-custom-section-world-exports-instance.wat.stderr b/tests/cli/component-wit-custom-section-world-exports-instance.wat.stderr
new file mode 100644
index 0000000000..046db9b6dd
--- /dev/null
+++ b/tests/cli/component-wit-custom-section-world-exports-instance.wat.stderr
@@ -0,0 +1,4 @@
+error: decoding custom section component-type:x
+
+Caused by:
+    0: cannot represent this component in WIT: export `a:b/w` is not a world
diff --git a/tests/cli/component-wit-custom-section-world-imports.wat b/tests/cli/component-wit-custom-section-world-imports.wat
new file mode 100644
index 0000000000..24bc273b80
--- /dev/null
+++ b/tests/cli/component-wit-custom-section-world-imports.wat
@@ -0,0 +1,9 @@
+;; FAIL: component wit %
+;;
+;; The custom section holds the binary of:
+;;
+;;     (component (type $c (component (import "a:b/c" (instance)) (export "a:b/w" (component)))) (export "w" (type $c)))
+
+(module
+    (@custom "component-type:x" "\00\61\73\6d\0d\00\01\00\07\1d\01\41\04\01\42\00\03\00\05\61\3a\62\2f\63\05\00\01\41\00\04\00\05\61\3a\62\2f\77\04\01\0b\07\01\00\01\77\03\00\00\00\16\0e\63\6f\6d\70\6f\6e\65\6e\74\2d\6e\61\6d\65\01\05\03\01\00\01\63")
+)
diff --git a/tests/cli/component-wit-custom-section-world-imports.wat.stderr b/tests/cli/component-wit-custom-section-world-imports.wat.stderr
new file mode 100644
index 0000000000..135818cbd0
--- /dev/null
+++ b/tests/cli/component-wit-custom-section-world-imports.wat.stderr
@@ -0,0 +1,4 @@
+error: decoding custom section component-type:x
+
+Caused by:
+    0: cannot represent this component in WIT: expected an exported component type with no imports and one export
diff --git a/tests/cli/component-wit-custom-section-world-no-export.wat b/tests/cli/component-wit-custom-section-world-no-export.wat
new file mode 100644
index 0000000000..d8e6f116e0
--- /dev/null
+++ b/tests/cli/component-wit-custom-section-world-no-export.wat
@@ -0,0 +1,9 @@
+;; FAIL: component wit %
+;;
+;; The custom section holds the binary of:
+;;
+;;     (component (type $c (component)) (export "x" (type $c)))
+
+(module
+    (@custom "component-type:x" "\00\61\73\6d\0d\00\01\00\07\03\01\41\00\0b\07\01\00\01\78\03\00\00\00\16\0e\63\6f\6d\70\6f\6e\65\6e\74\2d\6e\61\6d\65\01\05\03\01\00\01\63")
+)
diff --git a/tests/cli/component-wit-custom-section-world-no-export.wat.stderr b/tests/cli/component-wit-custom-section-world-no-export.wat.stderr
new file mode 100644
index 0000000000..135818cbd0
--- /dev/null
+++ b/tests/cli/component-wit-custom-section-world-no-export.wat.stderr
@@ -0,0 +1,4 @@
+error: decoding custom section component-type:x
+
+Caused by:
+    0: cannot represent this component in WIT: expected an exported component type with no imports and one export
diff --git a/tests/cli/component-wit-func-with-interface-name.wat b/tests/cli/component-wit-func-with-interface-name.wat
new file mode 100644
index 0000000000..4cab34b860
--- /dev/null
+++ b/tests/cli/component-wit-func-with-interface-name.wat
@@ -0,0 +1,5 @@
+;; FAIL: component wit %
+
+(component
+    (import "a:b/c" (func))
+)
diff --git a/tests/cli/component-wit-func-with-interface-name.wat.stderr b/tests/cli/component-wit-func-with-interface-name.wat.stderr
new file mode 100644
index 0000000000..14bd0f3d9a
--- /dev/null
+++ b/tests/cli/component-wit-func-with-interface-name.wat.stderr
@@ -0,0 +1,4 @@
+error: failed to decode function from import `a:b/c`
+
+Caused by:
+    0: cannot represent this component in WIT: function `a:b/c` has an interface's name
diff --git a/tests/cli/component-wit-implements-redefines-type-as-instance-type.wat b/tests/cli/component-wit-implements-redefines-type-as-instance-type.wat
new file mode 100644
index 0000000000..2fc9e8614c
--- /dev/null
+++ b/tests/cli/component-wit-implements-redefines-type-as-instance-type.wat
@@ -0,0 +1,12 @@
+;; FAIL: component wit %
+
+(component
+    (import "x" (implements "a:b/c") (instance
+        (type $t u32)
+        (export "t" (type (eq $t)))
+    ))
+    (import "y" (implements "a:b/c") (instance
+        (type $i (instance))
+        (export "t" (type (eq $i)))
+    ))
+)
diff --git a/tests/cli/component-wit-implements-redefines-type-as-instance-type.wat.stderr b/tests/cli/component-wit-implements-redefines-type-as-instance-type.wat.stderr
new file mode 100644
index 0000000000..e8aac07149
--- /dev/null
+++ b/tests/cli/component-wit-implements-redefines-type-as-instance-type.wat.stderr
@@ -0,0 +1,4 @@
+error: failed to decode WIT from import `y`
+
+Caused by:
+    0: cannot represent this component in WIT: type `t` is a component, instance or function type
diff --git a/tests/cli/component-wit-import-component-type.wat b/tests/cli/component-wit-import-component-type.wat
new file mode 100644
index 0000000000..92803cf7d2
--- /dev/null
+++ b/tests/cli/component-wit-import-component-type.wat
@@ -0,0 +1,6 @@
+;; FAIL: component wit %
+
+(component
+    (type $c (component))
+    (import "t" (type (eq $c)))
+)
diff --git a/tests/cli/component-wit-import-component-type.wat.stderr b/tests/cli/component-wit-import-component-type.wat.stderr
new file mode 100644
index 0000000000..5c32fdffef
--- /dev/null
+++ b/tests/cli/component-wit-import-component-type.wat.stderr
@@ -0,0 +1,4 @@
+error: failed to decode type from export `t`
+
+Caused by:
+    0: cannot represent this component in WIT: type `t` is a component, instance or function type
diff --git a/tests/cli/component-wit-instance-exports-func-type.wat b/tests/cli/component-wit-instance-exports-func-type.wat
new file mode 100644
index 0000000000..5c8b0868a5
--- /dev/null
+++ b/tests/cli/component-wit-instance-exports-func-type.wat
@@ -0,0 +1,8 @@
+;; FAIL: component wit %
+
+(component
+    (import "i" (instance
+        (type $f (func))
+        (export "t" (type (eq $f)))
+    ))
+)
diff --git a/tests/cli/component-wit-instance-exports-func-type.wat.stderr b/tests/cli/component-wit-instance-exports-func-type.wat.stderr
new file mode 100644
index 0000000000..9e26828071
--- /dev/null
+++ b/tests/cli/component-wit-instance-exports-func-type.wat.stderr
@@ -0,0 +1,5 @@
+error: failed to decode WIT from import `i`
+
+Caused by:
+    0: failed to register type export 't'
+    1: cannot represent this component in WIT: type `t` is a component, instance or function type
diff --git a/tests/cli/component-wit-instance-type-with-interface-name.wat b/tests/cli/component-wit-instance-type-with-interface-name.wat
new file mode 100644
index 0000000000..8a1dba6ae7
--- /dev/null
+++ b/tests/cli/component-wit-instance-type-with-interface-name.wat
@@ -0,0 +1,8 @@
+;; FAIL: component wit %
+
+(component
+    (import "i" (instance
+        (type (list u8))
+        (export "a:b/c@1.0.0" (type (eq 0)))
+    ))
+)
diff --git a/tests/cli/component-wit-instance-type-with-interface-name.wat.stderr b/tests/cli/component-wit-instance-type-with-interface-name.wat.stderr
new file mode 100644
index 0000000000..77195df3ed
--- /dev/null
+++ b/tests/cli/component-wit-instance-type-with-interface-name.wat.stderr
@@ -0,0 +1,5 @@
+error: failed to decode WIT from import `i`
+
+Caused by:
+    0: failed to register type export 'a:b/c@1.0.0'
+    1: cannot represent this component in WIT: type `a:b/c@1.0.0` doesn't have a plain name
diff --git a/tests/cli/component-wit-method-on-used-resource.wat b/tests/cli/component-wit-method-on-used-resource.wat
new file mode 100644
index 0000000000..3644e7690b
--- /dev/null
+++ b/tests/cli/component-wit-method-on-used-resource.wat
@@ -0,0 +1,12 @@
+;; FAIL: component wit %
+
+(component
+    (import "a:b/d" (instance $d
+        (export "r" (type (sub resource)))
+    ))
+    (alias export $d "r" (type $r))
+    (import "a:b/e" (instance
+        (export "r" (type $x (eq $r)))
+        (export "[method]r.f" (func (param "self" (borrow $x))))
+    ))
+)
diff --git a/tests/cli/component-wit-method-on-used-resource.wat.stderr b/tests/cli/component-wit-method-on-used-resource.wat.stderr
new file mode 100644
index 0000000000..bc1650a50e
--- /dev/null
+++ b/tests/cli/component-wit-method-on-used-resource.wat.stderr
@@ -0,0 +1,4 @@
+error: failed to decode WIT from import `a:b/e`
+
+Caused by:
+    0: cannot represent this component in WIT: function `[method]r.f` is not next to the definition of its resource
diff --git a/tests/cli/component-wit-type-with-interface-name.wat b/tests/cli/component-wit-type-with-interface-name.wat
new file mode 100644
index 0000000000..196ec6564f
--- /dev/null
+++ b/tests/cli/component-wit-type-with-interface-name.wat
@@ -0,0 +1,6 @@
+;; FAIL: component wit %
+
+(component
+    (type (list u8))
+    (import "a:b/c" (type (eq 0)))
+)
diff --git a/tests/cli/component-wit-type-with-interface-name.wat.stderr b/tests/cli/component-wit-type-with-interface-name.wat.stderr
new file mode 100644
index 0000000000..fb9810f929
--- /dev/null
+++ b/tests/cli/component-wit-type-with-interface-name.wat.stderr
@@ -0,0 +1,4 @@
+error: failed to decode type from export `a:b/c`
+
+Caused by:
+    0: cannot represent this component in WIT: type `a:b/c` doesn't have a plain name